chromium/mojo/public/cpp/system/handle.h

// Copyright 2014 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef MOJO_PUBLIC_CPP_SYSTEM_HANDLE_H_
#define MOJO_PUBLIC_CPP_SYSTEM_HANDLE_H_

#include <stdint.h>
#include <limits>

#include "base/check_op.h"
#include "mojo/public/c/system/functions.h"
#include "mojo/public/c/system/types.h"
#include "mojo/public/cpp/system/handle_signals_state.h"

namespace mojo {

// OVERVIEW
//
// |Handle| and |...Handle|:
//
// |Handle| is a simple, copyable wrapper for the C type |MojoHandle| (which is
// just an integer). Its purpose is to increase type-safety, not provide
// lifetime management. For the same purpose, we have trivial *subclasses* of
// |Handle|, e.g., |MessagePipeHandle| and |DataPipeProducerHandle|. |Handle|
// and its subclasses impose *no* extra overhead over using |MojoHandle|s
// directly.
//
// Note that though we provide constructors for |Handle|/|...Handle| from a
// |MojoHandle|, we do not provide, e.g., a constructor for |MessagePipeHandle|
// from a |Handle|. This is for type safety: If we did, you'd then be able to
// construct a |MessagePipeHandle| from, e.g., a |DataPipeProducerHandle| (since
// it's a |Handle|).
//
// |ScopedHandleBase| and |Scoped...Handle|:
//
// |ScopedHandleBase<HandleType>| is a templated scoped wrapper, for the handle
// types above (in the same sense that a C++11 |unique_ptr<T>| is a scoped
// wrapper for a |T*|). It provides lifetime management, closing its owned
// handle on destruction. It also provides move semantics, again along the lines
// of C++11's |unique_ptr|. A moved-from |ScopedHandleBase<HandleType>| sets its
// handle value to MOJO_HANDLE_INVALID.
//
// |ScopedHandle| is just (a typedef of) a |ScopedHandleBase<Handle>|.
// Similarly, |ScopedMessagePipeHandle| is just a
// |ScopedHandleBase<MessagePipeHandle>|. Etc. Note that a
// |ScopedMessagePipeHandle| is *not* a (subclass of) |ScopedHandle|.
//
// Wrapper functions:
//
// We provide simple wrappers for the |Mojo...()| functions (in
// mojo/public/c/system/core.h -- see that file for details on individual
// functions).
//
// The general guideline is functions that imply ownership transfer of a handle
// should take (or produce) an appropriate |Scoped...Handle|, while those that
// don't take a |...Handle|. For example, |CreateMessagePipe()| has two
// |ScopedMessagePipe| "out" parameters, whereas |Wait()| and |WaitMany()| take
// |Handle| parameters. Some, have both: e.g., |DuplicatedBuffer()| takes a
// suitable (unscoped) handle (e.g., |SharedBufferHandle|) "in" parameter and
// produces a suitable scoped handle (e.g., |ScopedSharedBufferHandle| a.k.a.
// |ScopedHandleBase<SharedBufferHandle>|) as an "out" parameter.
//
// An exception are some of the |...Raw()| functions. E.g., |CloseRaw()| takes a
// |Handle|, leaving the user to discard the wrapper.
//
// ScopedHandleBase ------------------------------------------------------------

// Scoper for the actual handle types defined further below. It's move-only,
// like the C++11 |unique_ptr|.
template <class HandleType>
class ScopedHandleBase { … };

template <typename HandleType>
inline ScopedHandleBase<HandleType> MakeScopedHandle(HandleType handle) { … }

// Handle ----------------------------------------------------------------------

const MojoHandle kInvalidHandleValue = …;

// Wrapper base class for |MojoHandle|.
class Handle { … };

// Should have zero overhead.
static_assert …;

// The scoper should also impose no more overhead.
ScopedHandle;
static_assert …;

// |Close()| takes ownership of the handle, since it'll invalidate it.
// Note: There's nothing to do, since the argument will be destroyed when it
// goes out of scope.
template <class HandleType>
inline void Close(ScopedHandleBase<HandleType> /*handle*/) { … }

// Most users should typically use |Close()| (above) instead.
inline MojoResult CloseRaw(Handle handle) { … }

// Strict weak ordering, so that |Handle|s can be used as keys in |std::map|s,
inline bool operator<(const Handle a, const Handle b) { … }

// Comparison, so that |Handle|s can be used as keys in hash maps.
inline bool operator==(const Handle a, const Handle b) { … }

}  // namespace mojo

#endif  // MOJO_PUBLIC_CPP_SYSTEM_HANDLE_H_