path: root/absl/utility/utility.h

// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// This header file contains C++11 versions of standard <utility> header
// abstractions available within C++14 and C++17, and are designed to be drop-in
// replacement for code compliant with C++14 and C++17.
//
// The following abstractions are defined:
//
//   * integer_sequence<T, Ints...>  == std::integer_sequence<T, Ints...>
//   * index_sequence<Ints...>       == std::index_sequence<Ints...>
//   * make_integer_sequence<T, N>   == std::make_integer_sequence<T, N>
//   * make_index_sequence<N>        == std::make_index_sequence<N>
//   * index_sequence_for<Ts...>     == std::index_sequence_for<Ts...>
//
// This header file also provides the tag types `in_place_t`, `in_place_type_t`,
// and `in_place_index_t`, as well as the constant `in_place`, and
// `constexpr` `std::move()` and `std::forward()` implementations in C++11.
//
// References:
//
//  http://en.cppreference.com/w/cpp/utility/integer_sequence
//  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3658.html
//
//
// Example:
//   // Unpack a tuple for use as a function call's argument list.
//
//   template <typename F, typename Tup, size_t... Is>
//   auto Impl(F f, const Tup& tup, absl::index_sequence<Is...>)
//       -> decltype(f(std::get<Is>(tup) ...)) {
//     return f(std::get<Is>(tup) ...);
//   }
//
//   template <typename Tup>
//   using TupIdxSeq = absl::make_index_sequence<std::tuple_size<Tup>::value>;
//
//   template <typename F, typename Tup>
//   auto ApplyFromTuple(F f, const Tup& tup)
//       -> decltype(Impl(f, tup, TupIdxSeq<Tup>{})) {
//     return Impl(f, tup, TupIdxSeq<Tup>{});
//   }
#ifndef ABSL_UTILITY_UTILITY_H_
#define ABSL_UTILITY_UTILITY_H_

#include <cstddef>
#include <cstdlib>
#include <utility>

#include "absl/base/config.h"
#include "absl/meta/type_traits.h"

namespace absl {

// integer_sequence
//
// Class template representing a compile-time integer sequence. An instantiation
// of `integer_sequence<T, Ints...>` has a sequence of integers encoded in its
// type through its template arguments (which is a common need when
// working with C++11 variadic templates). `absl::integer_sequence` is designed
// to be a drop-in replacement for C++14's `std::integer_sequence`.
//
// Example:
//
//   template< class T, T... Ints >
//   void user_function(integer_sequence<T, Ints...>);
//
//   int main()
//   {
//     // user_function's `T` will be deduced to `int` and `Ints...`
//     // will be deduced to `0, 1, 2, 3, 4`.
//     user_function(make_integer_sequence<int, 5>());
//   }
template <typename T, T... Ints>
struct integer_sequence {
  using value_type = T;
  static constexpr size_t size() noexcept { return sizeof...(Ints); }
};

// index_sequence
//
// A helper template for an `integer_sequence` of `size_t`,
// `absl::index_sequence` is designed to be a drop-in replacement for C++14's
// `std::index_sequence`.
template <size_t... Ints>
using index_sequence = integer_sequence<size_t, Ints...>;

namespace utility_internal {

template <typename Seq, size_t SeqSize, size_t Rem>
struct Extend;

// Note that SeqSize == sizeof...(Ints). It's passed explicitly for efficiency.
template <typename T, T... Ints, size_t SeqSize>
struct Extend<integer_sequence<T, Ints...>, SeqSize, 0> {
  using type = integer_sequence<T, Ints..., (Ints + SeqSize)...>;
};

template <typename T, T... Ints, size_t SeqSize>
struct Extend<integer_sequence<T, Ints...>, SeqSize, 1> {
  using type = integer_sequence<T, Ints..., (Ints + SeqSize)..., 2 * SeqSize>;
};

// Recursion helper for 'make_integer_sequence<T, N>'.
// 'Gen<T, N>::type' is an alias for 'integer_sequence<T, 0, 1, ... N-1>'.
template <typename T, size_t N>
struct Gen {
  using type =
      typename Extend<typename Gen<T, N / 2>::type, N / 2, N % 2>::type;
};

template <typename T>
struct Gen<T, 0> {
  using type = integer_sequence<T>;
};

}  // namespace utility_internal

// Compile-time sequences of integers

// make_integer_sequence
//
// This template alias is equivalent to
// `integer_sequence<int, 0, 1, ..., N-1>`, and is designed to be a drop-in
// replacement for C++14's `std::make_integer_sequence`.
template <typename T, T N>
using make_integer_sequence = typename utility_internal::Gen<T, N>::type;

// make_index_sequence
//
// This template alias is equivalent to `index_sequence<0, 1, ..., N-1>`,
// and is designed to be a drop-in replacement for C++14's
// `std::make_index_sequence`.
template <size_t N>
using make_index_sequence = make_integer_sequence<size_t, N>;

// index_sequence_for
//
// Converts a typename pack into an index sequence of the same length, and
// is designed to be a drop-in replacement for C++14's
// `std::index_sequence_for()`
template <typename... Ts>
using index_sequence_for = make_index_sequence<sizeof...(Ts)>;

// Tag types

#ifdef ABSL_HAVE_STD_OPTIONAL

using std::in_place_t;
using std::in_place;

#else  // ABSL_HAVE_STD_OPTIONAL

// in_place_t
//
// Tag type used to specify in-place construction, such as with
// `absl::optional`, designed to be a drop-in replacement for C++17's
// `std::in_place_t`.
struct in_place_t {};
extern const in_place_t in_place;

#endif  // ABSL_HAVE_STD_OPTIONAL

#ifdef ABSL_HAVE_STD_ANY
using std::in_place_type_t;
#else

// in_place_type_t
//
// Tag type used for in-place construction when the type to construct needs to
// be specified, such as with `absl::any`, designed to be a drop-in replacement
// for C++17's `std::in_place_type_t`.
template <typename T>
struct in_place_type_t {};
#endif  // ABSL_HAVE_STD_ANY

// in_place_index_t
//
// Tag type used for in-place construction when the type to construct needs to
// be specified, such as with `absl::any`, designed to be a drop-in replacement
// for C++17's `std::in_place_index_t`.
template <size_t I>
struct in_place_index_t {};

// Constexpr move and forward

// move()
//
// A constexpr version of `std::move()`, designed to be a drop-in replacement
// for C++14's `std::move()`.
template <typename T>
constexpr absl::remove_reference_t<T>&& move(T&& t) noexcept {
  return static_cast<absl::remove_reference_t<T>&&>(t);
}

// forward()
//
// A constexpr version of `std::forward()`, designed to be a drop-in replacement
// for C++14's `std::forward()`.
template <typename T>
constexpr T&& forward(
    absl::remove_reference_t<T>& t) noexcept {  // NOLINT(runtime/references)
  return static_cast<T&&>(t);
}

}  // namespace absl

#endif  // ABSL_UTILITY_UTILITY_H_