C++ Style Guide

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Give as descriptive a name as possible, within reason. Do not worry about saving horizontal space as it is far more important to make your code immediately understandable by a new reader. Do not use abbreviations that are ambiguous or unfamiliar to readers outside your project, and do not abbreviate by deleting letters within a word. Short names, such as x and i, are meaningful when used conventionally; that is, x should be a local variable or a parameter and i should be a loop index.

int number_particles_;   // No abbreviation.
int num_errors;          // "num" is a widespread convention.
int num_mpm_point;       // Most people know what "mpm" stands for.
for (int i = 0; ...      // Used conventionally as a loop index.

int n;                   // Meaningless.
int nerr;                // Ambiguous abbreviation.
int n_comp_conns;        // Ambiguous abbreviation.
int lem_connections;     // Only your group knows what this stands for.
int pc_reader;           // Lots of things can be abbreviated "pc".
int prtcl_id;            // Deletes internal letters.
int the_number_of_elements_in_a_cell
                         // A bit too long.

Examples of acceptable file names:

particle_soil_mpm.cc
particlesoilmpm.cc
particlesoilmpm_test.cc // _unittest/_regtest are deprecated.


Do not use filenames that already exist in /usr/include , such as error.h .

In general, make your filenames very specific. For example, use particle_soil_mpm.h rather than particle.h . A very common case is to have a pair of files called, e.g., foo_bar.h and foo_bar.cc , defining a class called FooBar .

Inline functions must be in a .h file. However, for shared template code the class definitions may go into a third file that ends in .tcc .

particle_soil_mpm.h       // The class declaration.
particle_soil_mpm.cc      // The class definition.
particle_soil_mpm.tcc     // Templatised class definitions.

Don't forget that header files require #define guard .

See .tcc files for more information on how to defined shared templatised classes.

The names of all types — classes, structs, typedefs, and enums — have the same naming convention. Type names should start with a capital letter and have a capital letter for each new word. No underscores.

When confronted with a situation where you could use an all upper case abbreviation instead use an initial upper case letter followed by all lower case letters. No matter what. For example:

// classes and structs
class ParticleSoilMpm { ...
class ParticleMpm { ...
struct ParticleMpmProperties { ...

// typedefs
typedef hash_map<ParticleMpm*, std::string> PropertiesMap;

// enums
enum MpmErrors { ...

Don't use names beginning with "my". The word "my" isn't generally meaningful, since it can refer to any human in the world. If you want to personalize a class or variable name, give your class or variable a name that describes its function, not its authorship.

Common Variable names

For example:

std::string material_name;  // OK - uses underscore.
std::string materialname;   // OK - all lowercase.

std::string materialName;   // Bad - mixed case.

Class Data Members

Data members (also called instance variables or member variables) are lowercase with optional underscores like regular variable names, but always end with a trailing underscore.

std::string material_name_;  // OK - underscore at end.
std::string materialname_;   // OK.

Struct Variables

Data members in structs should be named like regular variables without the trailing underscores that data members in classes have.

struct ParticleMpmProperties {
  std::string name;
  int num_points;
}

See Structs vs. Classes for a discussion of when to use a struct versus a class.

Enumerator Variables

Enumerator variables in enum should be named like regular variables.

enum MpmMaterials { linear_elastic, bingham, camclay };

Global Variables

Really? are you going to use global variables in your code? If your code requires global variables, think again, and restructure your code. In worst case, define your global variables as const inside a namespace.

Regular Functions

Functions should be lower case. Use underscores to seperate words.

If your function crashes upon an error, you should append _or_die to the function name. This only applies to functions which could be used by production code and to errors that are reasonably likely to occur during normal operation.

add_new_material()
delete_material_point()
open_file_or_die()

Accessors and Mutators

Accessors and mutators (get and set functions) should match the name of the variable they are getting and setting. This shows an excerpt of a class whose instance variable is num_materials_ .

class MpmMaterials {
public:
...
int num_materials() const
{
    return num_materials_;
}
void set_num_materials(int num_materials)
{
  num_materials_ = num_materials;
}

private:
int num_materials_;
};

You may also use lowercase letters for other very short inlined functions. For example if a function were so cheap you would not cache the value if you were calling it in a loop, then lowercase naming would be acceptable.

See Namespaces for a discussion of namespaces and how to name them.

Please see the description of macros ; in general macros should not be used. However, the only exception being #define guards, then they should be named with all capitals and underscores. Macros should not be used for any other purpose.

Use facilities from the C++ language proper, such as inline functions, templates, constructors (for initialization), destructors (for cleanup), exceptions (for exiting contexts), etc instead of using Macros.

#define _MPM2D_
#define SRC_CARTESIAN_BINGHAM_FLUID_H_

bigopen()

function name, follows form of open()

uint

typedef

bigpos

struct or class , follows form of pos

sparse_hash_map

STL-like entity; follows STL naming conventions

LONGLONG_MAX

a constant, as in INT_MAX

To guarantee uniqueness, they should be based on the full path in a project's source tree. For example, the file mpm/src/material/bingham.h in project mpm should have the following guard:

#ifndef MPM_SRC_MATERIAL_BINGHAM_H_
#define MPM_SRC_MATERIAL_BINGHAM_H_

...

#endif  // MPM_SRC_MATERIAL_BINGHAM_H_

Definition: A "forward declaration" is a declaration of a class, function, or template without an associated definition. #include lines can often be replaced with forward declarations of whatever symbols are actually used by the client code.

Pros:

• Unnecessary #include s force the compiler to open more files and process more input.
• They can also force your code to be recompiled more often, due to changes in the header.

Cons:

• It can be difficult to determine the correct form of a forward declaration in the presence of features like templates, typedefs, default parameters, and using declarations.
• It can be difficult to determine whether a forward declaration or a full #include is needed for a given piece of code, particularly when implicit conversion operations are involved. In extreme cases, replacing an #include with a forward declaration can silently change the meaning of code.
• Forward declaring multiple symbols from a header can be more verbose than simply #include ing the header.
• Forward declarations of functions and templates can prevent the header owners from making otherwise-compatible changes to their APIs; for example, widening a parameter type, or adding a template parameter with a default value.
• Forward declaring symbols from namespace std:: usually yields undefined behavior.
• Structuring code to enable forward declarations (e.g. using pointer members instead of object members) can make the code slower and more complex.
• The practical efficiency benefits of forward declarations are unproven.

Decision:

• When using a function declared in a header file, always #include that header.
• When using a class template, prefer to #include its header file.
• When using an ordinary class, relying on a forward declaration is OK, but be wary of situations where a forward declaration may be insufficient or incorrect; when in doubt, just #include the appropriate header.
• Do not replace data members with pointers just to avoid an #include .

Definition: You can declare functions in a way that allows the compiler to expand them inline rather than calling them through the usual function call mechanism.

Pros: Inlining a function can generate more efficient object code, as long as the inlined function is small. Feel free to inline accessors and mutators, and other short, performance-critical functions.

Cons: Overuse of inlining can actually make programs slower. Depending on a function's size, inlining it can cause the code size to increase or decrease. Inlining a very small accessor function will usually decrease code size while inlining a very large function can dramatically increase code size. On modern processors smaller code usually runs faster due to better use of the instruction cache.

Decision:

The definition of an inline function needs to be in a header file, so that the compiler has the definition available for inlining at the call sites. However, implementation code properly belongs in .cc files, and we do not like to have much actual code in .h files unless there is a readability or performance advantage. For example, accessors and mutators should certainly be inside a class definition.

A decent rule of thumb is to not inline a function if it is more than 5 lines long. Beware of destructors, which are often longer than they appear because of implicit member- and base-destructor calls!

Another useful rule of thumb: it's typically not cost effective to inline functions with loops or switch statements (unless, in the common case, the loop or switch statement is never executed).

It is important to know that functions are not always inlined even if they are declared as such; for example, virtual and recursive functions are not normally inlined. Usually recursive functions should not be inline. The main reason for making a virtual function inline is to place its definition in the class, either for convenience or to document its behavior, e.g., for accessors and mutators.

A template is not a class or a function. A template is a "pattern" that the compiler uses to generate a family of classes or functions.

In order for the compiler to generate the code, it must see both the template definition (not just declaration) and the specific types/whatever used to "fill in" the template. For example, if you're trying to use a Foo<int> , the compiler must see both the Foo template and the fact that you're trying to make a specific Foo<int> .

A common solution to this is to write the template declaration in a header file, then implement the class in an implementation file (for example .tcc), and include this implementation file at the end of the header.

// foo.h
template <typename T>
struct Foo {
    void do_something(T param);
};

#include "foo.tcc"

// foo.tcc
template <typename T>
void Foo<T>::do_something(T param)
{
    //implementation
}

This way, implementation is still separated from declaration, but is accessible to the compiler.

Parameters to C/C++ functions are either input to the function, output from the function, or both. Input parameters are usually values or const references, while output and input/output parameters will be non- const pointers. When ordering function parameters, put all input-only parameters before any output parameters. In particular, do not add new parameters to the end of the function just because they are new; place new input-only parameters before the output parameters.

This is not a hard-and-fast rule. Parameters that are both input and output (often classes/structs) muddy the waters, and, as always, consistency with related functions may require you to bend the rule.

All of a project's header files should be listed as descendants of the project's source directory without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory). For example, mpm/src/material/bingham.h should be included as

#include "material/bingham.h"

In dir/foo .cc or dir/foo_test .cc , whose main purpose is to implement or test the stuff in dir2/foo2 .h , order your includes as follows:

1. dir2/foo2 .h (preferred location — see details below).
2. C system files.
3. C++ system files.
4. Other libraries' .h files.
5. Your project's .h files.

With the preferred ordering, if dir2/foo2 .h omits any necessary includes, the build of dir/foo .cc or dir/foo _test.cc will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages.

dir/foo .cc and dir2/foo2 .h are often in the same directory (e.g. base/basictypes_test.cc and base/basictypes.h ), but can be in different directories too.

Within each section the includes should be ordered alphabetically. Note that older code might not conform to this rule and should be fixed when convenient.

For example, the includes in mpm/src/material/base/materialbase.cc might look like this:

#include "mpm/base/materialbase.h"  // Preferred location.

#include <sys/types.h>
#include <unistd.h>
#include <std::vector>

#include "base/materialbase.h"
#include "base/materialcontainer.h"

Exception: sometimes, system-specific code needs conditional includes. Such code can put conditional includes after other includes. Of course, keep your system-specific code small and localized.

Definition: Namespaces subdivide the global scope into distinct, named scopes, and so are useful for preventing name collisions in the global scope.

Pros:

Namespaces provide a (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provided by classes.

For example, if two different projects have a class Foo in the global scope, these symbols may collide at compile time or at runtime. If each project places their code in a namespace, project1::Foo and project2::Foo are now distinct symbols that do not collide.

Inline namespaces automatically place their names in the enclosing scope. Consider the following snippet, for example:

namespace x {
inline namespace y {
void foo();
}
}

The expressions x::y::foo() and x::foo() are interchangeable. Inline namespaces are primarily intended for ABI compatibility across versions.

Cons:

Namespaces can be confusing, because they provide an additional (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provided by classes.

Inline namespaces, in particular, can be confusing because names aren't actually restricted to the namespace where they are declared. They are only useful as part of some larger versioning policy.

Use of unnamed namespaces in header files can easily cause violations of the C++ One Definition Rule (ODR).

Decision:

Use namespaces according to the policy described below. Terminate namespaces with comments as shown in the given examples.

Unnamed Namespaces

• Unnamed namespaces are allowed and even encouraged in .cc files, to avoid runtime naming conflicts:

  namespace {                           // This is in .cc file.
  
  // The content of a namespace is not indented
  enum { unused, eof, error };          // Commonly used
                                        // tokens.
  bool AtEof() { return pos_ == eof; }  // Uses our namespace's
                                        // EOF.
  
  }  // namespace

  However, file-scope declarations that are associated with a particular class may be declared in that class as types, static data members or static member functions rather than as members of an unnamed namespace.

• Do not use unnamed namespaces in .h files.

Named Namespaces

Named namespaces should be used as follows:

• Namespaces wrap the entire source file after includes, definitions/declarations, and forward declarations of classes from other namespaces:

  // In the .h file
  namespace material {
  
  // All declarations are within the namespace scope.
  // Notice the lack of indentation.
  class Bingham {
   public:
    ...
    void foo();
  };
  
  }  // namespace material

  // In the .cc file
  namespace material {
  
  // Definition of functions is within scope of the namespace.
  void Bingham::foo() {
    ...
  }
  
  }  // namespace material

  The typical .cc file might have more complex detail, including the need to reference classes in other namespaces.

  #include "a.h"
  
  class C;  // Forward declaration of class C
            // in the global namespace.
  namespace a { class A; }  // Forward declaration of a::A.
  
  namespace b {
  
  ...code for b...         // Code goes against the left margin.
  
  }  // namespace b

• Do not declare anything in namespace std , not even forward declarations of standard library classes. Declaring entities in namespace std is undefined behavior, i.e., not portable. To declare entities from the standard library, include the appropriate header file.
• You may not use a using-directive to make all names from a namespace available.

  // Forbidden -- This pollutes the namespace.
  using namespace foo;
  using namespace std;

• You may use a using-declaration anywhere in a .cc file, and in functions, methods or classes in .h files.

  // OK in .cc files.
  // Must be in a function, method or class in .h files.
  using ::foo::bar;

• Namespace aliases are allowed anywhere in a .cc file, anywhere inside the named namespace that wraps an entire .h file, and in functions and methods.

  // Shorten access to some commonly used names in .cc files.
  namespace fbz = ::foo::bar::baz;
  
  // Shorten access to some commonly used names (in a .h file).
  namespace librarian {
  // The following alias is available to all files including
  // this header (in namespace librarian):
  // alias names should therefore be chosen consistently
  // within a project.
  namespace pd_s = ::pipeline_diagnostics::sidetable;
  
  inline void mpm_inline_function() {
    // namespace alias local to a function (or method).
    namespace fbz = ::foo::bar::baz;
    ...
  }
  }  // namespace librarian

  Note that an alias in a .h file is visible to everyone #including that file, so public headers (those available outside a project) and headers transitively #included by them, should avoid defining aliases, as part of the general goal of keeping public APIs as small as possible.

• Do not use inline namespaces.

Definition: A class can define another class within it; this is also called a member class .

class Foo {

 private:
  // Bar is a member class, nested within Foo.
  class Bar {
    ...
  };

};

Pros: This is useful when the nested (or member) class is only used by the enclosing class; making it a member puts it in the enclosing class scope rather than polluting the outer scope with the class name. Nested classes can be forward declared within the enclosing class and then defined in the .cc file to avoid including the nested class definition in the enclosing class declaration, since the nested class definition is usually only relevant to the implementation.

Cons: Nested classes can be forward-declared only within the definition of the enclosing class. Thus, any header file manipulating a Foo::Bar* pointer will have to include the full class declaration for Foo .

Decision: Do not make nested classes public unless they are actually part of the interface, e.g., a class that holds a set of options for some method.

Pros: Nonmember and static member functions can be useful in some situations. Putting nonmember functions in a namespace avoids polluting the global namespace.

Cons: Nonmember and static member functions may make more sense as members of a new class, especially if they access external resources or have significant dependencies.

Decision:

Sometimes it is useful, or even necessary, to define a function not bound to a class instance. Such a function can be either a static member or a nonmember function. Nonmember functions should not depend on external variables, and should nearly always exist in a namespace. Rather than creating classes only to group static member functions which do not share static data, use namespaces instead.

Functions defined in the same compilation unit as production classes may introduce unnecessary coupling and link-time dependencies when directly called from other compilation units; static member functions are particularly susceptible to this. Consider extracting a new class, or placing the functions in a namespace possibly in a separate library.

If you must define a nonmember function and it is only needed in its .cc file, use an unnamed namespace or static linkage (eg static int Foo() {...} ) to limit its scope.

C++ allows you to declare variables anywhere in a function. We encourage you to declare them in as local a scope as possible, and as close to the first use as possible. This makes it easier for the reader to find the declaration and see what type the variable is and what it was initialized to. In particular, initialization should be used instead of declaration and assignment, e.g.

int i;
i = f();      // Bad -- initialization separate from declaration.

int j = g();  // Good -- declaration has initialization.

std::vector<int> v;
v.push_back(1);  // Prefer initializing using brace initialization.
v.push_back(2);

std::vector<int> v = {1, 2};  // Good -- v starts initialized.

Note that gcc implements for (int i = 0; i < 10; ++i) correctly (the scope of i is only the scope of the for loop), so you can then reuse i in another for loop in the same scope. It also correctly scopes declarations in if and while statements, e.g.

while (const char* p = strchr(str, '/')) str = p + 1;

There is one caveat: if the variable is an object, its constructor is invoked every time it enters scope and is created, and its destructor is invoked every time it goes out of scope.

// Inefficient implementation:
for (int i = 0; i < 1000000; ++i) {
  Foo f;  // Constructor & Destructor get called 1000000 times each.
  f.do_something(i);
}

It may be more efficient to declare such a variable used in a loop outside that loop:

Foo f;  // ctor and dtor get called once each.
for (int i = 0; i < 1000000; ++i) {
  f.do_something(i);
}

Global variables are allowed, only when you declare them as const and are defined inside a namespace.

Objects with static storage duration, including global variables, static variables, static class member variables, and function static variables, must be Plain Old Data (POD): only ints, chars, floats, or pointers, or arrays/structs of POD.

The order in which class constructors and initializers for static variables are called is only partially specified in C++ and can even change from build to build, which can cause bugs that are difficult to find. Therefore in addition to banning globals of class type, we do not allow static POD variables to be initialized with the result of a function, unless that function (such as getenv(), or getpid()) does not itself depend on any other globals.

Likewise, global and static variables are destroyed when the program terminates, regardless of whether the termination is by returning from main() or by calling exit() . The order in which destructors are called is defined to be the reverse of the order in which the constructors were called. Since constructor order is indeterminate, so is destructor order. For example, at program-end time a static variable might have been destroyed, but code still running — perhaps in another thread — tries to access it and fails. Or the destructor for a static std::string variable might be run prior to the destructor for another variable that contains a reference to that string.

One way to alleviate the destructor problem is to terminate the program by calling quick_exit() instead of exit() . The difference is that quick_exit() does not invoke destructors and does not invoke any handlers that were registered by calling atexit() . If you have a handler that needs to run when a program terminates via quick_exit() (flushing logs, for example), you can register it using at_quick_exit() . (If you have a handler that needs to run at both exit() and quick_exit() , you need to register it in both places.)

As a result we only allow static variables to contain POD data. This rule completely disallows std::vector (use C arrays instead), or std::string (use const char [] ).

If you need a static or global variable of a class type, consider initializing a pointer (which will never be freed), from either your main() function or from pthread_once(). Note that this must be a raw pointer, not a "smart" pointer, since the smart pointer's destructor will have the order-of-destructor issue that we are trying to avoid.

Definition: It is possible to perform initialization in the body of the constructor.

Pros: Convenience in typing. No need to worry about whether the class has been initialized or not.

Cons: The problems with doing work in constructors are:

• There is no easy way for constructors to signal errors, short of using exceptions (which are forbidden ).
• If the work fails, we now have an object whose initialization code failed, so it may be an indeterminate state.
• If the work calls virtual functions, these calls will not get dispatched to the subclass implementations. Future modification to your class can quietly introduce this problem even if your class is not currently subclassed, causing much confusion.
• If someone creates a global variable of this type (which is against the rules, but still), the constructor code will be called before main() , possibly breaking some implicit assumptions in the constructor code.

Decision: Constructors should never call virtual functions or attempt to raise non-fatal failures. If your object requires non-trivial initialization, consider using a factory function or Init() method.

Definition: The default constructor is called when we new a class object with no arguments. It is always called when calling new[] (for arrays). In-class member initialization means declaring a member variable using a construction like int num_errors_ = 17; or std::string error_name_{"convergence"}; , as opposed to just int count_; or std::string name_; .

Pros:

A user defined default constructor is used to initialize an object if no initializer is provided. It can ensure that an object is always in a valid and usable state as soon as it's constructed; it can also ensure that an object is initially created in an obviously "impossible" state, to aid debugging.

In-class member initialization ensures that a member variable will be initialized appropriately without having to duplicate the initialization code in multiple constructors. This can reduce bugs where you add a new member variable, initialize it in one constructor, and forget to put that initialization code in another constructor.

Cons:

Explicitly defining a default constructor is extra work for you, the code writer.

In-class member initialization is potentially confusing if a member variable is initialized as part of its declaration and also initialized in a constructor, since the value in the constructor will override the value in the declaration.

Decision:

Use in-class member initialization for simple initializations, especially when a member variable must be initialized the same way in more than one constructor.

If your class defines member variables that aren't initialized in-class, and if it has no other constructors, you must define a default constructor (one that takes no arguments). It should preferably initialize the object in such a way that its internal state is consistent and valid.

The reason for this is that if you have no other constructors and do not define a default constructor, the compiler will generate one for you. This compiler generated constructor may not initialize your object sensibly.

If your class inherits from an existing class but you add no new member variables, you are not required to have a default constructor.

Definition: Normally, if a constructor takes one argument, it can be used as a conversion. For instance, if you define Foo::Foo(std::string name) and then pass a string to a function that expects a Foo , the constructor will be called to convert the string into a Foo and will pass the Foo to your function for you. This can be convenient but is also a source of trouble when things get converted and new objects created without you meaning them to. Declaring a constructor explicit prevents it from being invoked implicitly as a conversion.

Pros: Avoids undesirable conversions.

Cons: None.

Decision:

We require all single argument constructors to be explicit. Always put explicit in front of one-argument constructors in the class definition: explicit Foo(std::string name);

The exception is copy constructors, which, in the rare cases when we allow them, should probably not be explicit . Classes that are intended to be transparent wrappers around other classes are also exceptions. Such exceptions should be clearly marked with comments.

Finally, constructors that take only an initializer_list may be non-explicit. This is to permit construction of your type using the assigment form for brace init lists (i.e. MpmType materials = {1, 2} ).

Definition: The copy constructor and assignment operator are used to create copies of objects. The copy constructor is implicitly invoked by the compiler in some situations, e.g. passing objects by value.

Pros: Copy constructors make it easy to copy objects. STL containers require that all contents be copyable and assignable. Copy constructors can be more efficient than CopyFrom() -style workarounds because they combine construction with copying, the compiler can elide them in some contexts, and they make it easier to avoid heap allocation.

Cons: Implicit copying of objects in C++ is a rich source of bugs and of performance problems. It also reduces readability, as it becomes hard to track which objects are being passed around by value as opposed to by reference, and therefore where changes to an object are reflected.

Decision:

Few classes need to be copyable. Most should have neither a copy constructor nor an assignment operator. In many situations, a pointer or reference will work just as well as a copied value, with better performance. For example, you can pass function parameters by reference or pointer instead of by value, and you can store pointers rather than objects in an STL container.

If your class needs to be copyable, prefer providing a copy method, such as CopyFrom() or Clone() , rather than a copy constructor, because such methods cannot be invoked implicitly. If a copy method is insufficient in your situation (e.g. for performance reasons, or because your class needs to be stored by value in an STL container), provide both a copy constructor and assignment operator.

If your class does not need a copy constructor or assignment operator, you must explicitly disable them. To do so, add dummy declarations for the copy constructor and assignment operator in the private: section of your class, but do not provide any corresponding definition (so that any attempt to use them results in a link error).

Definition:

Delegating and inheriting constructors are two different features, both introduced in C++11, for reducing code duplication in constructors. Delegating constructors allow one of a class's constructors to forward work to one of the class's other constructors, using a special variant of the initialization list syntax. For example:

X::X(const std::string& name) : name_(name) {
  ...
}

X::X() : X("") { }

Inheriting constructors allow a derived class to have its base class's constructors available directly, just as with any of the base class's other member functions, instead of having to redeclare them. This is especially useful if the base has multiple constructors. For example:

class Base {
public:
  Base();
  Base(int n);
  Base(const std::string& s);
  ...
};

class Derived : public Base {
public:
  using Base::Base;  // Base's constructors are redeclared here.
};

This is especially useful when Derived 's constructors don't have to do anything more than calling Base 's constructors.

Pros:

Delegating and inheriting constructors reduce verbosity and boilerplate, which can improve readability.

Delegating constructors are familiar to Java programmers.

Cons:

It's possible to approximate the behavior of delegating constructors by using a helper function.

Inheriting constructors may be confusing if a derived class introduces new member variables, since the base class constructor doesn't know about them.

Decision:

Use delegating and inheriting constructors when they reduce boilerplate and improve readability. Be cautious about inheriting constructors when your derived class has new member variables. Inheriting constructors may still be appropriate in that case if you can use in-class member initialization for the derived class's member variables.

The struct and class keywords behave almost identically in C++. We add our own semantic meanings to each keyword, so you should use the appropriate keyword for the data-type you're defining.

structs should be used for passive objects that carry data, and may have associated constants, but lack any functionality other than access/setting the data members. The accessing/setting of fields is done by directly accessing the fields rather than through method invocations. Methods should not provide behavior but should only be used to set up the data members, e.g., constructor, destructor, Initialize() , Reset() , Validate() .

If more functionality is required, a class is more appropriate. If in doubt, make it a class .

For consistency with STL, you can use struct instead of class for functors and traits.

Note that member variables in structs and classes have different naming rules .

Definition: When a sub-class inherits from a base class, it includes the definitions of all the data and operations that the parent base class defines. In practice, inheritance is used in two major ways in C++: implementation inheritance, in which actual code is inherited by the child, and interface inheritance , in which only method names are inherited.

Pros: Implementation inheritance reduces code size by re-using the base class code as it specializes an existing type. Because inheritance is a compile-time declaration, you and the compiler can understand the operation and detect errors. Interface inheritance can be used to programmatically enforce that a class expose a particular API. Again, the compiler can detect errors, in this case, when a class does not define a necessary method of the API.

Cons: For implementation inheritance, because the code implementing a sub-class is spread between the base and the sub-class, it can be more difficult to understand an implementation. The sub-class cannot override functions that are not virtual, so the sub-class cannot change implementation. The base class may also define some data members, so that specifies physical layout of the base class.

Decision:

All inheritance should be public . If you want to do private inheritance, you should be including an instance of the base class as a member instead.

Do not overuse implementation inheritance. Composition is often more appropriate. Try to restrict use of inheritance to the "is-a" case: Bar subclasses Foo if it can reasonably be said that Bar "is a kind of" Foo .

Make your destructor virtual if necessary. If your class has virtual methods, its destructor should be virtual.

Limit the use of protected to those member functions that might need to be accessed from subclasses. Note that data members should be private .

When redefining an inherited virtual function, explicitly declare it virtual in the declaration of the derived class. Rationale: If virtual is omitted, the reader has to check all ancestors of the class in question to determine if the function is virtual or not.

Definition: Multiple inheritance allows a sub-class to have more than one base class. We distinguish between base classes that are pure interfaces and those that have an implementation .

Pros: Multiple implementation inheritance may let you re-use even more code than single inheritance (see Inheritance ).

Cons: Only very rarely is multiple implementation inheritance actually useful. When multiple implementation inheritance seems like the solution, you can usually find a different, more explicit, and cleaner solution.

Decision: Multiple inheritance is allowed only when all superclasses, with the possible exception of the first one, are pure interfaces .

Definition:

A class is a interface or an abstract base class if it meets the following requirements:

• It has only public pure virtual (" = 0 ") methods and static methods (but see below for destructor).
• It may not have non-static data members.
• It need not have any constructors defined. If a constructor is provided, it must take no arguments and it must be protected.
• If it is a subclass, it may only be derived from classes that satisfy these conditions.

An interface class can never be directly instantiated because of the pure virtual method(s) it declares. To make sure all implementations of the interface can be destroyed correctly, the interface must also declare a virtual destructor (in an exception to the first rule, this should not be pure). See Stroustrup, The C++ Programming Language , 3rd edition, section 12.4 for details.

Definition: A class can define that operators such as + and / operate on the class as if it were a built-in type. An overload of operator"" allows the built-in literal syntax to be used to create objects of class types.

Pros:

Operator overloading can make code appear more intuitive because a class will behave in the same way as built-in types (such as int ). Overloaded operators are more playful names for functions that are less-colorfully named, such as Equals() or Add() .

For some template functions to work correctly, you may need to define operators.

User-defined literals are a very concise notation for creating objects of user-defined types.

Cons: While operator overloading can make code more intuitive, it has several drawbacks:

• It can fool our intuition into thinking that expensive operations are cheap, built-in operations.
• It is much harder to find the call sites for overloaded operators. Searching for Equals() is much easier than searching for relevant invocations of == .
• Some operators work on pointers too, making it easy to introduce bugs. Foo + 4 may do one thing, while &Foo + 4 does something totally different. The compiler does not complain for either of these, making this very hard to debug.
• User-defined literals allow creating new syntactic forms that are unfamiliar even to experienced C++ programmers.

Decision:

In general, do not overload operators. The assignment operator ( operator= ), in particular, is insidious and should be avoided. You can define functions like Equals() and CopyFrom() if you need them. Likewise, avoid the dangerous unary operator& at all costs, if there's any possibility the class might be forward-declared.

Do not overload operator"" , i.e. do not introduce user-defined literals.

However, there may be rare cases where you need to overload an operator to interoperate with templates or "standard" C++ classes (such as operator<<(ostream&, const T&) for logging). These are acceptable if fully justified, but you should try to avoid these whenever possible. In particular, do not overload operator== or operator< just so that your class can be used as a key in an STL container; instead, you should create equality and comparison functor types when declaring the container.

Some of the STL algorithms do require you to overload operator== , and you may do so in these cases, provided you document why.

See also Copy Constructors and Function Overloading .

The definitions of accessors are usually inlined in the header file.

See also Inheritance and Function Names .

Your class definition should start with its public: section, followed by its protected: section and then its private: section. If any of these sections are empty, omit them.

Within each section, the declarations generally should be in the following order:

• Typedefs and Enums
• Constants ( static const data members)
• Constructors
• Destructor
• Methods, including static methods
• Data Members (except static const data members)

Friend declarations should always be in the private section. It should be the last thing in the class. See Copy Constructors .

Method definitions in the corresponding .cc file should be the same as the declaration order, as much as possible.

Do not put large method definitions inline in the class definition. Usually, only trivial or performance-critical, and very short, methods may be defined inline. See Inline Functions for more details.

We recognize that long functions are sometimes appropriate, so no hard limit is placed on functions length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program.

Even if your long function works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code.

You could find long and complicated functions when working with some code. Do not be intimidated by modifying existing code: if working with such a function proves to be difficult, you find that errors are hard to debug, or you want to use a piece of it in several different contexts, consider breaking up the function into smaller and more manageable pieces.

Definition: In C, if a function needs to modify a variable, the parameter must use a pointer, e.g., int foo(int *pval) . In C++, the function can alternatively declare a reference parameter: int foo(int &val) .

Pros: Defining a parameter as reference avoids ugly code like (*pval)++ . Necessary for some applications like copy constructors. Makes it clear, unlike with pointers, that a null pointer is not a possible value.

Cons: References can be confusing, as they have value syntax but pointer semantics.

Decision:

Within function parameter lists all references must be const :

void foo(const std::string &in, std::string *out);

The input arguments are values or const references, while output arguments are pointers. Input parameters may be const pointers, but never use non- const reference parameters except when required by convention, e.g., swap() .

However, there are some instances where using const T* is preferable to const T& for input parameters. For example:

• You want to pass in a null pointer.
• The function saves a pointer or reference to the input.

Definition:

You may write a function that takes a const std::string& and overload it with another that takes const char* .

class MpmClass {
 public:
  void analyze(const std::string &text);
  void analyze(const char *text, size_t textlen);
};

Pros: Overloading can make code more intuitive by allowing an identically-named function to take different arguments. It may be necessary for templatized code, and it can be convenient for Visitors.

Cons: If a function is overloaded by the argument types alone, a reader may have to understand C++'s complex matching rules in order to tell what's going on. Also many people are confused by the semantics of inheritance if a derived class overrides only some of the variants of a function.

Decision: If you want to overload a function, consider qualifying the name with some information about the arguments, e.g., append_string() , append_int() rather than just append() .

In terms of time and space, an array is just about the optimal construct for accessing a sequence of objects in memory. It is, however, also a very low level data structure with a vast potential for misuse and errors and in essentially all cases there are better alternatives, such as vectors.

Cons: The two fundamental problems with arrays are that

• An array doesn't know its own size.
• The name of an array converts to a pointer to its first element at the slightest provocation.

Decision: Do not use arrays, essentially in all cases use vectors.

Friends should usually be defined in the same file so that the reader does not have to look in another file to find uses of the private members of a class. A common use of friend is to have a FooBuilder class be a friend of Foo so that it can construct the inner state of Foo correctly, without exposing this state to the world. In some cases it may be useful to make a unittest class a friend of the class it tests.

Friends extend, but do not break, the encapsulation boundary of a class. In some cases this is better than making a member public when you want to give only one other class access to it. However, most classes should interact with other classes solely through their public members.

Pros:

• Exceptions allow higher levels of an application to decide how to handle "can't happen" failures in deeply nested functions, without the obscuring and error-prone bookkeeping of error codes.
• Exceptions are the only way for a constructor to fail. We can simulate this with a factory function or an Init() method, but these require heap allocation or a new "invalid" state, respectively.
• Exceptions are really handy in testing frameworks.

Cons:

• When you add a throw statement to an existing function, you must examine all of its transitive callers.
• More generally, exceptions make the control flow of programs difficult to evaluate by looking at code.

Decision:

Try to use C++ exceptions whenever possible, especially when parsing user inputs and validating those input parameters.

Definition: With the exception of dynamic_cast, the use of cast implies the possibility o a type error or the truncation of a numeric value. Even an innocent-looking cast can become a serious problem if, during development or maintenance, one of the types involved is changed. C++ introduced a different cast system from C that distinguishes the types of cast operations.

Decision:

Casts are generally best avoided. In worst case, use these C++-style casts. Limit your usage to the following.

• Use static_cast as the equivalent of a C-style cast that does value conversion, or when you need to explicitly up-cast a pointer from a class to its superclass.
• Use const_cast to remove the const qualifier (see const ).

Casts really are mostly avoidable in modern C++.

Definition: When a variable is incremented ( ++i or i++ ) or decremented ( --i or i-- ) and the value of the expression is not used, one must decide whether to preincrement (decrement) or postincrement (decrement).

Pros: When the return value is ignored, the "pre" form ( ++i ) is never less efficient than the "post" form ( i++ ), and is often more efficient. This is because post-increment (or decrement) requires a copy of i to be made, which is the value of the expression. If i is an iterator or other non-scalar type, copying i could be expensive. Since the two types of increment behave the same when the value is ignored, why not just always pre-increment?

Cons: The traditionally, post-increment is used when the expression value is not used, especially in for loops. Some find post-increment easier to read, since the "subject" ( i ) precedes the "verb" ( ++ ), just like in English.

Decision: For simple scalar (non-object) values there is no reason to prefer one form and we allow either. For iterators and other template types, use pre-increment.

Definition: Declared variables and parameters can be preceded by the keyword const to indicate the variables are not changed (e.g., const int foo ). Class functions can have the const qualifier to indicate the function does not change the state of the class member variables (e.g., class Foo { int Bar(char c) const; }; ).

Pros: Easier for people to understand how variables are being used. Allows the compiler to do better type checking, and, conceivably, generate better code. Helps people convince themselves of program correctness because they know the functions they call are limited in how they can modify your variables. Helps people know what functions are safe to use without locks in multi-threaded programs.

Cons: const is viral: if you pass a const variable to a function, that function must have const in its prototype (or the variable will need a const_cast ). This can be a particular problem when calling library functions.

Decision:

const variables, data members, methods and arguments add a level of compile-time type checking; it is better to detect errors as soon as possible. Therefore we strongly recommend that you use const whenever it makes sense to do so:

• If a function does not modify an argument passed by reference or by pointer, that argument should be const .
• Declare methods to be const whenever possible. Accessors should almost always be const . Other methods should be const if they do not modify any data members, do not call any non- const methods, and do not return a non- const pointer or non- const reference to a data member.
• Consider making data members const whenever they do not need to be modified after construction.

The mutable keyword is allowed but is unsafe when used with threads, so thread safety should be carefully considered first.

Where to put the const

Use const int* foo as it is more idiomatic. Putting the const first is arguably more readable, since it follows English in putting the "adjective" ( const ) before the "noun" ( int ).

Note that in const pointer, "const" always comes after the "*". For example:

int *const p1 = q;  // constant pointer to int variable
const int* p2 = q;  // pointer to constant int
int const* p3 = q;  // pointer to constant int

Use const first, excpet for const pointers.

Definition: Some variables can be declared constexpr to indicate the variables are true constants, i.e. fixed at compilation/link time. Some functions and constructors can be declared constexpr which enables them to be used in defining a constexpr variable.

Pros: Use of constexpr enables definition of constants with floating-point expressions rather than just literals; definition of constants of user-defined types; and definition of constants with function calls.

Cons: Prematurely marking something as constexpr may cause migration problems if later on it has to be downgraded. Current restrictions on what is allowed in constexpr functions and constructors may invite obscure workarounds in these definitions.

Decision:

constexpr definitions enable a more robust specification of the constant parts of an interface. Use constexpr to specify true constants and the functions that support their definitions. Avoid complexifying function definitions to enable their use with constexpr . Do not use constexpr to force inlining.

Definition: C++ does not specify the sizes of its integer types. Typically people assume that short is 16 bits, int is 32 bits, long is 32 bits and long long is 64 bits.

Pros: Uniformity of declaration.

Cons: The sizes of integral types in C++ can vary based on compiler and architecture.

Decision:

<stdint.h> defines types like int16_t , int64_t , etc. You should always use those in preference to short , unsigned long long and the like, when you need a guarantee on the size of an integer.

Use int , for integers we know are not going to be too big, e.g., loop counters.

For integers we know can be "big", use int64_t .

Do not use unsigned types to say a number will never be negative. Instead, use assertions for this.

When in doubt, use a larger type rather than a smaller type.

On Unsigned Integers

Some textbook authors, recommend using unsigned types to represent numbers that are never negative. This is intended as a form of self-documentation. Consider:

for (unsigned int i = foo.Length()-1; i >= 0; --i) ...

This code will never terminate! Sometimes gcc will notice this bug and warn you, but often it will not. Equally bad bugs can occur when comparing signed and unsigned variables.

So, document that a variable is non-negative using assertions. Don't use an unsigned type.

Macros mean that the code you see is not the same as the code the compiler sees. This can introduce unexpected behavior, especially since macros have global scope. Macros do not obey the C++ scope and type rules. Consequently, C++ provides alternatives that fit better with the rest of C++, such as inline functions, templates, and namespaces.

• Instead of using a macro to store a constant, use a const variable.
• Instead of using a macro to "abbreviate" a long variable name, use a reference.
• Instead of using a macro to conditionally compile code ... well, don't do that at all (except, of course, for the #define guards to prevent double inclusion of header files). It makes testing much more difficult.

For pointers, when using C++11, use nullptr . For C++0x projects, use NULL because it looks like a pointer.

Use sizeof( varname ) when you take the size of a particular variable. sizeof( varname ) will update appropriately if someone changes the variable type either now or later.

Struct data;
memset(&data, 0, sizeof(data));

memset(&data, 0, sizeof(Struct));

You may use sizeof( type ) for code unrelated to any particular variable, such as code that manages an external or internal data format where a variable of an appropriate C++ type is not convenient.

if (raw_size < sizeof(int)) {
  LOG(ERROR) << "compressed record not big enough for count: " << raw_size;
  return false;
}

Definition: In C++11, a variable whose type is given as auto will be given a type that matches that of the expression used to initialize it. You can use auto either to initialize a variable by copying, or to bind a reference.

std::vector<std::string> v;
...
for (auto it = v.begin(); it != v.end(); ++it)
...

Decision:

auto is permitted, for local variables only, preferably only foriterators. Do not use auto for file-scope or namespace-scope variables, or for class members. Never assign a braced initializer list to an auto -typed variable.

In C++11, we now have range-based for loops. If you are iterating through all the elements try to use a range based for loop, if possible. For example:

std::vector<int> material_ids;
for (auto i : material_ids) {
    std::cout << i;
}

An example of std::for_each

void increment_all(vector<int>& v)
{
    for_each(v.begin(),v.end(), [](int& x) {++x;});
}

An example of iterator based for loop:

// Compute the Mesh Parameters and check for Mesh discontinuity
std::vector<Mesh>::iterator gridItr;
for (gridItr = grid.begin(); gridItr != grid.end(); ++gridItr)
    gridItr->computeMeshParameters(*gridItr, tolerance);

In C++0x, aggregate types (arrays and structs with no constructor) could be initialized using braces.

struct Point { int x; int y; };
Point p = {1, 2};

In C++11, this syntax has been expanded for use with all other datatypes. The brace initialization form is called braced-init-list . For example:

// Vector takes lists of elements.
std::vector<std::string> v{"foo", "bar"};

Never assign a braced-init-list to an auto local variable. In the single element case, what this means can be confusing.

auto d = {1.23};        // d is an initializer_list<double>

auto d = double{1.23};  // Good: d is a double, not an initializer_list.

Definition:

"Ownership" is a bookkeeping technique for managing dynamically allocated memory (and other resources). The owner of a dynamically allocated object is an object or function that is responsible for ensuring that it is deleted when no longer needed. Ownership can sometimes be shared, in which case the last owner is typically responsible for deleting it. Even when ownership is not shared, it can be transferred from one piece of code to another.

"Smart" pointers are classes that act like pointers, e.g. by overloading the * and -> operators. Some smart pointer types can be used to automate ownership bookkeeping, to ensure these responsibilities are met. std::unique_ptr is a smart pointer type introduced in C++11, which expresses exclusive ownership of a dynamically allocated object; the object is deleted when the std::unique_ptr goes out of scope. It cannot be copied, but can be moved to represent ownership transfer. shared_ptr is a smart pointer type which expresses shared ownership of a dynamically allocated object. shared_ptr s can be copied; ownership of the object is shared among all copies, and the object is deleted when the last shared_ptr is destroyed.

Pros:

• It's virtually impossible to manage dynamically allocated memory without some sort of ownership logic.
• Transferring ownership of an object can be cheaper than copying it (if copying it is even possible).
• Transferring ownership can be simpler than 'borrowing' a pointer or reference, because it reduces the need to coordinate the lifetime of the object between the two users.
• Smart pointers can improve readability by making ownership logic explicit, self-documenting, and unambiguous.
• Smart pointers can eliminate manual ownership bookkeeping, simplifying the code and ruling out large classes of errors.
• For const objects, shared ownership can be a simple and efficient alternative to deep copying.

Cons:

• Ownership must be represented and transferred via pointers (whether smart or plain). Pointer semantics are more complicated than value semantics, especially in APIs: you have to worry not just about ownership, but also aliasing, lifetime, and mutability, among other issues.
• The performance costs of value semantics are often overestimated, so the performance benefits of ownership transfer might not justify the readability and complexity costs.
• APIs that transfer ownership force their clients into a single memory management model.
• Code using smart pointers is less explicit about where the resource releases take place.
• std::unique_ptr expresses ownership transfer using C++11's move semantics, which are generally forbidden in Google code, and may confuse some programmers.
• Shared ownership can be a tempting alternative to careful ownership design, obfuscating the design of a system.
• Shared ownership requires explicit bookkeeping at run-time, which can be costly.
• In some cases (e.g. cyclic references), objects with shared ownership may never be deleted.
• Smart pointers are not perfect substitutes for plain pointers.

Decision:

If dynamic allocation is necessary, prefer to keep ownership with the code that allocated it. If other code needs access to the object, consider passing it a copy, or passing a pointer or reference without transferring ownership. Prefer to use std::unique_ptr to make ownership transfer explicit. For example:

std::unique_ptr<Foo> FooFactory();
void FooConsumer(std::unique_ptr<Foo> ptr);

Do not design your code to use shared ownership without a very good reason. One such reason is to avoid expensive copy operations, but you should only do this if the performance benefits are significant, and the underlying object is immutable (i.e. shared_ptr<const Foo> ). If you do use shared ownership, prefer to use shared_ptr .

Do not use scoped_ptr in new code unless you need to be compatible with older versions of C++. Never use linked_ptr or std::auto_ptr . In all three cases, use std::unique_ptr instead.

Definition: The Boost library collection is a popular collection of peer-reviewed, free, open-source C++ libraries.

Decision:

The following libraries are discouraged because they've been superseded by the standard libraries in C++11:

• Array from boost/array.hpp : use std::array instead.
• Pointer Container from boost/ptr_container : use containers of std::unique_ptr instead.

C++11 is the latest ISO C++ standard. It contains significant changes both to the language and libraries. Consider portability to other environments before using C++11 features in your project.

Decision:

C++11 features may be used unless specified otherwise. In addition to what's described in the rest of the style guide, the following C++11 features may not be used:

• Functions with trailing return types, e.g. writing auto foo()->int; instead of int foo(); , because of a desire to preserve stylistic consistency with the many existing function declarations.
• The <cfenv> and <fenv.h> headers, because many compilers do not support those features reliably.

Use the // syntax for comments, which is more common than /* ... */ . Use // syntax even for multi-line comments.

Every file should have a comment at the top describing its contents.

Generally a .h file will describe the classes that are declared in the file with an overview of what they are for and how they are used. A .cc file should contain more information about implementation details or discussions of tricky algorithms. If you feel the implementation details or a discussion of the algorithms would be useful for someone reading the .h , feel free to put it there instead, but mention in the .cc that the documentation is in the .h file.

Do not duplicate comments in both the .h and the .cc . Duplicated comments diverge.

An example of file comment:

// File: ParticleCloudMpm.ipp
// Details: Implementation of container class for the MPM particles
// Dependency: MPM_ITEMS contains all template parameters
// Author: Krishna Kumar and Samila Bandara, University of Cambridge
// Version: 2.0 - 2014; 1.0 - 2011.

// \brief Base class container for the MPM particles
// \details Has containers with pointers to the particles
// \param MPM_ITEMS: the main template file which includes all the classes
// \param PARTICLE_TYPE - the type of particle (soil/water) to be used

template<typename Mpm_Items, typename Particle_Type>
class mpm::ParticleCloudMpm {
    ...
};

If you have already described a class in detail in the comments at the top of your file feel free to simply state "See comment at top of file for a complete description", but be sure to have some sort of comment.

Document the synchronization assumptions the class makes, if any. If an instance of the class can be accessed by multiple threads, take extra care to document the rules and invariants surrounding multithreaded use.

Function Declarations

Every function declaration should have comments immediately preceding it that describe what the function does and how to use it. These comments should be descriptive ("Opens the file ... ") rather than imperative ("Open the file"); the comment describes the function, it does not tell the function what to do. In general, these comments do not describe how the function performs its task. Instead, that should be left to comments in the function definition.

Types of things to mention in comments at the function declaration:

• What the inputs and outputs are.
• For class member functions: whether the object remembers reference arguments beyond the duration of the method call, and whether it will free them or not.
• If the function allocates memory that the caller must free.
• Whether any of the arguments can be a null pointer.
• If there are any performance implications of how a function is used.
• If the function is re-entrant. What are its synchronization assumptions?

Here is an example:

// Apply a given functor to all particles making use of std::for_each
// \tparam OP         Type of the particle operation
// \param[in,out] op  Specific operation applied to all particles
// \retval OP         for_each returns the operator
template<typename Op>
Op mpm::iterateOverParticles(Op op) {
    return std::for_each(particles_.begin(), particles_.end(), op);
}

However, do not be unnecessarily verbose or state the completely obvious. Notice below that it is not necessary to say "returns false otherwise" because this is implied.

// Returns true if the table cannot hold any more entries.
bool is_table_full();

When commenting constructors and destructors, remember that the person reading your code knows what constructors and destructors are for, so comments that just say something like "destroys this object" are not useful. Document what constructors do with their arguments (for example, if they take ownership of pointers), and what cleanup the destructor does. If this is trivial, just skip the comment. It is quite common for destructors not to have a header comment.

Function Definitions

If there is anything tricky about how a function does its job, the function definition should have an explanatory comment. For example, in the definition comment you might describe any coding tricks you use, give an overview of the steps you go through, or explain why you chose to implement the function in the way you did rather than using a viable alternative. For instance, you might mention why it must acquire a lock for the first half of the function but why it is not needed for the second half.

Note you should not just repeat the comments given with the function declaration, in the .h file or wherever. It's okay to recapitulate briefly what the function does, but the focus of the comments should be on how it does it.

Class Data Members

Each class data member (also called an instance variable or member variable) should have a comment describing what it is used for. If the variable can take sentinel values with special meanings, such as a null pointer or -1, document this. For example:

private:
 // Keeps track of the total number of materials in the model.
 // Used to ensure we do not go over the limit. -1 means
 // that we don't yet know how many materials the model has.
 int num_total_materials_;

Global Variables

As with data members, all global variables should have a comment describing what they are and what they are used for. For example:

// The tolerance value for the Newton-Raphson approach.
const double tolerance = 1.0e-6;

Class Data Members

Tricky or complicated code blocks should have comments before them. Example:

// Divide result by two, taking into account that x
// contains the carry from the add.
for (int i = 0; i < result->size(); i++) {
  x = (x << 8) + (*result)[i];
  (*result)[i] = x >> 1;
  x &= 1;
}

Line Comments

Also, lines that are non-obvious should get a comment at the end of the line. These end-of-line comments should be separated from the code by 2 spaces.

Note that there are both comments that describe what the code is doing, and comments that mention that an error has already been logged when the function returns.

If you have several comments on subsequent lines, it can often be more readable to line them up:

do_something();                  // Comments line up here.
do_somethingElseThatIsLonger();  // Comment here so there are 2 spaces
                                 // between the code and the comment.

nullptr/NULL, true/false, 1, 2, 3...

When you pass in a null pointer, boolean, or literal integer values to functions, you should consider adding a comment about what they are, or make your code self-documenting by using constants. For example, compare:

bool success = calculate_something(interesting_value,
                                  10,
                                  false,
                                  NULL);  // What are these arguments??

versus:

bool success = calculate_something(interesting_value,
                                  10,     // Default base value.
                                  false,  // Not the first time we're calling this.
                                  NULL);  // No callback.

Don'ts

Note that you should never describe the code itself. Assume that the person reading the code knows C++ better than you do, even though he or she does not know what you are trying to do:

// Now go through the b array and make sure that if i occurs,
// the next element is i+1.
// Geez.  What a useless comment.

Comments should be as readable as narrative text, with proper capitalization and punctuation. In many cases, complete sentences are more readable than sentence fragments. Shorter comments, such as comments at the end of a line of code, can sometimes be less formal, but you should be consistent with your style.

Although it can be frustrating to have a code reviewer point out that you are using a comma when you should be using a semicolon, it is very important that source code maintain a high level of clarity and readability. Proper punctuation, spelling, and grammar help with that goal.

TODO s should include the string TODO in all caps, followed by the name, e-mail address, or other identifier of the person who can best provide context about the problem referenced by the TODO . A colon is optional. The main purpose is to have a consistent TODO format that can be searched to find the person who can provide more details upon request. A TODO is not a commitment that the person referenced will fix the problem. Thus when you create a TODO , it is almost always your name that is given.

// TODO(kks32[at]cam.ac.uk): Use a "*" here for concatenation operator.
// TODO(ks207) change this to use relations.

If your TODO is of the form "At a future date do something" make sure that you either include a very specific date ("Fix by November 2005") or a very specific event ("Remove this code when all clients can handle XML responses.").

You can mark an interface as deprecated by writing a comment containing the word DEPRECATED in all caps. The comment goes either before the declaration of the interface or on the same line as the declaration.

After the word DEPRECATED , write your name, e-mail address, or other identifier in parentheses.

A deprecation comment must include simple, clear directions for people to fix their callsites. In C++, you can implement a deprecated function as an inline function that calls the new interface point.

Marking an interface point DEPRECATED will not magically cause any callsites to change. If you want people to actually stop using the deprecated facility, you will have to fix the callsites yourself or recruit a crew to help you.

New code should not contain calls to deprecated interface points. Use the new interface point instead. If you cannot understand the directions, find the person who created the deprecation and ask them for help using the new interface point.

We recognize that this rule is controversial, but so much existing code already adheres to it, and we feel that consistency is important.

Pros: Those who favor this rule argue that it is rude to force them to resize their windows and there is no need for anything longer. Some folks are used to having several code windows side-by-side, and thus don't have room to widen their windows in any case. People set up their work environment assuming a particular maximum window width, and 80 columns has been the traditional standard. Why change it?

Cons: Proponents of change argue that a wider line can make code more readable. The 80-column limit is an hidebound throwback to 1960s mainframes; modern equipment has wide screens that can easily show longer lines.

Decision:

80 characters is the maximum.

Exception: if a comment line contains an example command or a literal URL longer than 80 characters, that line may be longer than 80 characters for ease of cut and paste.

Exception: an #include statement with a long path may exceed 80 columns. Try to avoid situations where this becomes necessary.

Exception: you needn't be concerned about header guards that exceed the maximum length.

You should use UTF-8, since that is an encoding understood by most tools able to handle more than just ASCII.

You shouldn't use the C++11 char16_t and char32_t character types, since they're for non-UTF-8 text. For similar reasons you also shouldn't use wchar_t .

We use spaces for indentation. Do not use tabs in your code. You should set your editor to emit spaces when you hit the tab key.

Functions look like this:

ReturnType ClassName::function_name(Type par_name1, Type par_name2) {
    do_something();  // 4 space indent
    ...
}

If you have too much text to fit on one line:

ReturnType ClassName::really_long_function(Type par_name1,
                                           Type par_name2,
                                           Type par_name3) {
    do_something();
    ...
}

or if you cannot fit even the first parameter:

ReturnType LongClassName::really_really_long_function_name(
                              Type var1,  // 4 space indent from scope
                              Type var2,
                              Type var3) {
     do_something();  // 4 space indent
     ...
}

Some points to note:

• If you cannot fit the return type and the function name on a single line, break between them.
• If you break after the return type of a function definition, do not indent.
• The open parenthesis is always on the same line as the function name.
• There is never a space between the function name and the open parenthesis.
• There is never a space between the parentheses and the parameters.
• The open curly brace is always at the end of the same line as the last parameter.
• The close curly brace is either on the last line by itself or (if other style rules permit) on the same line as the open curly brace.
• There should be a space between the close parenthesis and the open curly brace.
• All parameters should be named, with identical names in the declaration and implementation.
• All parameters should be aligned if possible.
• Default indentation is 2 spaces.
• Wrapped parameters have a 4 space indent.

If some parameters are unused, comment out the variable name in the function definition:

// Always have named parameters in interfaces.
class Shape {
 public:
  virtual void rotate(double radians) = 0;
}

// Always have named parameters in the declaration.
class Circle : public Shape {
 public:
  virtual void rotate(double radians);
}

// Comment out unused named parameters in definitions.
void Circle::rotate(double /*radians*/) {}

// Bad - if someone wants to implement later, it's not clear what the
// variable means.
void Circle::rotate(double) {}

Function calls have the following format:

bool retval = do_something(argument1, argument2, argument3);

If the arguments do not all fit on one line, they should be broken up onto multiple lines, with each subsequent line aligned with the first argument. Do not add spaces after the open paren or before the close paren:

bool retval = do_something(averyveryveryverylongargument1,
                           argument2, argument3);

If the function has many arguments, consider having one per line if this makes the code more readable:

bool retval = do_something(argument1,
                           argument2,
                           argument3,
                           argument4);

Arguments may optionally all be placed on subsequent lines, with one line per argument:

if (...) {
    ...
    ...
    if (...) {
        do_something(
            argument1,  // 4 space indent
            argument2,
            argument3,
            argument4);
    }
    ...
}

In particular, this should be done if the function signature is so long that it cannot fit within the maximum line length .

If the braced list follows a name (e.g. a type or variable name), format as if the {} were the parentheses of a function call with that name. If there is no name, assume a zero-length name.

// Examples of braced init list on a single line.
return {foo, bar};
functioncall({foo, bar});
pair<int, int> p{foo, bar};

// When you have to wrap.
some_function(
    {"assume a zero-length name before {"},
    some_other_function_parameter);
SomeType variable{
    some, other, values,
    {"assume a zero-length name before {"},
    SomeOtherType{
        "Very long string requiring the surrounding breaks.",
        some, other values},
    SomeOtherType{"Slightly shorter string",
                  some, other, values}};
SomeType variable{
    "This is too long to fit all in one line"};
SomeType m = {  // Here, you could also break before {.
    superlongvariablename1,
    superlongvariablename2,
    {short, interior, list},
    {interiorwrappinglist,
     interiorwrappinglist2}};

The most common form is without spaces.

if (condition) {  // no spaces inside parentheses
    ...  // 4 space indent.
}
else if (...) {  // The else goes on the same line as the closing brace.
    ...
}
else {
    ...
}

Note: you must have a space between the if and the open parenthesis. You must also have a space between the close parenthesis and the curly brace, if you're using one.

if(condition)     // Bad - space missing after IF.
if (condition){   // Bad - space missing before {.
if(condition){    // Doubly bad.

if (condition) {  // Good - proper space after IF and before {.

Short conditional statements may be written on one line if this enhances readability. You may use this only when the line is brief and the statement does not use the else clause.

if (x == kFoo) return new Foo();
if (x == kBar) return new Bar();

This is not allowed when the if statement has an else :

// Not allowed - IF statement on one line when there is an ELSE clause
if (x) DoThis();
else DoThat();

In general, curly braces are not required for single-line statements, but they are allowed if you like them; conditional or loop statements with complex conditions or statements may be more readable with curly braces. Some projects require that an if must always always have an accompanying brace.

if (condition)
    do_something();  // 4 space indent.

if (condition) {
    do_something();  // 4 space indent.
}

However, if one part of an if - else statement uses curly braces, the other part must too:

// Not allowed - curly on IF but not ELSE
if (condition) {
    foo;
}
else
    bar;

// Not allowed - curly on ELSE but not IF
if (condition)
    foo;
else {
    bar;
}

// Curly braces around both IF and ELSE required because
// one of the clauses used braces.
if (condition) {
    foo;
}
else {
    bar;
}

case blocks in switch statements can have curly braces or not, depending on your preference. If you do include curly braces they should be placed as shown below.

If not conditional on an enumerated value, switch statements should always have a default case (in the case of an enumerated value, the compiler will warn you if any values are not handled). If the default case should never execute, simply assert :

switch (var) {
case 0: {    // no indent
    ...      // 4 space indent
    break;
}
case 1: {
    ...
    break;
}
default: {
    assert(false);
}
}

Empty loop bodies should use {} or continue , but not a single semicolon.

while (condition) {
  // Repeat test until it returns false.
}
for (int i = 0; i < kSomeNumber; ++i) {}  // Good - empty body.
while (condition) continue;  // Good - continue indicates no logic.

while (condition);  // Bad - looks like part of do/while loop.

The following are examples of correctly-formatted pointer and reference expressions:

x = *p;
p = &x;
x = r.y;
x = r->y;

Note that:

• There are no spaces around the period or arrow when accessing a member.
• Pointer operators have no space after the * or & .

When declaring a pointer variable or argument, you may place the asterisk adjacent to either the type or to the variable name:

// These are fine, space preceding.
char *c;
const std::string &str;

// These are fine, space following.
char* c;    // but remember to do "char* c, *d, *e, ...;"!
const std::string& str;

char * c;  // Bad - spaces on both sides of *
const std::string & str;  // Bad - spaces on both sides of &

You should do this consistently within a single file, so, when modifying an existing file, use the style in that file.

In this example, the logical AND operator is always at the end of the lines:

if (this_one_thing > this_other_thing &&
    a_third_thing == a_fourth_thing &&
    yet_another && last_one) {
    ...
}

Note that when the code wraps in this example, both of the && logical AND operators are at the end of the line. This is more common in Google code, though wrapping all operators at the beginning of the line is also allowed. Feel free to insert extra parentheses judiciously because they can be very helpful in increasing readability when used appropriately. Also note that you should always use the punctuation operators, such as && and ~ , rather than the word operators, such as and and compl .

Use parentheses in return expr; only where you would use them in x = expr; .

return result;                  // No parentheses in the simple case.
return (some_long_condition &&  // Parentheses ok - complex
        another_condition);     //     expression more readable.

return (value);                // You wouldn't write var = (value);
return(result);                // return is not a function!

You may choose between = , () , and {} ; the following are all correct:

int x = 3;
int x(3);
int x{3};
std::string name = "Some Name";
std::string name("Some Name");
std::string name{"Some Name"};

Be careful when using the {} on a type that takes an initializer_list in one of its constructors. The {} syntax prefers the initializer_list constructor whenever possible. To get the non- initializer_list constructor, use () .

std::vector<int> v(100, 1);  // A vector of 100 1s.
std::vector<int> v{100, 1};  // A vector of 100, 1.

Also, the brace form prevents narrowing of integral types. This can prevent some types of programming errors.

int pi(3.14);  // OK -- pi == 3.
int pi{3.14};  // Compile error: narrowing conversion.

Even when preprocessor directives are within the body of indented code, the directives should start at the beginning of the line.

// Good - directives at beginning of line
if (lopsided_score) {
#if DISASTER_PENDING      // Correct -- Starts at beginning of line
    drop_everything();
#if NOTIFY
    notify_client();
#endif
#endif
    back_to_normal();
}

// Bad - indented directives
if (lopsided_score) {
    #if DISASTER_PENDING  // Wrong! "#if" should be at beginning of line
    drop_everything();
    #endif                // Wrong!  Do not indent "#endif"
    back_to_normal();
}

The basic format for a class declaration (lacking the comments, see Class Comments for a discussion of what comments are needed) is:

class MpmClass : public OtherClass {
        public:      // Note no indent!
            MpmClass();  // Regular 4 space indent.
            explicit MpmClass(int var);
            ~MpmClass() {}

            void some_function();
            void some_function_that_does_nothing() {
                ...
	    }

            void set_some_var(int var) { some_var_ = var; }
            int some_var() const { return some_var_; }

       private:
            bool some_internal_function();

            int some_var_;
           int some_other_var_;
        };

Things to note:

• Any base class name should be on the same line as the subclass name, subject to the 80-column limit.
• The public: , protected: , and private: keywords should be indented one space.
• Except for the first instance, these keywords should be preceded by a blank line. This rule is optional in small classes.
• Do not leave a blank line after these keywords.
• The public section should be first, followed by the protected and finally the private section.
• See Declaration Order for rules on ordering declarations within each of these sections.

There are two acceptable formats for initializer lists:

// When it all fits on one line:
MpmClass::MpmClass(int var) : some_var_(var),
                              some_other_var_(var + 1) {}

or

// When it requires multiple lines, indent 4 spaces,
// putting the colon on the first initializer line:
MpmClass::MpmClass(int var)
    : some_var_(var),             // 4 space indent
      some_other_var_(var + 1) {  // lined up
    ...
    do_something();
    ...
}

Namespaces do not add an extra level of indentation. For example, use:

namespace {

void foo() {  // Correct.  No extra indentation within namespace.
    ...
}

}  // namespace

Do not indent within a namespace:

namespace {

  // Wrong.  Indented when it should not be.
  void foo() {
      ...
  }

}  // namespace

When declaring nested namespaces, put each namespace on its own line.

namespace foo {
namespace bar {

General

void f(bool b) {  // Open braces should always have a space before them.
    ...
int i = 0;        // Semicolons usually have no space before them.
int x[] = { 0 };  // Spaces inside braces for braced-init-list are
int x[] = {0};    // optional.  If you use them, put them on both sides!

// Spaces around the colon in inheritance and initializer lists.
class Foo : public Bar {
public:

    // For inline function implementations, put spaces
    //  between the braces and the implementation itself.
    foo(int b) : Bar(), baz_(b) {}  // No spaces inside empty braces.
    void reset() { baz_ = 0; }      // Spaces separating braces
                                    // from implementation.
    ...

Adding trailing whitespace can cause extra work for others editing the same file, when they merge, as can removing existing trailing whitespace. So: Don't introduce trailing whitespace. Remove it if you're already changing that line, or do it in a separate clean-up operation (preferably when no-one else is working on the file).

Loops and Conditionals

if (b) {          // Space after the keyword in conditions and loops.
        }
	else {            // Spaces around else.
        }
        while (test) {}   // There is usually no space inside parentheses.
        switch (i) {
        for (int i = 0; i < 5; ++i) { //space after a semicolon
            ...
        for (auto x : counts) {          // Range-based for loops always have a
            ...                          // space before and after the colon.
        }
        switch (i) {
        case 1:          // No space before colon in a switch case.
            ...
        case 2: break;   // Use a space after a colon if there's code after it.

Operators

x = 0;              // Assignment operators always have spaces around
                    // them.
x = -5;             // No spaces separating unary operators and their
++x;                // arguments.
if (x && !y)
  ...
v = w * x + y / z;  // Binary operators usually have spaces around them,
v = w*x + y/z;      // but it's okay to remove spaces around factors.
v = w * (x + z);    // Parentheses should have no spaces inside them.

Templates and Casts

std::vector<std::string> x;   // No spaces inside the angle
y = static_cast<char*>(x);    // brackets (< and >), before
                                    // <, or between >( in a cast.

set<list<std::string>> x;    // Permitted in C++11 code.
set<list<std::string> > x;   // C++03 required a space
                                         // in > >.
set< list<std::string> > x;  // You may optionally use
                                         // symmetric spacing
                                         // in < <.

This is more a principle than a rule: don't use blank lines when you don't have to. In particular, don't put more than one or two blank lines between functions, resist starting functions with a blank line, don't end functions with a blank line, and be discriminating with your use of blank lines inside functions.

The basic principle is: The more code that fits on one screen, the easier it is to follow and understand the control flow of the program. Of course, readability can suffer from code being too dense as well as too spread out, so use your judgement. But in general, minimize use of vertical whitespace.

Some rules of thumb to help when blank lines may be useful:

• Blank lines at the beginning or end of a function very rarely help readability.
• Blank lines inside a chain of if-else blocks may well help readability.
• Two blank lines before you begin a function declaration.

You can automatically format your code to confirm to this style guide by running

./astyle --style=stroustrup --break-closing-brackets  *.cc
./astyle --style=stroustrup --break-closing-brackets  *.h

or

./astyle -A4 -y *.cc
./astyle -A4 -y *.h

Copy the lisp commands from cued.emacs to your .emacs file to initialise emacs with style settings described in this guide. This helps you write your code in a consistent manner.

It is easy to install the auto-completion tool using the install.el script. Load it using M+x load-file RET ~/path/to/auto-complete-x.x/etc/install.el . Wait for it to compile and update the ~/.emacs file.