type_traits.hpp

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// Copyright 2009-present MongoDB, Inc.
//
// 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.
 
#pragma once
 
#include <type_traits>
#include <utility>
 
#include <bsoncxx/config/prelude.hpp>
 
namespace bsoncxx {
namespace detail {
 
// Obtain the nested ::type of the given type argument
template <typename T>
using type_t = typename T::type;
 
// Obtain the value_type member type of the given argument
template <typename T>
using value_type_t = typename T::value_type;
 
template <bool B, typename T = void>
using enable_if_t = typename std::enable_if<B, T>::type;
 
#pragma push_macro("DECL_ALIAS")
#undef DECL_ALIAS
#define DECL_ALIAS(Name)  \
    template <typename T> \
    using Name##_t = type_t<std::Name<T>>
DECL_ALIAS(decay);
DECL_ALIAS(make_signed);
DECL_ALIAS(make_unsigned);
DECL_ALIAS(remove_reference);
DECL_ALIAS(remove_const);
DECL_ALIAS(remove_volatile);
DECL_ALIAS(remove_pointer);
DECL_ALIAS(remove_cv);
DECL_ALIAS(add_pointer);
DECL_ALIAS(add_const);
DECL_ALIAS(add_volatile);
DECL_ALIAS(add_lvalue_reference);
DECL_ALIAS(add_rvalue_reference);
#pragma pop_macro("DECL_ALIAS")
 
template <typename... Ts>
using common_type_t = type_t<std::common_type<Ts...>>;
 
// Remove top-level const+volatile+reference qualifiers from the given type.
template <typename T>
using remove_cvref_t = remove_cv_t<remove_reference_t<T>>;
 
// Create a reference-to-const for the given type
template <typename T>
using const_reference_t = add_lvalue_reference_t<const remove_cvref_t<T>>;
 
// Workaround for CWG issue 1558.
template <typename...>
struct just_void {
    using type = void;
};
 
// A "do-nothing" alias template that always evaluates to void.
//
// @tparam Ts Zero or more type arguments, all discarded
template <typename... Ts>
using void_t =
#if defined(_MSC_VER) && _MSC_VER < 1910
    // Old MSVC requires that the type parameters actually be "used" to trigger SFINAE at caller.
    // This was resolved by CWG issue 1558.
    typename just_void<Ts...>::type;
#else
    void;
#endif
 
// Alias for integral_constant<bool, B>.
template <bool B>
using bool_constant = std::integral_constant<bool, B>;
 
// Holds a list of types.
//
// This template is never defined, so cannot be used in contexts that require a complete type.
template <typename...>
struct mp_list;
 
// Details for implementing the C++11 detection idiom.
namespace impl_detection {
 
// Implementation of detection idiom for is_detected: true case
template <
    // A metafunction to try and apply
    template <class...>
    class Oper,
    // The arguments to be given. These are deduced from the mp_list argument
    typename... Args,
    // Apply the arguments to the metafunction. If this yields a type, this function
    // will be viable. If substitution fails, this function is discarded from the
    // overload set.
    typename SfinaeHere = Oper<Args...>>
std::true_type is_detected_f(mp_list<Args...>*);
 
// Failure case for is_detected. Because this function takes an elipsis, this is
// less preferred than the above overload that accepts a pointer type directly.
template <template <class...> class Oper>
std::false_type is_detected_f(...);
 
// Provides the detected_or impl
template <bool IsDetected>
struct detection;
 
// Non-detected case:
template <>
struct detection<false> {
    // We just return the default, since the metafunction will not apply
    template <typename Default, template <class...> class, typename...>
    using f = Default;
};
 
// Detected case:
template <>
struct detection<true> {
    template <typename, template <class...> class Oper, typename... Args>
    using f = Oper<Args...>;
};
 
// Workaround: MSVC 14.0 forgets whether a type resulting from the evaluation
// of a template-template parameter to an alias template is a reference.
template <typename Dflt, typename Void, template <class...> class Oper, typename... Args>
struct vc140_detection {
    using type = Dflt;
};
 
template <typename Dflt, template <class...> class Oper, typename... Args>
struct vc140_detection<Dflt, void_t<Oper<Args...>>, Oper, Args...> {
    using type = Oper<Args...>;
};
 
}  // namespace impl_detection
 
// The type yielded by detected_t if the given type operator does not yield a type.
struct nonesuch {
    ~nonesuch() = delete;
    nonesuch(nonesuch const&) = delete;
    void operator=(nonesuch const&) = delete;
};
 
// Results in true_type if the given metafunction yields a valid type when applied to the given
// arguments, otherwise yields false_type.
//
// @tparam Oper A template that evaluates to a type
// @tparam Args Some number of arguments to apply to Oper
template <template <class...> class Oper, typename... Args>
struct is_detected
    : decltype(impl_detection::is_detected_f<Oper>(static_cast<mp_list<Args...>*>(nullptr))) {};
 
// If Oper<Args...> evaluates to a type, yields that type. Otherwise, yields the Dflt type.
//
// @tparam Dflt The default type to return if the metafunction does not apply
// @tparam Oper A metafunction to speculatively apply
// @tparam Args The arguments to give to the Oper metafunction
template <typename Dflt, template <class...> class Oper, typename... Args>
using detected_or =
#if defined(_MSC_VER) && _MSC_VER < 1910
    typename impl_detection::vc140_detection<Dflt, void, Oper, Args...>::type
#else
    typename impl_detection::detection<
        is_detected<Oper, Args...>::value>::template f<Dflt, Oper, Args...>
#endif
    ;
 
// If Oper<Args...> evaluates to a type, yields that type. Otherwise, yields the sentinel type
// `nonesuch`.
//
// @tparam Oper A metafunction to try to apply.
// @tparam Args The metafunction arguments to apply to Oper.
template <template <class...> class Oper, typename... Args>
using detected_t = detected_or<nonesuch, Oper, Args...>;
 
// Impl of conditional_t.
//
// Separating the boolean from the type arguments results in significant speedup to compilation due
// to type memoization.
template <bool B>
struct conditional {
    template <typename IfTrue, typename>
    using f = IfTrue;
};
 
template <>
struct conditional<false> {
    template <typename, typename IfFalse>
    using f = IfFalse;
};
 
// Pick one of two types based on a boolean.
//
// @tparam B A boolean value
// @tparam T If `B` is true, pick this type
// @tparam F If `B` is false, pick this type
template <bool B, typename T, typename F>
using conditional_t = typename conditional<B>::template f<T, F>;
 
// Impl for conjunction+disjunction
namespace impl_logic {
 
template <typename FalseType, typename Opers>
struct conj;
 
template <typename H, typename... Tail>
struct conj<bool_constant<H::value || !sizeof...(Tail)>, mp_list<H, Tail...>> : H {};
 
template <typename F, typename H, typename... Tail>
struct conj<F, mp_list<H, Tail...>> : conj<F, mp_list<Tail...>> {};
 
template <typename H>
struct conj<std::false_type, mp_list<H>> : H {};
 
template <>
struct conj<std::false_type, mp_list<>> : std::true_type {};
 
template <typename TrueType, typename Opers>
struct disj;
 
template <typename H, typename... Tail>
struct disj<bool_constant<H::value && sizeof...(Tail)>, mp_list<H, Tail...>> : H {};
 
template <typename F, typename H, typename... Tail>
struct disj<F, mp_list<H, Tail...>> : disj<F, mp_list<Tail...>> {};
 
template <typename H>
struct disj<std::true_type, mp_list<H>> : H {};
 
template <>
struct disj<std::true_type, mp_list<>> : std::false_type {};
 
}  // namespace impl_logic
 
// Inherits unambiguously from the first of `Ts...` for which `Ts::value` is a valid expression
// equal to `false`, or the last of `Ts...` otherwise.
//
// conjunction<> (given no arguments) inherits from std::true_type.
//
// If any of `Ts::value == false`, then no subsequent `Ts::value` will be instantiated.
//
template <typename... Cond>
struct conjunction : impl_logic::conj<std::false_type, mp_list<Cond...>> {};
 
// Inherits unambiguous from the first of `Ts...` where `Ts::value` is `true`, or the last of
// `Ts...` otherwise.
//
// Given no arguments, inherits from std::false_type.
//
// If any of `Ts::value == true`, then no subsequent `Ts::value` will be instantiated.
template <typename... Cond>
struct disjunction : impl_logic::disj<std::true_type, mp_list<Cond...>> {};
 
// A type trait that produces the negation of the given boolean type trait.
//
// @tparam T A type trait with a static member ::value.
template <typename T>
struct negation : bool_constant<!T::value> {};
 
// Yields std::true_type, regardless of type arguments.
//
// Useful for wrapping potential decltype() substitution failures in positions
// that expect a bool_constant type.
template <typename...>
using true_t = std::true_type;
 
namespace impl_requires {
 
template <typename R>
R norm_conjunction(const R&);
 
template <typename R, typename... Cs>
conjunction<Cs...> norm_conjunction(const conjunction<Cs...>&);
 
template <typename T>
using norm_conjunction_t = decltype(norm_conjunction<T>(std::declval<const T&>()));
 
template <typename Constraint, typename = void>
struct requirement;
 
template <typename FailingRequirement>
struct failed_requirement {
    failed_requirement(int) = delete;
 
    template <typename T>
    static T explain(failed_requirement);
};
 
template <typename... SubRequirements>
struct failed_requirement<conjunction<SubRequirements...>> {
    failed_requirement(int) = delete;
 
    template <typename T>
    static auto explain(int)
        -> common_type_t<decltype(requirement<SubRequirements>::test::template explain<T>(0))...>;
};
 
template <typename Constraint, typename>
struct requirement {
    using test = failed_requirement<impl_requires::norm_conjunction_t<Constraint>>;
};
 
template <typename Constraint>
struct requirement<Constraint, enable_if_t<Constraint::value>> {
    struct test {
        template <typename T>
        static T explain(int);
    };
};
 
}  // namespace impl_requires
 
// If none of `Ts::value is 'false'`, yields the type `Type`, otherwise this type is undefined.
//
// Use this to perform enable-if style template constraints.
//
// @tparam Type The type to return upon success
// @tparam Traits A list of type traits with nested ::value members
template <typename Type, typename... Traits>
#if defined _MSC_VER && _MSC_VER < 1920
// VS 2015 has trouble with expression SFINAE.
using requires_t = enable_if_t<conjunction<Traits...>::value, Type>;
#else
// Generates better error messages in case of substitution failure than a plain enable_if_t:
using requires_t =
    decltype(impl_requires::requirement<conjunction<Traits...>>::test::template explain<Type>(0));
#endif
 
// If any of `Ts::value` is 'true', this type is undefined, otherwise yields the type `Type`.
//
// Use this to perform enable-if template contraints.
//
// @tparam Type The type to return upon success
// @tparam Traits A list of type traits with nested ::value members
template <typename Type, typename... Traits>
using requires_not_t = requires_t<Type, negation<disjunction<Traits...>>>;
 
// Impl: invoke/is_invocable
namespace impl_invoke {
 
template <bool IsMemberObject, bool IsMemberFunction>
struct invoker {
    template <typename F, typename... Args>
    constexpr static auto apply(F&& fun, Args&&... args)
        BSONCXX_RETURNS(static_cast<F&&>(fun)(static_cast<Args&&>(args)...));
};
 
template <>
struct invoker<false, true> {
    template <typename F, typename Self, typename... Args>
    constexpr static auto apply(F&& fun, Self&& self, Args&&... args)
        BSONCXX_RETURNS((static_cast<Self&&>(self).*fun)(static_cast<Args&&>(args)...));
};
 
template <>
struct invoker<true, false> {
    template <typename F, typename Self>
    constexpr static auto apply(F&& fun, Self&& self)
        BSONCXX_RETURNS(static_cast<Self&&>(self).*fun);
};
 
}  // namespace impl_invoke
 
static constexpr struct invoke_fn {
    // Invoke the given object with the given arguments.
    //
    // @param fn An invocable: A callable, member object pointer, or member function pointer.
    // @param args The arguments to use for invocation.
    // @cond DOXYGEN_DISABLE "Found ';' while parsing initializer list!"
    template <typename F, typename... Args, typename Fd = remove_cvref_t<F>>
    constexpr auto operator()(F&& fn, Args&&... args) const
        BSONCXX_RETURNS(impl_invoke::invoker<std::is_member_object_pointer<Fd>::value,
                                             std::is_member_function_pointer<Fd>::value>  //
                        ::apply(static_cast<F&&>(fn), static_cast<Args&&>(args)...));
    // @endcond
} invoke;
 
// Yields the type that would result from invoking F with the given arguments.
//
// @tparam F A invocable: A function pointer or callable object, or a member pointer
// @tparam Args The arguments to apply
template <typename F, typename... Args>
using invoke_result_t = decltype(invoke(std::declval<F>(), std::declval<Args>()...));
 
// Trait type to detect if the given object can be "invoked" using the given arguments.
//
// @tparam F A invocable: A function pointer or callable object, or a member pointer
// @tparam Args The arguments to match against
template <typename F, typename... Args>
#if defined(_MSC_VER) && _MSC_VER < 1910
using is_invocable = is_detected<invoke_result_t, F, Args...>;
#else
struct is_invocable : is_detected<invoke_result_t, F, Args...> {
};
#endif
 
// Trait detects whether the given types are the same after the removal of top-level CV-ref
// qualifiers
template <typename T, typename U>
struct is_alike : std::is_same<remove_cvref_t<T>, remove_cvref_t<U>> {};
 
// Tag type for creating ranked overloads to force disambiguation.
//
// @tparam N The ranking of the overload. A higher value is ranked greater than
// lower values.
template <std::size_t N>
struct rank :
    // @cond DOXYGEN_DISABLE " Detected potential recursive class relation ..."
    rank<N - 1>
// @endcond
{};
 
template <>
struct rank<0> {};
 
namespace swap_detection {
 
using std::swap;
 
// Declare an unusable variadic swap. If not present, MSVC 19.00 (VS2015) errors in
// this header and complains "'std::swap': function does not take 1 arguments" (???).
void swap(...) = delete;
 
template <typename T, typename U>
auto is_swappable_f(rank<0>) -> std::false_type;
 
template <typename T, typename U>
auto is_swappable_f(rank<1>)                                       //
    noexcept(noexcept(swap(std::declval<T>(), std::declval<U>()))  //
             && noexcept(swap(std::declval<U>(), std::declval<T>())))
        -> true_t<decltype(swap(std::declval<T>(), std::declval<U>())),
                  decltype(swap(std::declval<U>(), std::declval<T>()))>;
 
template <typename T, typename U>
auto is_nothrow_swappable_f(rank<0>) -> std::false_type;
 
template <typename T, typename U>
auto is_nothrow_swappable_f(rank<1>)  //
    -> bool_constant<noexcept(swap(std::declval<T>(), std::declval<U>())) &&
                     noexcept(swap(std::declval<U>(), std::declval<T>()))>;
 
}  // namespace swap_detection
 
template <typename T, typename U>
struct is_swappable_with : decltype(swap_detection::is_swappable_f<T, U>(rank<1>{})) {};
 
template <typename T, typename U>
struct is_nothrow_swappable_with
    : decltype(swap_detection::is_nothrow_swappable_f<T, U>(rank<1>{})) {};
 
template <typename T>
struct is_swappable : is_swappable_with<T&, T&> {};
 
template <typename T>
struct is_nothrow_swappable : is_nothrow_swappable_with<T&, T&> {};
 
}  // namespace detail
}  // namespace bsoncxx
 
#include <bsoncxx/config/postlude.hpp>