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C++ Style Guide

Table of Contents

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;
};

snake_case_ for C-like structs

For plain C-like structs snake_case can be used.

struct foo_bar {
  int name;
}

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

Using auto

Being explicit about types is usually preferred over using auto.

Use auto to avoid type names that are noisy, obvious, or unimportant. When doing so, keep in mind that explicit types often help with readability and verifying the correctness of code.

for (const auto& item : some_map) {
  const KeyType& key = item.first;
  const ValType& value = item.second;
  // The rest of the loop can now just refer to key and value,
  // a reader can see the types in question, and we've avoided
  // the too-common case of extra copies in this iteration.
}

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 C++

When there is a need to throw errors from a C++ binding method, try to return the data necessary for constructing the errors to JavaScript, then construct and throw the errors using lib/internal/errors.js.

Note that in general, type-checks on arguments should be done in JavaScript before the arguments are passed into C++. Then in the C++ binding, simply using CHECK assertions to guard against invalid arguments should be enough.

If the return value of the binding cannot be used to signal failures or return the necessary data for constructing errors in JavaScript, pass a context object to the binding and put the necessary data inside in C++. For example:

void Foo(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  // Let the JavaScript handle the actual type-checking,
  // only assertions are placed in C++
  CHECK_EQ(args.Length(), 2);
  CHECK(args[0]->IsString());
  CHECK(args[1]->IsObject());

  int err = DoSomethingWith(args[0].As<String>());
  if (err) {
    // Put the data inside the error context
    Local<Object> ctx = args[1].As<Object>();
    Local<String> key = FIXED_ONE_BYTE_STRING(env->isolate(), "code");
    ctx->Set(env->context(), key, err).FromJust();
  } else {
    args.GetReturnValue().Set(something_to_return);
  }
}

// In the initialize function
env->SetMethod(target, "foo", Foo);
exports.foo = function(str) {
  // Prefer doing the type-checks in JavaScript
  if (typeof str !== 'string') {
    throw new errors.codes.ERR_INVALID_ARG_TYPE('str', 'string');
  }

  const ctx = {};
  const result = binding.foo(str, ctx);
  if (ctx.code !== undefined) {
    throw new errors.codes.ERR_ERROR_NAME(ctx.code);
  }
  return result;
};

Avoid throwing JavaScript errors in nested C++ methods

When you have to throw the errors from C++, try to do it at the top level and not inside of nested calls.

Using C++ throw is not allowed.