Blink C++ Style Guide

This document is a list of differences from the overall Chromium Style Guide, which is in turn a set of differences from the Google C++ Style Guide. The long-term goal is to make both Chromium and Blink style more similar to Google style over time, so this document aims to highlight areas where Blink style differs from Google style.

Use references for all non-null pointer arguments

Pointer arguments that can never be null should be passed as a reference, even if this results in a mutable reference argument.

Note: Even though Google style prohibits mutable reference arguments, Blink style explicitly permits their use.

Good:

// Passed by mutable reference since |frame| is assumed to be non-null.
FrameLoader::FrameLoader(LocalFrame& frame)
    : frame_(&frame),
      progress_tracker_(ProgressTracker::Create(frame)) {
  // ...
}

// Optional arguments should still be passed by pointer.
void LocalFrame::SetDOMWindow(LocalDOMWindow* dom_window) {
  if (dom_window)
    GetScriptController().ClearWindowProxy();

  if (this->DomWindow())
    this->DomWindow()->Reset();
  dom_window_ = dom_window;
}

Bad:

// Since the constructor assumes that |frame| is never null, it should be
// passed as a mutable reference.
FrameLoader::FrameLoader(LocalFrame* frame)
    : frame_(frame),
      progress_tracker_(ProgressTracker::Create(frame)) {
  DCHECK(frame_);
  // ...
}

Prefer WTF types over STL types

Outside of //third_party/blink/common, Blink should use WTF types. STL string and container types should only be used at the boundary to interoperate with ‘//base’, //third_party/blink/common, and other Chromium-side code. Similarly, Blink should prefer KURL over GURL and SecurityOrigin over url::Origin.

Good:

  String title;
  Vector<KURL> urls;
  HashMap<int, Deque<RefPtr<SecurityOrigin>>> origins;

Bad:

  std::string title;
  std::vector<GURL> urls;
  std::unordered_map<int, std::deque<url::Origin>> origins;

Naming

Use CamelCase for all function names

All function names should use CamelCase()-style names, beginning with an uppercase letter.

As an exception, method names for web-exposed bindings begin with a lowercase letter to match JavaScript.

Good:

class Document {
 public:
  // Function names should begin with an uppercase letter.
  virtual void Shutdown();

  // However, web-exposed function names should begin with a lowercase letter.
  LocalDOMWindow* defaultView();

  // ...
};

Bad:

class Document {
 public:
  // Though this is a getter, all Blink function names must use camel case.
  LocalFrame* frame() const { return frame_; }

  // ...
};

Precede boolean values with words like “is” and “did”

bool is_valid;
bool did_send_data;

Bad:

bool valid;
bool sent_data;

Precede setters with the word “Set”; use bare words for getters

Precede setters with the word “set”. Prefer bare words for getters. Setter and getter names should match the names of the variable being accessed/mutated.

If a getter’s name collides with a type name, prefix it with “Get”.

Good:

class FrameTree {
 public:
  // Prefer to use the bare word for getters.
  Frame* FirstChild() const { return first_child_; }
  Frame* LastChild() const { return last_child_; }

  // However, if the type name and function name would conflict, prefix the
  // function name with “Get”.
  Frame* GetFrame() const { return frame_; }

  // ...
};

Bad:

class FrameTree {
 public:
  // Should not be prefixed with “Get” since there's no naming conflict.
  Frame* GetFirstChild() const { return first_child_; }
  Frame* GetLastChild() const { return last_child_; }

  // ...
};

Precede getters that return values via out-arguments with the word “Get”

Good:

class RootInlineBox {
 public:
  Node* GetLogicalStartBoxWithNode(InlineBox*&) const;
  // ...
}

Bad:

class RootInlineBox {
 public:
  Node* LogicalStartBoxWithNode(InlineBox*&) const;
  // ...
}

May leave obvious parameter names out of function declarations

Google C++ Style Guide allows us to leave parameter names out only if the parameter is not used. In Blink, you may leave obvious parameter names out of function declarations for historical reason. A good rule of thumb is if the parameter type name contains the parameter name (without trailing numbers or pluralization), then the parameter name isn’t needed.

Good:

class Node {
 public:
  Node(TreeScope* tree_scope, ConstructionType construction_type);
  // You may leave them out like:
  // Node(TreeScope*, ConstructionType);

  // The function name makes the meaning of the parameters clear.
  void SetActive(bool);
  void SetDragged(bool);
  void SetHovered(bool);

  // Parameters are not obvious.
  DispatchEventResult DispatchDOMActivateEvent(int detail,
                                               Event& underlying_event);
};

Bad:

class Node {
 public:
  // ...

  // Parameters are not obvious.
  DispatchEventResult DispatchDOMActivateEvent(int, Event&);
};

Prefer enums to bools for function parameters

Prefer enums to bools for function parameters if callers are likely to be passing constants, since named constants are easier to read at the call site. An exception to this rule is a setter function, where the name of the function already makes clear what the boolean is.

Good:

// An named enum value makes it clear what the parameter is for.
if (frame_->Loader().ShouldClose(CloseType::kNotForReload)) {
  // No need to use enums for boolean setters, since the meaning is clear.
  frame_->SetIsClosing(true);

  // ...

Bad:

// Not obvious what false means here.
if (frame_->Loader().ShouldClose(false)) {
  frame_->SetIsClosing(ClosingState::kTrue);

  // ...

Comments

Please follow the standard Chromium Documentation Guidelines. In particular, most classes should have a class-level comment describing the purpose, while non-trivial code should have comments describing why the code is written the way it is. Often, what is apparent when the code is written is not so obvious a year later.

From Google C++ Style Guide: Comments:

Giving sensible names to types and variables is much better than obscure names that must be explained through comments.

Use README.md to document high-level components

Documentation for a related-set of classes and how they interact should be done with a README.md file in the root directory of a component.

TODO style

Comments for future work should use TODO and have a name or bug attached.

From Google C++ Style Guide: TODO Comments:

The person named is not necessarily the person who must fix it.

Good:

// TODO(dcheng): Clean up after the Blink rename is done.
// TODO(https://crbug.com/675877): Clean up after the Blink rename is done.

Bad:

// FIXME(dcheng): Clean up after the Blink rename is done.
// FIXME: Clean up after the Blink rename is done.
// TODO: Clean up after the Blink rename is done.