CPP_STYLE_GUIDE.md

C++ Style Guide

Table of Contents

• Formatting
  • Left-leaning (C++ style) asterisks for pointer declarations
  • C++ style comments
  • 2 spaces of indentation for blocks or bodies of conditionals
  • 4 spaces of indentation for statement continuations
  • Align function arguments vertically
  • Initialization lists
  • CamelCase for methods, functions, and classes
  • snake_case for local variables and parameters
  • snake_case_ for private class fields
  • Space after template
• Memory Management
  • Memory allocation
  • Use nullptr instead of NULL or 0
  • Ownership and Smart Pointers
• Others
  • Type casting
  • Do not include *.h if *-inl.h has already been included
  • Avoid throwing JavaScript errors in nested C++ methods

Unfortunately, the C++ linter (based on Google’s cpplint), which can be run explicitly via make lint-cpp, does not currently catch a lot of rules that are specific to the Node.js C++ code base. This document explains the most common of these rules:

Formatting

Left-leaning (C++ style) asterisks for pointer declarations

char* buffer; instead of char *buffer;

C++ style comments

Use C++ style comments (//) for both single-line and multi-line comments. Comments should also start with uppercase and finish with a dot.

Examples:

// A single-line comment.

// Multi-line comments
// should also use C++
// style comments.

The codebase may contain old C style comments (/* */) from before this was the preferred style. Feel free to update old comments to the preferred style when working on code in the immediate vicinity or when changing/improving those comments.

2 spaces of indentation for blocks or bodies of conditionals

if (foo)
  bar();

or

if (foo) {
  bar();
  baz();
}

Braces are optional if the statement body only has one line.

namespaces receive no indentation on their own.

4 spaces of indentation for statement continuations

VeryLongTypeName very_long_result = SomeValueWithAVeryLongName +
    SomeOtherValueWithAVeryLongName;

Operators are before the line break in these cases.

Align function arguments vertically

void FunctionWithAVeryLongName(int parameter_with_a_very_long_name,
                               double other_parameter_with_a_very_long_name,
                               ...);

If that doesn’t work, break after the ( and use 4 spaces of indentation:

void FunctionWithAReallyReallyReallyLongNameSeriouslyStopIt(
    int okay_there_is_no_space_left_in_the_previous_line,
    ...);

Initialization lists

Long initialization lists are formatted like this:

HandleWrap::HandleWrap(Environment* env,
                       Local<Object> object,
                       uv_handle_t* handle,
                       AsyncWrap::ProviderType provider)
    : AsyncWrap(env, object, provider),
      state_(kInitialized),
      handle_(handle) {

CamelCase for methods, functions, and classes

Exceptions are simple getters/setters, which are named property_name() and set_property_name(), respectively.

class FooBar {
 public:
  void DoSomething();
  static void DoSomethingButItsStaticInstead();

  void set_foo_flag(int flag_value);
  int foo_flag() const;  // Use const-correctness whenever possible.
};

snake_case for local variables and parameters

int FunctionThatDoesSomething(const char* important_string) {
  const char* pointer_into_string = important_string;
}

snake_case_ for private class fields

class Foo {
 private:
  int counter_ = 0;
};

Space after template

template <typename T>
class FancyContainer {
 ...
}

Memory Management

Memory allocation

• Malloc(), Calloc(), etc. from util.h abort in Out-of-Memory situations
• UncheckedMalloc(), etc. return nullptr in OOM situations

Use nullptr instead of NULL or 0

What it says in the title.

Ownership and Smart Pointers

"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. std::shared_ptr is a smart pointer type that expresses shared ownership of a dynamically allocated object. std::shared_ptrs can be copied; ownership of the object is shared among all copies, and the object is deleted when the last std::shared_ptr is destroyed.

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);

Never use std::auto_ptr. Instead, use std::unique_ptr.

Others

Type casting

• Always avoid C-style casts ((type)value)
• dynamic_cast does not work because RTTI is not enabled
• Use static_cast for casting whenever it works
• reinterpret_cast is okay if static_cast is not appropriate

Do not include *.h if *-inl.h has already been included

Do

#include "util-inl.h"  // already includes util.h

instead of

#include "util.h"
#include "util-inl.h"

Avoid throwing JavaScript errors in nested C++ methods

If you need to throw JavaScript errors from a C++ binding method, try to do it at the top level and not inside of nested calls.

A lot of code inside Node.js is written so that typechecking etc. is performed in JavaScript.

Using C++ throw is not allowed.