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AFunctionObject type is the type of an object that can be used on the left of the function call operator.
Contents |
The typeT satisfiesFunctionObject if
T satisfiesstd::is_object, andGiven
f, a value of typeT orconst T,args, suitable argument list, which may be empty.The following expressions must be valid:
| Expression | Requirements |
|---|---|
| f(args) | performs a function call |
Functions and references to functions are not function object types, but can be used where function object types are expected due to function-to-pointerimplicit conversion.
Demonstrates different types of function objects.
#include <functional>#include <iostream> void foo(int x){std::cout<<"foo("<< x<<")\n";}void bar(int x){std::cout<<"bar("<< x<<")\n";} int main(){void(*fp)(int)= foo; fp(1);// calls foo using the pointer to function std::invoke(fp,2);// all FunctionObject types are Callable auto fn=std::function(foo);// see also the rest of <functional> fn(3); fn.operator()(3);// the same effect as fn(3) struct S{void operator()(int x)const{std::cout<<"S::operator("<< x<<")\n";}} s; s(4);// calls s.operator() s.operator()(4);// the same as s(4) auto lam=[](int x){std::cout<<"lambda("<< x<<")\n";}; lam(5);// calls the lambda lam.operator()(5);// the same as lam(5) struct T{using FP=void(*)(int); operator FP()const{return bar;}} t; t(6);// t is converted to a function pointerstatic_cast<void(*)(int)>(t)(6);// the same as t(6) t.operator T::FP()(6);// the same as t(6)}
Output:
foo(1)foo(2)foo(3)foo(3)S::operator(4)S::operator(4)lambda(5)lambda(5)bar(6)bar(6)bar(6)
| a type for which the invoke operation is defined (named requirement) |
