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C++ functional argument unwrap traits. Extracts argument and return types of functions, std:.function and std::tuple's
* Copyright (C) 2015 Naios <>
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* GNU General Public License for more details.
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <>.
* The functional unwrap trait provides the possibility to extract
* various types out of c++ functional types, like std::function, functors and more.
#include <functional>
#include <type_traits>
namespace fu
/// Identity class which is used to carry parameter packs.
template<typename... Args>
struct identity { };
namespace detail
template<typename Function>
struct unwrap_function_impl;
template<typename _RTy, typename... _ATy>
struct unwrap_function_impl<_RTy(_ATy...)>
/// The return type of the function.
typedef _RTy return_type;
/// The argument types of the function as pack in fu::identity.
typedef identity<_ATy...> argument_type;
/// The function provided as std::function
typedef std::function<_RTy(_ATy...)> function_type;
/// The function provided as function_ptr
typedef _RTy(*function_ptr)(_ATy...);
/// STL: std::function
template<typename _RTy, typename... _ATy>
struct unwrap_function_impl<std::function<_RTy(_ATy...)>>
: unwrap_function_impl<_RTy(_ATy...)> { };
/// STL: std::tuple
template<typename _RTy, typename... _ATy>
struct unwrap_function_impl<std::tuple<_RTy, _ATy...>>
: unwrap_function_impl<_RTy(_ATy...)> { };
/// Function pointers
template<typename _RTy, typename... _ATy>
struct unwrap_function_impl<_RTy(*const)(_ATy...)>
: unwrap_function_impl<_RTy(_ATy...)> { };
/// Class Method pointers
template<typename _CTy, typename _RTy, typename... _ATy>
struct unwrap_function_impl<_RTy(_CTy::*)(_ATy...) const>
: unwrap_function_impl<_RTy(_ATy...)> { };
/// Pack in fu::identity
template<typename _RTy, typename... _ATy>
struct unwrap_function_impl<identity<_RTy, _ATy...>>
: unwrap_function_impl<_RTy(_ATy...)> { };
/// Unwrap through pointer of functor.
template<typename Function>
static auto select_best_unwrap_unary_arg(int)
-> unwrap_function_impl<decltype(&Function::operator())>;
/// Unwrap through plain type.
template<typename Function>
static auto select_best_unwrap_unary_arg(long)
-> unwrap_function_impl<Function>;
template<typename... _FTy>
struct select_best_unwrap;
/// Enable only if 1 template argument is given.
template<typename _FTy>
struct select_best_unwrap<_FTy>
typedef decltype(select_best_unwrap_unary_arg<_FTy>(0)) type;
// Enable if more then 1 template argument is given.
// (Handles lazy typing)
template<typename _RTy, typename... _ATy>
struct select_best_unwrap<_RTy, _ATy...>
typedef unwrap_function_impl<_RTy(_ATy...)> type;
template <typename>
struct to_true
: std::true_type { };
/// std::true_type if unwrappable
template<typename... Function>
static auto test_unwrappable(int)
-> to_true<typename select_best_unwrap<Function...>::type::function_type>;
/// std::false_type if not unwrappable
template<typename... Function>
static auto test_unwrappable(long)
-> std::false_type;
} // detail
/// Trait to unwrap function parameters of various types:
/// Function style definition: Result(Parameters...)
/// STL `std::function` : std::function<Result(Parameters...)>
/// STL `std::tuple` : std::tuple<Result, Parameters...>
/// C++ Function pointers: `Result(*)(Parameters...)`
/// C++ Class method pointers: `Result(Class::*)(Parameters...)`
/// Lazy typed signatures: `Result, Parameters...`
/// Also provides optimized unwrap of functors and functional objects.
template<typename... Function>
using unwrap_function =
typename detail::select_best_unwrap<Function...>::type;
/// Trait which defines the return type of the function.
template<typename... Function>
using return_type_of_t =
typename detail::select_best_unwrap<Function...>::type::return_type;
/// Trait which defines the argument types of the function packed in std::tuple.
template<typename... Function>
using argument_type_of_t =
typename detail::select_best_unwrap<Function...>::type::argument_type;
/// Trait which defines the std::function type of the function.
template<typename... Function>
using function_type_of_t =
typename detail::select_best_unwrap<Function...>::type::function_type;
/// Trait which defines the function pointer type of the function.
template<typename... Function>
using function_ptr_of_t =
typename detail::select_best_unwrap<Function...>::type::function_ptr;
/// Trait which defines if the given function is unwrappable or not.
template<typename... Function>
struct is_unwrappable
: decltype(detail::test_unwrappable<Function...>(0)) { };
} // fu
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