std::is_function

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Standard library
Standard library headers
Named requirements
Feature test macros(C++20)
Language support library
Concepts library(C++20)
Diagnostics library
Memory management library
Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
Technical specifications
Symbols index
External libraries

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Metaprogramming library

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

(C++11)

(C++11)

(C++11)

(C++11)

(C++14)

(C++11)

(C++17)

(C++23)

(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)

(C++11)

(C++11)

(C++20)

(C++20)

(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)

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

std::is_function is aUnaryTypeTrait.

Checks whetherT is a function type. Types likestd::function, lambdas, classes with overloadedoperator() and pointers to functions don't count as function types. Provides the member constantvalue which is equal totrue, ifT is a function type. Otherwise,value is equal tofalse.

If the program adds specializations forstd::is_function orstd::is_function_v, the behavior is undefined.

[edit]Template parameters

a type to check

[edit]Helper variable template

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

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Inherited fromstd::integral_constant

Member constants

value

[static]

true ifT is a function type,false otherwise
(public static member constant)

Member functions

operator bool	converts the object tobool, returnsvalue (public member function)
operator() (C++14)	returnsvalue (public member function)

Member types

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

[edit]Notes

std::is_function can be implemented in much simpler ways. Implementations similar to the following one are used by new versions oflibc++,libstdc++ andMS 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.

[edit]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{};

[edit]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>);}

[edit]See also

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