cppbestpractices/03-Style.md

# Style

Consistency of style is more important. Second most importance 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 variables. 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.

## Common C++ Naming Conventions

 * Types start with capitals: `MyClass`
 * functions and variables start with lower case: `myMethod`
 * constants are all capital: `const double PI=3.14159265358979323;`

C++ Standard Library (and other well-known C++ libraries like [Boost](http://www.boost.org/)) use these guidelines:

 * Macro names use uppercase 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

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.

By using `t_` for parameters and `m_` for module data, we can have consistency with both public members of structs and private members of classes.

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](https://github.com/lefticus/cppbestpractices/issues/11).

```cpp
struct Size
{
  int width;
  int height;

  ValueType(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; }
    ValueType(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 broaching on names reserved for implementation use:

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


## Well-Formed Example

```cpp
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 build folder, not the source folder


## Use `nullptr`

C++11 introduces `nullptr` which is a special type denoting a null pointer value. 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.

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

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

```cpp
/*
// 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 name space you are `using` to be pulled into the namespace of the header file.
It litters 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.

```cpp
#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.

```cpp
// Bad Idea
// this compiles and does what you want, but can lead to confusing
// errors if 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

```cpp
// 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](http://blog2.emptycrate.com/content/when-use-include-verses-include).

```cpp
// 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

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

private:
  int m_value;
};


// Good Idea
// C++'s member initializer list is unique to the language and leads to
// cleaner code and potential performance gains that other languages cannot
// match
class MyClass
{
public:
  MyClass(int t_value)
    : m_value(t_value)
  {
  }

private:
  int m_value;
};
```

In C++11 you may consider always giving each member a default value, e.g. by writing
```cpp
// ... //
private:
  int m_value = 0;
// ... //
```
inside the class body. This makes sure that no constructor ever "forgets" to initialize a member object.
Forgetting to initialize a member is a source of undefined behavior bugs which are often extremely hard to find.


## Always Use Namespaces

There is almost never a reason to declare an identifier in the global namespaces. 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 stdlib Features

The standard library generally returns `size_t` for anything related to size. What exactly `size_t` is 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++ stdlib. It might not warn on the platform you are currently using, but it probably will when you change platforms.

## 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](https://github.com/NREL/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.

## Never Put Code with Side Effects Inside an assert()

```cpp
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](http://en.wikipedia.org/wiki/Don%27t_repeat_yourself).
They should be preferred to macros, because macros do not honor namespaces, etc.

## Use Operator Overloads Judiciously