THUAI6/CAPI/cpp/grpc/include/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
//
//      https://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...>
//   * apply<Functor, Tuple>         == std::apply<Functor, Tuple>
//   * exchange<T>                   == std::exchange<T>
//   * make_from_tuple<T>            == std::make_from_tuple<T>
//
// 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:
//
//  https://en.cppreference.com/w/cpp/utility/integer_sequence
//  https://en.cppreference.com/w/cpp/utility/apply
//  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3658.html

#ifndef ABSL_UTILITY_UTILITY_H_
#define ABSL_UTILITY_UTILITY_H_

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

#include "absl/base/config.h"
#include "absl/base/internal/inline_variable.h"
#include "absl/base/internal/invoke.h"
#include "absl/meta/type_traits.h"

namespace absl
{
    ABSL_NAMESPACE_BEGIN

    // 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>;
        };

        template<typename T>
        struct InPlaceTypeTag
        {
            explicit InPlaceTypeTag() = delete;
            InPlaceTypeTag(const InPlaceTypeTag&) = delete;
            InPlaceTypeTag& operator=(const InPlaceTypeTag&) = delete;
        };

        template<size_t I>
        struct InPlaceIndexTag
        {
            explicit InPlaceIndexTag() = delete;
            InPlaceIndexTag(const InPlaceIndexTag&) = delete;
            InPlaceIndexTag& operator=(const InPlaceIndexTag&) = delete;
        };

    }  // 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_USES_STD_OPTIONAL

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

#else  // ABSL_USES_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
    {
    };

    ABSL_INTERNAL_INLINE_CONSTEXPR(in_place_t, in_place, {});

#endif  // ABSL_USES_STD_OPTIONAL

#if defined(ABSL_USES_STD_ANY) || defined(ABSL_USES_STD_VARIANT)
    using std::in_place_type;
    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>
    using in_place_type_t = void (*)(utility_internal::InPlaceTypeTag<T>);

    template<typename T>
    void in_place_type(utility_internal::InPlaceTypeTag<T>)
    {
    }
#endif  // ABSL_USES_STD_ANY || ABSL_USES_STD_VARIANT

#ifdef ABSL_USES_STD_VARIANT
    using std::in_place_index;
    using std::in_place_index_t;
#else

    // 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>
    using in_place_index_t = void (*)(utility_internal::InPlaceIndexTag<I>);

    template<size_t I>
    void in_place_index(utility_internal::InPlaceIndexTag<I>)
    {
    }
#endif  // ABSL_USES_STD_VARIANT

    // 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 utility_internal
    {
        // Helper method for expanding tuple into a called method.
        template<typename Functor, typename Tuple, std::size_t... Indexes>
        auto apply_helper(Functor&& functor, Tuple&& t, index_sequence<Indexes...>)
            -> decltype(absl::base_internal::invoke(
                absl::forward<Functor>(functor),
                std::get<Indexes>(absl::forward<Tuple>(t))...
            ))
        {
            return absl::base_internal::invoke(
                absl::forward<Functor>(functor),
                std::get<Indexes>(absl::forward<Tuple>(t))...
            );
        }

    }  // namespace utility_internal

    // apply
    //
    // Invokes a Callable using elements of a tuple as its arguments.
    // Each element of the tuple corresponds to an argument of the call (in order).
    // Both the Callable argument and the tuple argument are perfect-forwarded.
    // For member-function Callables, the first tuple element acts as the `this`
    // pointer. `absl::apply` is designed to be a drop-in replacement for C++17's
    // `std::apply`. Unlike C++17's `std::apply`, this is not currently `constexpr`.
    //
    // Example:
    //
    //   class Foo {
    //    public:
    //     void Bar(int);
    //   };
    //   void user_function1(int, std::string);
    //   void user_function2(std::unique_ptr<Foo>);
    //   auto user_lambda = [](int, int) {};
    //
    //   int main()
    //   {
    //       std::tuple<int, std::string> tuple1(42, "bar");
    //       // Invokes the first user function on int, std::string.
    //       absl::apply(&user_function1, tuple1);
    //
    //       std::tuple<std::unique_ptr<Foo>> tuple2(absl::make_unique<Foo>());
    //       // Invokes the user function that takes ownership of the unique
    //       // pointer.
    //       absl::apply(&user_function2, std::move(tuple2));
    //
    //       auto foo = absl::make_unique<Foo>();
    //       std::tuple<Foo*, int> tuple3(foo.get(), 42);
    //       // Invokes the method Bar on foo with one argument, 42.
    //       absl::apply(&Foo::Bar, tuple3);
    //
    //       std::tuple<int, int> tuple4(8, 9);
    //       // Invokes a lambda.
    //       absl::apply(user_lambda, tuple4);
    //   }
    template<typename Functor, typename Tuple>
    auto apply(Functor&& functor, Tuple&& t)
        -> decltype(utility_internal::apply_helper(
            absl::forward<Functor>(functor), absl::forward<Tuple>(t), absl::make_index_sequence<std::tuple_size<typename std::remove_reference<Tuple>::type>::value>{}
        ))
    {
        return utility_internal::apply_helper(
            absl::forward<Functor>(functor), absl::forward<Tuple>(t), absl::make_index_sequence<std::tuple_size<typename std::remove_reference<Tuple>::type>::value>{}
        );
    }

    // exchange
    //
    // Replaces the value of `obj` with `new_value` and returns the old value of
    // `obj`.  `absl::exchange` is designed to be a drop-in replacement for C++14's
    // `std::exchange`.
    //
    // Example:
    //
    //   Foo& operator=(Foo&& other) {
    //     ptr1_ = absl::exchange(other.ptr1_, nullptr);
    //     int1_ = absl::exchange(other.int1_, -1);
    //     return *this;
    //   }
    template<typename T, typename U = T>
    T exchange(T& obj, U&& new_value)
    {
        T old_value = absl::move(obj);
        obj = absl::forward<U>(new_value);
        return old_value;
    }

    namespace utility_internal
    {
        template<typename T, typename Tuple, size_t... I>
        T make_from_tuple_impl(Tuple&& tup, absl::index_sequence<I...>)
        {
            return T(std::get<I>(std::forward<Tuple>(tup))...);
        }
    }  // namespace utility_internal

    // make_from_tuple
    //
    // Given the template parameter type `T` and a tuple of arguments
    // `std::tuple(arg0, arg1, ..., argN)` constructs an object of type `T` as if by
    // calling `T(arg0, arg1, ..., argN)`.
    //
    // Example:
    //
    //   std::tuple<const char*, size_t> args("hello world", 5);
    //   auto s = absl::make_from_tuple<std::string>(args);
    //   assert(s == "hello");
    //
    template<typename T, typename Tuple>
    constexpr T make_from_tuple(Tuple&& tup)
    {
        return utility_internal::make_from_tuple_impl<T>(
            std::forward<Tuple>(tup),
            absl::make_index_sequence<
                std::tuple_size<absl::decay_t<Tuple>>::value>{}
        );
    }

    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_UTILITY_UTILITY_H_