std::is_function

 

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Type traits
Type categories is_void (C++11) is_null_pointer (C++11)(DR*) is_array (C++11) is_pointer (C++11) is_enum (C++11) is_union (C++11) is_class (C++11) is_function (C++11) is_reference (C++11)  is_object (C++11) is_scalar (C++11) is_compound (C++11) is_integral (C++11) is_floating_point (C++11)   is_fundamental (C++11) is_arithmetic (C++11) is_lvalue_reference (C++11) is_rvalue_reference (C++11) is_member_pointer (C++11) is_member_object_pointer (C++11) is_member_function_pointer (C++11) Type properties is_const (C++11) is_volatile (C++11) is_empty (C++11) is_polymorphic (C++11) is_final (C++14) is_abstract (C++11) is_aggregate (C++17) is_implicit_lifetime (C++23) is_trivial (C++11)(deprecated in C++26) is_trivially_copyable (C++11) is_standard_layout (C++11) is_literal_type (C++11)(until C++20*) is_pod (C++11)(deprecated in C++20) is_signed (C++11) is_unsigned (C++11) is_bounded_array (C++20) is_unbounded_array (C++20) is_scoped_enum (C++23) has_unique_object_representations (C++17) Type trait constants integral_constantbool_constanttrue_typefalse_type (C++11)(C++17)(C++11)(C++11) Metafunctions conjunction (C++17) disjunction (C++17) negation (C++17)	Supported operations is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11) is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11) is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11) is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11) is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11) is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11) is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11) is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11) has_virtual_destructor (C++11) is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17) Relationships and property queries is_same (C++11) is_convertibleis_nothrow_convertible (C++11)(C++20) is_layout_compatible (C++20) is_pointer_interconvertible_base_of (C++20) is_pointer_interconvertible_with_class (C++20)   is_corresponding_member (C++20) reference_constructs_from_temporary (C++23) reference_converts_from_temporary (C++23) is_base_of (C++11) is_virtual_base_of (C++26) alignment_of (C++11) rank (C++11) extent (C++11) is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17) Type modifications remove_cvremove_constremove_volatile (C++11)(C++11)(C++11) add_cvadd_constadd_volatile (C++11)(C++11)(C++11) make_signed (C++11) make_unsigned (C++11) remove_reference (C++11) add_lvalue_referenceadd_rvalue_reference (C++11)(C++11) remove_pointer (C++11) add_pointer (C++11) remove_extent (C++11) remove_all_extents (C++11) Type transformations aligned_storage (C++11)(deprecated in C++23) aligned_union (C++11)(deprecated in C++23) decay (C++11) remove_cvref (C++20) result_ofinvoke_result (C++11)(until C++20*)(C++17) conditional (C++11) common_type (C++11) common_reference (C++20) underlying_type (C++11) type_identity (C++20) enable_if (C++11) void_t (C++17)
Compile-time rational arithmetic
Compile-time integer sequences
integer_sequence (C++14)

 

Defined in header <type_traits>
template<class T > struct is_function;		(since C++11)

std::is_function is a UnaryTypeTrait.

Checks whether T is a function type. Types like std::function, lambdas, classes with overloaded operator() and pointers to functions don't count as function types. Provides the member constant value which is equal to true, if T is a function type. Otherwise, value is equal to false.

If the program adds specializations for std::is_function or std::is_function_v, the behavior is undefined.

Template parameters

T

-

a type to check

Helper variable template

template<class T > constexprbool is_function_v = is_function<T>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

true if T is a function type, 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>

Notes

std::is_function can be implemented in much simpler ways. Implementations similar to the following one are used by new versions of libc++, libstdc++ and MS STL:

template<class T>struct is_function :std::integral_constant<bool, !std::is_const<const T>::value&&!std::is_reference<T>::value>{};

The implementation shown below is for pedagogical purposes, since it exhibits the myriad kinds of function types.

Possible implementation

// primary templatetemplate<class>struct is_function :std::false_type{};   // specialization for regular functionstemplate<class Ret, class... Args>struct is_function<Ret(Args...)>:std::true_type{};   // specialization for variadic functions such as std::printftemplate<class Ret, class... Args>struct is_function<Ret(Args......)>:std::true_type{};   // specialization for function types that have cv-qualifierstemplate<class Ret, class... Args>struct is_function<Ret(Args...)const>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)volatile>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)constvolatile>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)const>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)volatile>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)constvolatile>:std::true_type{};   // specialization for function types that have ref-qualifierstemplate<class Ret, class... Args>struct is_function<Ret(Args...)&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)const&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)volatile&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)constvolatile&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)const&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)volatile&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)constvolatile&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)&&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)const&&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)volatile&&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)constvolatile&&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)&&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)const&&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)volatile&&>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)constvolatile&&>:std::true_type{};   // specializations for noexcept versions of all the above (C++17 and later)template<class Ret, class... Args>struct is_function<Ret(Args...)noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)constnoexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)volatilenoexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)constvolatilenoexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)constnoexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)volatilenoexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)constvolatilenoexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)const&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)volatile&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)constvolatile&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)const&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)volatile&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)constvolatile&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)&&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)const&&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)volatile&&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args...)constvolatile&&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)&&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)const&&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)volatile&&noexcept>:std::true_type{};template<class Ret, class... Args>struct is_function<Ret(Args......)constvolatile&&noexcept>:std::true_type{};

Example

Run this code

#include <functional>#include <type_traits>   int f(); static_assert(std::is_function_v<decltype(f)>);   static_assert(std::is_function_v<int(int)>); static_assert(!std::is_function_v<int>); static_assert(!std::is_function_v<decltype([]{})>); static_assert(!std::is_function_v<std::function<void()>>);   struct O {void operator()(){}}; static_assert(std::is_function_v<O()>);   struct A {staticint foo();int fun()const&;}; static_assert(!std::is_function_v<A>); static_assert(std::is_function_v<decltype(A::foo)>); static_assert(!std::is_function_v<decltype(&A::fun)>);   template<typename>struct PM_traits {};template<class T, class U>struct PM_traits<U T::*>{using member_type = U;};   int main(){using T = PM_traits<decltype(&A::fun)>::member_type;// T is int() const& static_assert(std::is_function_v<T>);}

See also

is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)	checks if a type can be invoked (as if by std::invoke) with the given argument types (class template)
is_object (C++11)	checks if a type is an object type (class template)
is_class (C++11)	checks if a type is a non-union class type (class template)