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03-Style.md

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Style

Consistency is the most important aspect of style. The second most important aspect is following a style that the average C++ programmer is used to reading.

C++ allows for arbitrary-length identifier names, so there's no reason to be terse when naming things. Use descriptive names, and be consistent in the style.

  • CamelCase
  • snake_case

are common examples. snake_case has the advantage that it can also work with spell checkers, if desired.

Establishing A Style Guideline

Whatever style guidelines you establish, be sure to implement a .clang-format file that specifies the style you expect. While this cannot help with naming, it is particularly important for an open source project to maintain a consistent style.

Every IDE and many editors have support for clang-format built in or easily installable with an add-in.

Common C++ Naming Conventions

  • Types start with upper case: MyClass.
  • Functions and variables start with lower case: myMethod.
  • Constants are all upper case: const double PI=3.14159265358979323;.

C++ Standard Library (and other well-known C++ libraries like Boost) use these guidelines:

  • Macro names use upper case with underscores: INT_MAX.
  • Template parameter names use camel case: InputIterator.
  • All other names use snake case: unordered_map.

Distinguish Private Object Data

Name private data with a m_ prefix to distinguish it from public data. m_ stands for "member" data.

Distinguish Function Parameters

The most important thing is consistency within your codebase; this is one possibility to help with consistency.

Name function parameters with an t_ prefix. t_ can be thought of as "the", but the meaning is arbitrary. The point is to distinguish function parameters from other variables in scope while giving us a consistent naming strategy.

Any prefix or postfix can be chosen for your organization. This is just one example. This suggestion is controversial, for a discussion about it see issue #11.

struct Size
{
  int width;
  int height;

  Size(int t_width, int t_height) : width(t_width), height(t_height) {}
};

// This version might make sense for thread safety or something,
// but more to the point, sometimes we need to hide data, sometimes we don't.
class PrivateSize
{
  public:
    int width() const { return m_width; }
    int height() const { return m_height; }
    PrivateSize(int t_width, int t_height) : m_width(t_width), m_height(t_height) {}

  private:
    int m_width;
    int m_height;
};

Don't Name Anything Starting With _

If you do, you risk colliding with names reserved for compiler and standard library implementation use:

http://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier

Well-Formed Example

class MyClass
{
public:
  MyClass(int t_data)
    : m_data(t_data)
  {
  }

  int getData() const
  {
    return m_data;
  }

private:
  int m_data;
};

Enable Out-of-Source-Directory Builds

Make sure generated files go into an output folder that is separate from the source folder.

Use nullptr

C++11 introduces nullptr which is a special value denoting a null pointer. This should be used instead of 0 or NULL to indicate a null pointer.

Comments

Comment blocks should use //, not /* */. Using // makes it much easier to comment out a block of code while debugging.

// this function does something
int myFunc()
{
}

To comment out this function block during debugging we might do:

/*
// this function does something
int myFunc()
{
}
*/

which would be impossible if the function comment header used /* */.

Never Use using namespace in a Header File

This causes the namespace you are using to be pulled into the namespace of all files that include the header file. It pollutes the namespace and it may lead to name collisions in the future. Writing using namespace in an implementation file is fine though.

Include Guards

Header files must contain a distinctly-named include guard to avoid problems with including the same header multiple times and to prevent conflicts with headers from other projects.

#ifndef MYPROJECT_MYCLASS_HPP
#define MYPROJECT_MYCLASS_HPP

namespace MyProject {
  class MyClass {
  };
}

#endif

You may also consider using the #pragma once directive instead which is quasi-standard across many compilers. It's short and makes the intent clear.

{} Are Required for Blocks.

Leaving them off can lead to semantic errors in the code.

// Bad Idea
// This compiles and does what you want, but can lead to confusing
// errors if modification are made in the future and close attention
// is not paid.
for (int i = 0; i < 15; ++i)
  std::cout << i << std::endl;

// Bad Idea
// The cout is not part of the loop in this case even though it appears to be.
int sum = 0;
for (int i = 0; i < 15; ++i)
  ++sum;
  std::cout << i << std::endl;


// Good Idea
// It's clear which statements are part of the loop (or if block, or whatever).
int sum = 0;
for (int i = 0; i < 15; ++i) {
  ++sum;
  std::cout << i << std::endl;
}

Keep Lines a Reasonable Length

// Bad Idea
// hard to follow
if (x && y && myFunctionThatReturnsBool() && caseNumber3 && (15 > 12 || 2 < 3)) {
}

// Good Idea
// Logical grouping, easier to read
if (x && y && myFunctionThatReturnsBool()
    && caseNumber3
    && (15 > 12 || 2 < 3)) {
}

Many projects and coding standards have a soft guideline that one should try to use less than about 80 or 100 characters per line. Such code is generally easier to read. It also makes it possible to have two separate files next to each other on one screen without having a tiny font.

Use "" for Including Local Files

... <> is reserved for system includes.

// Bad Idea. Requires extra -I directives to the compiler
// and goes against standards.
#include <string>
#include <includes/MyHeader.hpp>

// Worse Idea
// Requires potentially even more specific -I directives and
// makes code more difficult to package and distribute.
#include <string>
#include <MyHeader.hpp>


// Good Idea
// Requires no extra params and notifies the user that the file
// is a local file.
#include <string>
#include "MyHeader.hpp"

Initialize Member Variables

...with the member initializer list.

For POD types, the performance of an initializer list is the same as manual initialization, but for other types there is a clear performance gain, see below.

// Bad Idea
class MyClass
{
public:
  MyClass(int t_value)
  {
    m_value = t_value;
  }

private:
  int m_value;
};

// Bad Idea
// This leads to an additional constructor call for m_myOtherClass
// before the assignment.
class MyClass
{
public:
  MyClass(MyOtherClass t_myOtherClass)
  {
    m_myOtherClass = t_myOtherClass;
  }

private:
  MyOtherClass m_myOtherClass;
};

// Good Idea
// There is no performance gain here but the code is cleaner.
class MyClass
{
public:
  MyClass(int t_value)
    : m_value(t_value)
  {
  }

private:
  int m_value;
};

// Good Idea
// The default constructor for m_myOtherClass is never called here, so
// there is a performance gain if MyOtherClass is not is_trivially_default_constructible.
class MyClass
{
public:
  MyClass(MyOtherClass t_myOtherClass)
    : m_myOtherClass(t_myOtherClass)
  {
  }

private:
  MyOtherClass m_myOtherClass;
};

In C++11 you can assign default values to each member (using = or using {}).

Assigning default values with =

// ... //
private:
  int m_value = 0; // allowed
  unsigned m_value_2 = -1; // narrowing from signed to unsigned allowed
// ... //

This ensures that no constructor ever "forgets" to initialize a member object.

Assigning default values with brace initialization

Using brace initialization does not allow narrowing at compile-time.

// Best Idea

// ... //
private:
  int m_value{ 0 }; // allowed
  unsigned m_value_2 { -1 }; // narrowing from signed to unsigned not allowed, leads to a compile time error
// ... //

Prefer {} initialization over = unless you have a strong reason not to.

Forgetting to initialize a member is a source of undefined behavior bugs which are often extremely hard to find.

If the member variable is not expected to change after the initialization, then mark it const.

class MyClass
{
public:
  MyClass(int t_value)
    : m_value{t_value}
  {
  }

private:
  const int m_value{0};
};

Since a const member variable cannot be assigned a new value, such a class may not have a meaningful copy assignment operator.

Always Use Namespaces

There is almost never a reason to declare an identifier in the global namespace. Instead, functions and classes should exist in an appropriately named namespace or in a class inside of a namespace. Identifiers which are placed in the global namespace risk conflicting with identifiers from other libraries (mostly C, which doesn't have namespaces).

Use the Correct Integer Type for Standard Library Features

The standard library generally uses std::size_t for anything related to size. The size of size_t is implementation defined.

In general, using auto will avoid most of these issues, but not all.

Make sure you stick with the correct integer types and remain consistent with the C++ standard library. It might not warn on the platform you are currently using, but it probably will when you change platforms.

Note that you can cause integer underflow when performing some operations on unsigned values. For example:

std::vector<int> v1{2,3,4,5,6,7,8,9};
std::vector<int> v2{9,8,7,6,5,4,3,2,1};
const auto s1 = v1.size();
const auto s2 = v2.size();
const auto diff = s1 - s2; // diff underflows to a very large number

Use .hpp and .cpp for Your File Extensions

Ultimately this is a matter of preference, but .hpp and .cpp are widely recognized by various editors and tools. So the choice is pragmatic. Specifically, Visual Studio only automatically recognizes .cpp and .cxx for C++ files, and Vim doesn't necessarily recognize .cc as a C++ file.

One particularly large project (OpenStudio) uses .hpp and .cpp for user-generated files and .hxx and .cxx for tool-generated files. Both are well recognized and having the distinction is helpful.

Never Mix Tabs and Spaces

Some editors like to indent with a mixture of tabs and spaces by default. This makes the code unreadable to anyone not using the exact same tab indentation settings. Configure your editor so this does not happen.

Never Put Code with Side Effects Inside an assert()

assert(registerSomeThing()); // make sure that registerSomeThing() returns true

The above code succeeds when making a debug build, but gets removed by the compiler when making a release build, giving you different behavior between debug and release builds. This is because assert() is a macro which expands to nothing in release mode.

Don't Be Afraid of Templates

They can help you stick to DRY principles. They should be preferred to macros, because macros do not honor namespaces, etc.

Use Operator Overloads Judiciously

Operator overloading was invented to enable expressive syntax. Expressive in the sense that adding two big integers looks like a + b and not a.add(b). Another common example is std::string, where it is very common to concatenate two strings with string1 + string2.

However, you can easily create unreadable expressions using too much or wrong operator overloading. When overloading operators, there are three basic rules to follow as described on stackoverflow.

Specifically, you should keep these things in mind:

  • Overloading operator=() when handling resources is a must. See Consider the Rule of Zero below.
  • For all other operators, only overload them when they are used in a context that is commonly connected to these operators. Typical scenarios are concatenating things with +, negating expressions that can be considered "true" or "false", etc.
  • Always be aware of the operator precedence and try to circumvent unintuitive constructs.
  • Do not overload exotic operators such as ~ or % unless implementing a numeric type or following a well recognized syntax in specific domain.
  • Never overload operator,() (the comma operator).
  • Use non-member functions operator>>() and operator<<() when dealing with streams. For example, you can overload operator<<(std::ostream &, MyClass const &) to enable "writing" your class into a stream, such as std::cout or an std::fstream or std::stringstream. The latter is often used to create a string representation of a value.
  • There are more common operators to overload described here.

More tips regarding the implementation details of your custom operators can be found here.

Avoid Implicit Conversions

Single Parameter Constructors

Single parameter constructors can be applied at compile time to automatically convert between types. This is handy for things like std::string(const char *) but should be avoided in general because they can add to accidental runtime overhead.

Instead mark single parameter constructors as explicit, which requires them to be explicitly called.

Conversion Operators

Similarly to single parameter constructors, conversion operators can be called by the compiler and introduce unexpected overhead. They should also be marked as explicit.

//bad idea
struct S {
  operator int() {
    return 2;
  }
};
//good idea
struct S {
  explicit operator int() {
    return 2;
  }
};

Consider the Rule of Zero

The Rule of Zero states that you do not provide any of the functions that the compiler can provide (copy constructor, copy assignment operator, move constructor, move assignment operator, destructor) unless the class you are constructing does some novel form of ownership.

The goal is to let the compiler provide optimal versions that are automatically maintained when more member variables are added.

This article provides a background and explains techniques for implementing nearly 100% of the time.