dlib C++ Library - type_traits.h

    // Copyright (C) 2016  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_TYPE_TRAITS_H_
    #define DLIB_TYPE_TRAITS_H_

    /*
        This header contains back-ports of C++14/17 type traits
        It also contains aliases for things found in <type_traits> which
        deprecate old dlib type traits.
    */

    #include <type_traits>
    #include <cstdint>

    namespace dlib
    {
    // ----------------------------------------------------------------------------------------

        /*!A is_pointer_type

            This is a template where is_pointer_type<T>::value == true when T is a pointer
            type and false otherwise.
        !*/
        template <typename T>
        using is_pointer_type = std::is_pointer<T>;

    // ----------------------------------------------------------------------------------------

        /*!A is_const_type

            This is a template where is_const_type<T>::value == true when T is a const
            type and false otherwise.
        !*/
        template <typename T>
        using is_const_type = std::is_const<std::remove_reference_t<T>>;

    // ----------------------------------------------------------------------------------------

        /*!A is_reference_type

            This is a template where is_const_type<T>::value == true when T is a reference
            type and false otherwise.
        !*/
        template <typename T>
        using is_reference_type = std::is_reference<std::remove_const_t<T>>;

    // ----------------------------------------------------------------------------------------

        /*!A is_function

            This is a template that allows you to determine if the given type is a function.

            For example,
                void funct();

                is_function<funct>::value == true
                is_function<int>::value == false
        !*/
        template<typename T>
        using is_function = std::is_function<T>;

    // ----------------------------------------------------------------------------------------

        /*!A is_same_type

            This is a template where is_same_type<T,U>::value == true when T and U are the
            same type and false otherwise.
        !*/
        template <typename T, typename U>
        using is_same_type = std::is_same<T,U>;

    // ----------------------------------------------------------------------------------------

        /*!A And

            This template takes a list of bool values and yields their logical and.  E.g.
            And<true,true,true>::value == true
            And<true,false,true>::value == false
        !*/
    #ifdef __cpp_fold_expressions
        template<bool... v>
        struct And : std::integral_constant<bool, (... && v)> {};
    #else
        template<bool First, bool... Rest>
        struct And : std::integral_constant<bool, First && And<Rest...>::value> {};

        template<bool Value>
        struct And<Value> : std::integral_constant<bool, Value>{};
    #endif

    // ----------------------------------------------------------------------------------------

        /*!A Or

            This template takes a list of bool values and yields their logical or.  E.g.
            Or<true,true,true>::value == true
            Or<true,false,true>::value == true
            Or<false,false,false>::value == false
        !*/
    #ifdef __cpp_fold_expressions
        template<bool... v>
        struct Or : std::integral_constant<bool, (... || v)> {};
    #else
        template<bool First, bool... Rest>
        struct Or : std::integral_constant<bool, First || Or<Rest...>::value> {};

        template<bool Value>
        struct Or<Value> : std::integral_constant<bool, Value>{};
    #endif

    // ----------------------------------------------------------------------------------------

        /*!A is_any_type

            This is a template where is_any_type<T,Rest...>::value == true when T is
            the same type as any one of the types in Rest...
        !*/

        template <typename T, typename... Types>
        struct is_any_type : Or<std::is_same<T,Types>::value...> {};

    // ----------------------------------------------------------------------------------------

        /*!A is_float_type

            This is a template that can be used to determine if a type is one of the built
            int floating point types (i.e. float, double, or long double).
        !*/
        template<typename T>
        using is_float_type = std::is_floating_point<T>;

    // ----------------------------------------------------------------------------------------

        /*!A is_unsigned_type

            This is a template where is_unsigned_type<T>::value == true when T is an unsigned
            scalar type and false when T is a signed scalar type.
        !*/
        template <typename T>
        using is_unsigned_type = std::is_unsigned<T>;

    // ----------------------------------------------------------------------------------------

        /*!A is_signed_type

            This is a template where is_signed_type<T>::value == true when T is a signed
            scalar type and false when T is an unsigned scalar type.
        !*/
        template <typename T>
        using is_signed_type = std::is_signed<T>;

    // ----------------------------------------------------------------------------------------

        /*!A is_built_in_scalar_type

            This is a template that allows you to determine if the given type is a built
            in scalar type such as an int, char, float, short, etc.

            For example, is_built_in_scalar_type<char>::value == true
            For example, is_built_in_scalar_type<std::string>::value == false
        !*/
        template <typename T>
        using is_built_in_scalar_type = std::is_arithmetic<T>;

    // ----------------------------------------------------------------------------------------

        /*!A is_byte

            Tells you if a type is one of the byte types in C++.  E.g.
            is_byte<char>::value == true
            is_byte<int>::value == false
        !*/
        template<class Byte>
        using is_byte = std::integral_constant<bool, std::is_same<Byte,char>::value
                                                  || std::is_same<Byte,int8_t>::value
                                                  || std::is_same<Byte,uint8_t>::value
    #ifdef __cpp_lib_byte
                                                  || std::is_same<Byte,std::byte>::value
    #endif
                                              >;

    // ----------------------------------------------------------------------------------------

        /*!A remove_cvref_t

            This is a template that takes a type and strips off any const, volatile, or reference
            qualifiers and gives you back the basic underlying type.  So for example:

            remove_cvref_t<const int&> == int
        !*/
        template< class T >
        using remove_cvref_t = std::remove_cv_t<std::remove_reference_t<T>>;

        /*!A basic_type

            This is a template that takes a type and strips off any const, volatile, or reference
            qualifiers and gives you back the basic underlying type.  So for example:

            basic_type<const int&>::type == int

            This is the same as remove_cvref_t and exists for backwards compatibility with older dlib clients,
            since basic_type has existed in dlib long before remove_cvref_t was added to the standard library.
        !*/
        template <typename T>
        struct basic_type { using type = remove_cvref_t<T>; };

    // ----------------------------------------------------------------------------------------

        /*!A conjunction

            Takes a list of type traits and gives you the logical AND of them.  E.g.

            conjunction<is_same_type<int,int>, is_same_type<char,char>>::value == true
            conjunction<is_same_type<int,int>, is_same_type<char,float>>::value == false
        !*/
        template<class...>
        struct conjunction : std::true_type {};

        template<class B1>
        struct conjunction<B1> : B1 {};

        template<class B1, class... Bn>
        struct conjunction<B1, Bn...> : std::conditional_t<bool(B1::value), conjunction<Bn...>, B1> {};

    // ----------------------------------------------------------------------------------------

        /*!A are_nothrow_move_constructible

            A type trait class telling you if all the types given to it are no-throw move constructable.

        !*/
        template <typename ...Types>
        struct are_nothrow_move_constructible : And<std::is_nothrow_move_constructible<Types>::value...> {};

    // ----------------------------------------------------------------------------------------

        /*!A are_nothrow_move_assignable

            A type trait class telling you if all the types given to it are no-throw move assignable.

        !*/
        template <typename ...Types>
        struct are_nothrow_move_assignable : And<std::is_nothrow_move_assignable<Types>::value...> {};

    // ----------------------------------------------------------------------------------------

        /*!A are_nothrow_copy_constructible

            A type trait class telling you if all the types given to it are no-throw copy constructable.

        !*/
        template <typename ...Types>
        struct are_nothrow_copy_constructible : And<std::is_nothrow_copy_constructible<Types>::value...> {};

    // ----------------------------------------------------------------------------------------

        /*!A are_nothrow_copy_assignable

            A type trait class telling you if all the types given to it are no-throw copy assignable.

        !*/
        template <typename ...Types>
        struct are_nothrow_copy_assignable : And<std::is_nothrow_copy_assignable<Types>::value...> {};

    // ----------------------------------------------------------------------------------------

        /*!A void_t

            Just always the void type.  Is useful in SFINAE expressions when the resulting type doesn't
            matter and you just need a place to put an expression where SFINAE can take effect.
        !*/
        template< class... >
        using void_t = void;

    // ----------------------------------------------------------------------------------------

        namespace swappable_details
        {
            using std::swap;

            template<class T, class = void>
            struct swap_traits
            {
                constexpr static bool is_swappable{false};
                constexpr static bool is_nothrow{false};
            };

            template<class T>
            struct swap_traits<T, void_t<decltype(swap(std::declval<T&>(), std::declval<T&>()))>>
            {
                constexpr static bool is_swappable{true};
                constexpr static bool is_nothrow{noexcept(swap(std::declval<T&>(), std::declval<T&>()))};
            };
        }

    // ----------------------------------------------------------------------------------------

        /*!A is_swappable

            A type trait telling you if T can be swapped by a global swap() function.
            I.e. if this would compile:
                T a, b;
                swap(a,b);
            Then is_swappable<T>::value == true.
        !*/
        template<class T>
        using is_swappable = std::integral_constant<bool, swappable_details::swap_traits<T>::is_swappable>;

    // ----------------------------------------------------------------------------------------

        /*!A is_nothrow_swappable

            A type trait telling you if T can be swapped by a global swap() function that is declared noexcept
            then is_nothrow_swappable<T>::value == true.
        !*/
        template<class T>
        using is_nothrow_swappable = std::integral_constant<bool, swappable_details::swap_traits<T>::is_nothrow>;

    // ----------------------------------------------------------------------------------------

        /*!A are_nothrow_swappable

            A type trait telling you if a list of types are all no-throw swappable.
        !*/
        template <typename ...Types>
        struct are_nothrow_swappable : And<is_nothrow_swappable<Types>::value...> {};

    // ----------------------------------------------------------------------------------------

        /*!A size_

            This is just a shorthand for making std::integral_constant of type size_t.
        !*/
        template<std::size_t I>
        using size_ = std::integral_constant<std::size_t, I>;

    // ----------------------------------------------------------------------------------------

        /*!A is_convertible

            This is a template that can be used to determine if one type is convertible
            into another type.

            For example:
                is_convertible<int,float>::value == true    // because ints are convertible to floats
                is_convertible<int*,float>::value == false  // because int pointers are NOT convertible to floats
        !*/
        template <typename from, typename to>
        using is_convertible = std::is_convertible<from, to>;

    // ----------------------------------------------------------------------------------------

        namespace details
        {
            template<class T, class AlwaysVoid = void>
            struct is_complete_type_impl : std::false_type{};

            template<class T>
            struct is_complete_type_impl<T, void_t<decltype(sizeof(T))>> : std::true_type{};
        }

        /*!A is_complete_type

            This is a template that can be used to determine if a type is a complete type.
            I.e. if T is a complete type then is_complete_type<T>::value == true.
        !*/
        template<class T>
        using is_complete_type = details::is_complete_type_impl<T>;

    // ----------------------------------------------------------------------------------------

        namespace details
        {
            template<typename Void, template <class...> class Op, class... Args>
            struct is_detected_impl : std::false_type{};

            template<template <class...> class Op, class... Args>
            struct is_detected_impl<dlib::void_t<Op<Args...>>, Op, Args...> : std::true_type {};
        }

        /*!A is_detected

           This is exactly the same as std::experimental::is_detected from library fundamentals v.

           It is a convenient way to test if the Args types satisfy some property, like having a certain
           member function.  For example, say you wanted to know if a type had a .size() method.  You
           could define:
              template<typename T>
              using has_a_size_member_function = decltype(std::declval<T>().size());
           And then
              is_detected<has_a_size_member_function, int>::value == false
              is_detected<has_a_size_member_function, std::string>::value == true
        !*/
        template<template <class...> class Op, class... Args>
        using is_detected = details::is_detected_impl<void, Op, Args...>;

    // ----------------------------------------------------------------------------------------

        template<typename... T>
        struct types_ {};
        /*!
            WHAT THIS OBJECT REPRESENTS
                This is a type list. You can use this for general-purpose meta-programming
                and it's used to pass types to the switch_() function.
        !*/

    // ----------------------------------------------------------------------------------------

        namespace details
        {
    #if defined(__has_builtin)
    #if __has_builtin(__type_pack_element)
        #define HAS_TYPE_PACK_ELEMENT 1
    #endif
    #endif

    #if HAS_TYPE_PACK_ELEMENT
            template<std::size_t I, class... Ts>
            struct nth_type_impl { using type = __type_pack_element<I,Ts...>; };
    #else
            template<std::size_t I, class... Ts>
            struct nth_type_impl;

            template<std::size_t I, class T0, class... Ts>
            struct nth_type_impl<I, T0, Ts...> { using type = typename nth_type_impl<I-1, Ts...>::type; };

            template<class T0, class... Ts>
            struct nth_type_impl<0, T0, Ts...> { using type = T0; };
    #endif
        }

        template<std::size_t I, class... Ts>
        struct nth_type;
        /*!
            WHAT THIS OBJECT REPRESENTS
                This is a type trait for getting the n'th argument of a parameter pack.
                In particular, nth_type<n, some_types...>::type is the nth type in some_types.
        !*/

        template<std::size_t I, class... Ts>
        struct nth_type<I, types_<Ts...>> : details::nth_type_impl<I,Ts...> {};

        template<std::size_t I, class... Ts>
        struct nth_type : details::nth_type_impl<I,Ts...> {};

        template<std::size_t I, class... Ts>
        using nth_type_t = typename nth_type<I,Ts...>::type;

    // ----------------------------------------------------------------------------------------

        namespace details
        {
            template<class AlwaysVoid, class F>
            struct callable_traits_impl
            {
                constexpr static bool is_callable = false;
            };

            template<class AlwaysVoid, class R, class... Args>
            struct callable_traits_impl<AlwaysVoid, R(Args...)>
            {
                using return_type = R;
                using args        = types_<Args...>;
                constexpr static std::size_t    nargs       = sizeof...(Args);
                constexpr static bool           is_callable = true;
            };

            template<class AlwaysVoid, class R, class... Args>
            struct callable_traits_impl<AlwaysVoid, R(*)(Args...)>
            : public callable_traits_impl<AlwaysVoid, R(Args...)>{};

            template<class AlwaysVoid, class C, class R, class... Args>
            struct callable_traits_impl<AlwaysVoid, R(C::*)(Args...)>
            : public callable_traits_impl<AlwaysVoid, R(Args...)>{};

            template<class AlwaysVoid, class C, class R, class... Args>
            struct callable_traits_impl<AlwaysVoid, R(C::*)(Args...) const>
            : public callable_traits_impl<AlwaysVoid, R(Args...)>{};

            template<class F>
            struct callable_traits_impl<void_t<decltype(&std::decay_t<F>::operator())>, F>
            : public callable_traits_impl<void, decltype(&std::decay_t<F>::operator())>{};
        }

        template<class F>
        struct callable_traits : details::callable_traits_impl<void, F> {};
        /*!
            WHAT THIS OBJECT REPRESENTS
                This is a type trait for callable types.

                If the template parameter F is function pointer, functor or lambda then
                it provides the following types:
                    return_type : the return type of the callable object
                    args        : a parameter pack packaged in a types_<> meta container containing
                                  all the function argument types
                It also provides the following static members:
                    nargs       : the number of function arguments
                    is_callable : a boolean which determines whether F is callable. In this case, it is true

                If the template parameter F is not function-like object, then it provides:
                    is_callable : false

                For example, a function type F with signature R(T1, T2, T3) has the following traits:
                    callable_traits<F>::return_type == R
                    callable_traits<F>::args        == types_<T1,T2,T3>
                    callable_traits<F>::nargs       == 3
                    callable_traits<F>::is_callable == true

                Another example:
                    callable_traits<int>::is_callable == false
                    callable_traits<int>::return_type == does not exist. Compile error
                    callable_traits<int>::args        == does not exist. Compile error
                    callable_traits<int>::nargs       == does not exist. Compile error
        !*/

        template<class Callable>
        using callable_args = typename callable_traits<Callable>::args;

        template<std::size_t I, class Callable>
        using callable_arg = nth_type_t<I, callable_args<Callable>>;

        template<class Callable>
        using callable_nargs = std::integral_constant<std::size_t, callable_traits<Callable>::nargs>;

        template<class Callable>
        using callable_return = typename callable_traits<Callable>::return_type;

        template<class F>
        using is_callable = std::integral_constant<bool, callable_traits<F>::is_callable>;

    // ----------------------------------------------------------------------------------------

    }

    #endif //DLIB_TYPE_TRAITS_H_