rtc_base/template_util.h

/*
 *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

// Borrowed from Chromium's src/base/template_util.h.

#ifndef RTC_BASE_TEMPLATE_UTIL_H_
#define RTC_BASE_TEMPLATE_UTIL_H_

#include <stddef.h>  // For size_t.

namespace rtc {

// Template definitions from tr1.

template <class T, T v>
struct integral_constant {
  static const T value = v;
  typedef T value_type;
  typedef integral_constant<T, v> type;
};

template <class T, T v>
const T integral_constant<T, v>::value;

typedef integral_constant<bool, true> true_type;
typedef integral_constant<bool, false> false_type;

template <class T>
struct is_pointer : false_type {};
template <class T>
struct is_pointer<T*> : true_type {};

template <class T, class U>
struct is_same : public false_type {};
template <class T>
struct is_same<T, T> : true_type {};

template <class>
struct is_array : public false_type {};
template <class T, size_t n>
struct is_array<T[n]> : public true_type {};
template <class T>
struct is_array<T[]> : public true_type {};

template <class T>
struct is_non_const_reference : false_type {};
template <class T>
struct is_non_const_reference<T&> : true_type {};
template <class T>
struct is_non_const_reference<const T&> : false_type {};

template <class T>
struct is_void : false_type {};
template <>
struct is_void<void> : true_type {};

// Helper useful for converting a tuple to variadic template function
// arguments.
//
// sequence_generator<3>::type will be sequence<0, 1, 2>.
template <int...>
struct sequence {};
template <int N, int... S>
struct sequence_generator : sequence_generator<N - 1, N - 1, S...> {};
template <int... S>
struct sequence_generator<0, S...> {
  typedef sequence<S...> type;
};

namespace internal {

// Types YesType and NoType are guaranteed such that sizeof(YesType) <
// sizeof(NoType).
typedef char YesType;

struct NoType {
  YesType dummy[2];
};

// This class is an implementation detail for is_convertible, and you
// don't need to know how it works to use is_convertible. For those
// who care: we declare two different functions, one whose argument is
// of type To and one with a variadic argument list. We give them
// return types of different size, so we can use sizeof to trick the
// compiler into telling us which function it would have chosen if we
// had called it with an argument of type From.  See Alexandrescu's
// _Modern C++ Design_ for more details on this sort of trick.

struct ConvertHelper {
  template <typename To>
  static YesType Test(To);

  template <typename To>
  static NoType Test(...);

  template <typename From>
  static From& Create();
};

// Used to determine if a type is a struct/union/class. Inspired by Boost's
// is_class type_trait implementation.
struct IsClassHelper {
  template <typename C>
  static YesType Test(void (C::*)(void));

  template <typename C>
  static NoType Test(...);
};

}  // namespace internal

// Inherits from true_type if From is convertible to To, false_type otherwise.
//
// Note that if the type is convertible, this will be a true_type REGARDLESS
// of whether or not the conversion would emit a warning.
template <typename From, typename To>
struct is_convertible
    : integral_constant<bool,
                        sizeof(internal::ConvertHelper::Test<To>(
                            internal::ConvertHelper::Create<From>())) ==
                            sizeof(internal::YesType)> {};

template <typename T>
struct is_class
    : integral_constant<bool,
                        sizeof(internal::IsClassHelper::Test<T>(0)) ==
                            sizeof(internal::YesType)> {};

}  // namespace rtc

#endif  // RTC_BASE_TEMPLATE_UTIL_H_