gecko-dev/mfbt/UniquePtrExtensions.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* Useful extensions to UniquePtr. */

#ifndef mozilla_UniquePtrExtensions_h
#define mozilla_UniquePtrExtensions_h

#include <type_traits>

#include "mozilla/Attributes.h"
#include "mozilla/fallible.h"
#include "mozilla/UniquePtr.h"

#ifdef XP_WIN
#  include <cstdint>
#endif

namespace mozilla {

/**
 * MakeUniqueFallible works exactly like MakeUnique, except that the memory
 * allocation performed is done fallibly, i.e. it can return nullptr.
 */
template <typename T, typename... Args>
typename detail::UniqueSelector<T>::SingleObject MakeUniqueFallible(
    Args&&... aArgs) {
  return UniquePtr<T>(new (fallible) T(std::forward<Args>(aArgs)...));
}

template <typename T>
typename detail::UniqueSelector<T>::UnknownBound MakeUniqueFallible(
    decltype(sizeof(int)) aN) {
  using ArrayType = std::remove_extent_t<T>;
  return UniquePtr<T>(new (fallible) ArrayType[aN]());
}

template <typename T, typename... Args>
typename detail::UniqueSelector<T>::KnownBound MakeUniqueFallible(
    Args&&... aArgs) = delete;

/**
 * MakeUniqueForOverwrite and MakeUniqueFallibleForOverwrite are like MakeUnique
 * and MakeUniqueFallible except they use default-initialization. This is
 * useful, for example, when you have a POD type array that will be overwritten
 * directly after construction and so zero-initialization is a waste.
 */
template <typename T, typename... Args>
typename detail::UniqueSelector<T>::SingleObject MakeUniqueForOverwrite() {
  return UniquePtr<T>(new T);
}

template <typename T>
typename detail::UniqueSelector<T>::UnknownBound MakeUniqueForOverwrite(
    decltype(sizeof(int)) aN) {
  using ArrayType = std::remove_extent_t<T>;
  return UniquePtr<T>(new ArrayType[aN]);
}

template <typename T, typename... Args>
typename detail::UniqueSelector<T>::KnownBound MakeUniqueForOverwrite(
    Args&&... aArgs) = delete;

template <typename T, typename... Args>
typename detail::UniqueSelector<T>::SingleObject
MakeUniqueForOverwriteFallible() {
  return UniquePtr<T>(new (fallible) T);
}

template <typename T>
typename detail::UniqueSelector<T>::UnknownBound MakeUniqueForOverwriteFallible(
    decltype(sizeof(int)) aN) {
  using ArrayType = std::remove_extent_t<T>;
  return UniquePtr<T>(new (fallible) ArrayType[aN]);
}

template <typename T, typename... Args>
typename detail::UniqueSelector<T>::KnownBound MakeUniqueForOverwriteFallible(
    Args&&... aArgs) = delete;

namespace detail {

template <typename T>
struct FreePolicy {
  void operator()(const void* ptr) { free(const_cast<void*>(ptr)); }
};

#if defined(XP_WIN)
// Can't include <windows.h> to get the actual definition of HANDLE
// because of namespace pollution.
typedef void* FileHandleType;
#elif defined(XP_UNIX)
typedef int FileHandleType;
#else
#  error "Unsupported OS?"
#endif

struct FileHandleHelper {
  MOZ_IMPLICIT FileHandleHelper(FileHandleType aHandle) : mHandle(aHandle) {}

  MOZ_IMPLICIT constexpr FileHandleHelper(std::nullptr_t)
      : mHandle(kInvalidHandle) {}

  bool operator!=(std::nullptr_t) const {
#ifdef XP_WIN
    // Windows uses both nullptr and INVALID_HANDLE_VALUE (-1 cast to
    // HANDLE) in different situations, but nullptr is more reliably
    // null while -1 is also valid input to some calls that take
    // handles.  So class considers both to be null (since neither
    // should be closed) but default-constructs as nullptr.
    if (mHandle == (void*)-1) {
      return false;
    }
#endif
    return mHandle != kInvalidHandle;
  }

  operator FileHandleType() const { return mHandle; }

#ifdef XP_WIN
  // NSPR uses an integer type for PROsfd, so this conversion is
  // provided for working with it without needing reinterpret casts
  // everywhere.
  operator std::intptr_t() const {
    return reinterpret_cast<std::intptr_t>(mHandle);
  }
#endif

  // When there's only one user-defined conversion operator, the
  // compiler will use that to derive equality, but that doesn't work
  // when the conversion is ambiguoug (the XP_WIN case above).
  bool operator==(const FileHandleHelper& aOther) const {
    return mHandle == aOther.mHandle;
  }

 private:
  FileHandleType mHandle;

#ifdef XP_WIN
  // See above for why this is nullptr.  (Also, INVALID_HANDLE_VALUE
  // can't be expressed as a constexpr.)
  static constexpr FileHandleType kInvalidHandle = nullptr;
#else
  static constexpr FileHandleType kInvalidHandle = -1;
#endif
};

struct FileHandleDeleter {
  typedef FileHandleHelper pointer;
  MFBT_API void operator()(FileHandleHelper aHelper);
};

}  // namespace detail

template <typename T>
using UniqueFreePtr = UniquePtr<T, detail::FreePolicy<T>>;

// A RAII class for the OS construct used for open files and similar
// objects: a file descriptor on Unix or a handle on Windows.
using UniqueFileHandle =
    UniquePtr<detail::FileHandleType, detail::FileHandleDeleter>;

// Helper for passing a UniquePtr to an old-style function that uses raw
// pointers for out params. Example usage:
//
//   void AllocateFoo(Foo** out) { *out = new Foo(); }
//   UniquePtr<Foo> foo;
//   AllocateFoo(getter_Transfers(foo));
template <typename T, typename D>
auto getter_Transfers(UniquePtr<T, D>& up) {
  class MOZ_TEMPORARY_CLASS UniquePtrGetterTransfers {
   public:
    using Ptr = UniquePtr<T, D>;
    explicit UniquePtrGetterTransfers(Ptr& p) : mPtr(p) {}
    ~UniquePtrGetterTransfers() { mPtr.reset(mRawPtr); }

    operator typename Ptr::ElementType**() { return &mRawPtr; }
    operator void**() { return reinterpret_cast<void**>(&mRawPtr); }
    typename Ptr::ElementType*& operator*() { return mRawPtr; }

   private:
    Ptr& mPtr;
    typename Ptr::Pointer mRawPtr = nullptr;
  };

  return UniquePtrGetterTransfers(up);
}

}  // namespace mozilla

#endif  // mozilla_UniquePtrExtensions_h