std::is_invocable, std::is_invocable_r, std::is_nothrow_invocable, std::is_nothrow_invocable_r

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Type support

Basic types
Fixed width integer types (C++11)
Fixed width floating-point types (C++23)
ptrdiff_t size_t max_align_t (C++11)     byte (C++17)	nullptr_t (C++11) offsetof NULL
Numeric limits
numeric_limits
C numeric limits interface
Runtime type information
type_info type_index (C++11)	bad_typeid bad_cast

 

Defined in header <type_traits>
template <class Fn, class... ArgTypes> struct is_invocable;	(1)	(since C++17)
template <class R, class Fn, class... ArgTypes> struct is_invocable_r;	(2)	(since C++17)
template <class Fn, class... ArgTypes> struct is_nothrow_invocable;	(3)	(since C++17)
template <class R, class Fn, class... ArgTypes> struct is_nothrow_invocable_r;	(4)	(since C++17)

1) Determines whether Fn can be invoked with the arguments ArgTypes.... Formally, determines whether INVOKE(declval<Fn>(), declval<ArgTypes>()...) is well formed when treated as an unevaluated operand, where INVOKE is the operation defined in Template:concept.

2) Determines whether Fn can be invoked with the arguments ArgTypes... to yield a result that is convertible to R. Formally, determines whether INVOKE<R>(declval<Fn>(), declval<ArgTypes>()...) is well formed when treated as an unevaluated operand, where INVOKE is the operation defined in Template:concept

3) Determines whether Fn is callable with the arguments ArgTypes... (same as (1)), and that such call is known not to throw any exceptions.

4) Determines whether Fn can be invoked with the arguments ArgTypes... to yield a result that is convertible to R (same as (2)), and that such call is known not to throw any exceptions.

If Fn, R or any type in the parameter pack ArgTypes is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

Helper variable templates

Defined in header <type_traits>
template <class Fn, class... ArgTypes> inline constexpr bool is_invocable_v = std::is_invocable<Fn, ArgTypes...>::value;	(1)	(since C++17)
template <class R, class Fn, class... ArgTypes> inline constexpr bool is_invocable_r_v = std::is_invocable_r<R, Fn, ArgTypes...>::value;	(2)	(since C++17)
template <class Fn, class... ArgTypes> inline constexpr bool is_nothrow_invocable_v = std::is_nothrow_invocable<Fn, ArgTypes...>::value;	(3)	(since C++17)
template <class R, class Fn, class... ArgTypes> inline constexpr bool is_nothrow_invocable_r_v = std::is_nothrow_invocable_r<R, Fn, ArgTypes...>::value;	(4)	(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

true if INVOKE<R>(declval<Fn>(), declval<ArgTypes>()...) is well formed when treated as an unevaluated operand , false otherwise (public static member constant)

Member functions

operator bool	converts the object to bool, returns value (public member function)
operator() (C++14)	returns value (public member function)

Member types

Type	Definition
value_type	bool
type	std::integral_constant<bool, value>

Possible implementation

Note: Possible implementation of INVOKE see std::invoke

namespace detail {
template <typename AlwaysVoid, typename, typename...>
struct is_invocable : std::false_type {};
 
template <typename F, typename... Args>
struct is_invocable<decltype(void(INVOKE(std::declval<F>(), 
		std::declval<Args>()...))), F, Args...> : std::true_type {};
 
template <bool cond, typename F, typename... Args>
struct is_nothrow_invocable : std::false_type {};
 
template <typename F, typename... Args>
struct is_nothrow_invocable<true, F, Args...> : std::integral_constant<bool, 
		noexcept(INVOKE(std::declval<F>(), std::declval<Args>()...))> {};
}  // namespace detail
 
template <typename F, typename... Args>
struct is_invocable : detail::is_invocable<void, F, Args...> {};
 
template <typename R, typename F, typename... Args>
struct is_invocable_r : std::integral_constant<bool, 
    is_invocable<F, Args...>::value && 
        std::is_convertible<invoke_result_t<F, Args...>, R>::value> {};
 
template <typename F, typename... Args>
struct is_nothrow_invocable : 
    detail::is_nothrow_invocable<is_invocable<F, Args...>::value, F, Args...> {};
 
template <typename R, typename F, typename... Args>
struct is_nothrow_invocable_r : std::integral_constant<bool, 
    is_nothrow_invocable<F, Args...>::value && 
        std::is_convertible<invoke_result_t<F, Args...>, R>::value> {};

Examples

Run this code

#include <type_traits>
#include <iostream>
 
class ClassType {};
 
auto func1() -> void
{
}
 
auto func2() -> int
{
    return 0;
}
 
auto func3(int a) -> void
{
}
 
auto func4(ClassType b) -> int
{
    return -1;
}
 
auto func5(ClassType b) -> int (*)()
{
    auto r = &func2;
    return r;
}
 
auto main() -> int
{
    static_assert( std::is_invocable<void ()>::value );
    static_assert( std::is_invocable<int ()>::value );
    static_assert( std::is_invocable_r<int, int ()>::value );
    static_assert( std::is_invocable_r<void, void (int), int>::value );
    static_assert( std::is_invocable_r<int(*)(), decltype(func5), ClassType>::value );
 
    return 0;
}

See also

invokeinvoke_r (C++17)(C++23)	invokes any Callable object with given arguments and possibility to specify return type(since C++23) (function template)
result_ofinvoke_result (C++11)(removed in C++20)(C++17)	deduces the result type of invoking a callable object with a set of arguments (class template)
declval (C++11)	obtains a reference to an object of the template type argument for use in an unevaluated context (function template)