std::invoke, std::invoke_r

 

C++

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(C++26) 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 Function objects Bit manipulation(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)

 

Function objects

Function wrappers function (C++11) move_only_function (C++23) copyable_function (C++26) function_ref (C++26) mem_fn (C++11) bad_function_call (C++11)	Partial function application bind_frontbind_back (C++20)(C++23) bind (C++11) is_bind_expression (C++11) is_placeholder (C++11) _1, _2, _3, ... (C++11)
Function invocation invokeinvoke_r (C++17)(C++23) Identity function object identity (C++20)	Reference wrappers reference_wrapper (C++11) refcref (C++11)(C++11) unwrap_referenceunwrap_ref_decay (C++20)(C++20)
Operator wrappers plus minus negate multiplies divides modulus bit_and bit_or bit_not (C++14)   bit_xor equal_to not_equal_to greater less greater_equal less_equal logical_and logical_or logical_not	Transparent operator wrappers plus<> (C++14) minus<> (C++14) negate<> (C++14) multiplies<> (C++14)   divides<> (C++14) modulus<> (C++14) bit_and<> (C++14) bit_or<> (C++14) bit_not<> (C++14) bit_xor<> (C++14) equal_to<> (C++14) not_equal_to<> (C++14) greater<> (C++14) less<> (C++14) greater_equal<> (C++14) less_equal<> (C++14) logical_and<> (C++14) logical_or<> (C++14) logical_not<> (C++14)
Negators not_fn (C++17) Searchers default_searcher (C++17) boyer_moore_searcher (C++17) boyer_moore_horspool_searcher (C++17)	Constrained comparators ranges::equal_to (C++20) ranges::not_equal_to (C++20) ranges::greater (C++20) ranges::less (C++20) ranges::greater_equal (C++20) ranges::less_equal (C++20) compare_three_way (C++20)
Old binders and adaptors
unary_function (until C++17*) binary_function (until C++17*) ptr_fun (until C++17*) pointer_to_unary_function (until C++17*) pointer_to_binary_function (until C++17*)   mem_fun (until C++17*) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) not1 (until C++20*) not2 (until C++20*)	binder1stbinder2nd (until C++17*)(until C++17*) bind1stbind2nd (until C++17*)(until C++17*) mem_fun_ref (until C++17*) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) unary_negate (until C++20*) binary_negate (until C++20*)

 

Defined in header <functional>
template<class F, class... Args> std::invoke_result_t<F, Args...>     invoke( F&& f, Args&&... args)noexcept(/* see below */);	(1)	(since C++17) (constexpr since C++20)
template<class R, class F, class... Args> constexpr R     invoke_r( F&& f, Args&&... args)noexcept(/* see below */);	(2)	(since C++23)

1) Invoke the Callable object f with the parameters args as by INVOKE(std::forward<F>(f), std::forward<Args>(args)...). This overload participates in overload resolution only if std::is_invocable_v<F, Args...> is true.

2) Invoke the Callable object f with the parameters args as by INVOKE<R>(std::forward<F>(f), std::forward<Args>(args)...). This overload participates in overload resolution only if std::is_invocable_r_v<R, F, Args...> is true.

Parameters

f	-	Callable object to be invoked
args	-	arguments to pass to f

Return value

1) The value returned by f.

2) The value returned by f, implicitly converted to R, if R is not (possibly cv-qualified) void. None otherwise.

Exceptions

1)

noexcept specification:  

noexcept(std::is_nothrow_invocable_v<F, Args...>)

2)

noexcept specification:  

noexcept(std::is_nothrow_invocable_r_v<R, F, Args...>)

Possible implementation

invoke (1)
namespace detail {template<class>constexprbool is_reference_wrapper_v =false;template<class U>constexprbool is_reference_wrapper_v<std::reference_wrapper<U>>=true;   template<class T>using remove_cvref_t =std::remove_cv_t<std::remove_reference_t<T>>;   template<class C, class Pointed, class Object, class... Args>constexpr decltype(auto) invoke_memptr(Pointed C::* member, Object&& object, Args&&... args){using object_t = remove_cvref_t<Object>;constexprbool is_member_function =std::is_function_v<Pointed>;constexprbool is_wrapped = is_reference_wrapper_v<object_t>;constexprbool is_derived_object =std::is_same_v<C, object_t>||std::is_base_of_v<C, object_t>;   ifconstexpr(is_member_function){ifconstexpr(is_derived_object)return(std::forward<Object>(object) .* member)(std::forward<Args>(args)...);elseifconstexpr(is_wrapped)return(object.get() .* member)(std::forward<Args>(args)...);elsereturn((*std::forward<Object>(object)) .* member)(std::forward<Args>(args)...);}else{ static_assert(std::is_object_v<Pointed>&& sizeof...(args)==0);ifconstexpr(is_derived_object)returnstd::forward<Object>(object) .* member;elseifconstexpr(is_wrapped)return object.get() .* member;elsereturn(*std::forward<Object>(object)) .* member;}}}// namespace detail   template<class F, class... Args>constexprstd::invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)noexcept(std::is_nothrow_invocable_v<F, Args...>){ifconstexpr(std::is_member_pointer_v<detail::remove_cvref_t<F>>)return detail::invoke_memptr(f, std::forward<Args>(args)...);elsereturnstd::forward<F>(f)(std::forward<Args>(args)...);}
invoke_r (2)
template<class R, class F, class... Args> requires std::is_invocable_r_v<R, F, Args...>constexpr R invoke_r(F&& f, Args&&... args)noexcept(std::is_nothrow_invocable_r_v<R, F, Args...>){ifconstexpr(std::is_void_v<R>) std::invoke(std::forward<F>(f), std::forward<Args>(args)...);elsereturn std::invoke(std::forward<F>(f), std::forward<Args>(args)...);}

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_invoke	201411L	(C++17)	std::invoke, (1)
__cpp_lib_invoke_r	202106L	(C++23)	std::invoke_r, (2)

Example

Run this code

#include <functional>#include <iostream>#include <type_traits>   struct Foo { Foo(int num): num_(num){}void print_add(int i)const{std::cout<< num_ + i <<'\n';}int num_;};   void print_num(int i){std::cout<< i <<'\n';}   struct PrintNum {void operator()(int i)const{std::cout<< i <<'\n';}};   int main(){std::cout<<"invoke a free function: "; std::invoke(print_num, -9);   std::cout<<"invoke a lambda: "; std::invoke([](){ print_num(42);});   std::cout<<"invoke a member function: ";const Foo foo(314159); std::invoke(&Foo::print_add, foo, 1);   std::cout<<"invoke (i.e., access) a data member num_: "<< std::invoke(&Foo::num_, foo)<<'\n';   std::cout<<"invoke a function object: "; std::invoke(PrintNum(), 18);   #if defined(__cpp_lib_invoke_r)auto add =[](int x, int y){return x + y;};std::cout<<"invoke a lambda converting result to float: ";auto ret = std::invoke_r<float>(add, 11, 22); static_assert(std::is_same<decltype(ret), float>());std::cout<<std::fixed<< ret <<"\ninvoke print_num: "; std::invoke_r<void>(print_num, 44);#endif}

Possible output:

invoke a free function: -9 invoke a lambda: 42 invoke a member function: 314160 invoke (i.e., access) a data member num_: 314159 invoke a function object: 18 invoke a lambda converting result to float: 33.000000 invoke print_num: 44

See also

mem_fn (C++11)	creates a function object out of a pointer to a member (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)
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)
apply (C++17)	calls a function with a tuple of arguments (function template)