C++ named requirements: FunctionObject

#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 pointer
 static_cast<void (*)(int)>(t)(6); // the same as t(6)
 t.operator T::FP()(6); // the same as t(6) 
}

foo(1)
foo(2)
foo(3)
foo(3)
S::operator(4)
S::operator(4)
lambda(5)
lambda(5)
bar(6)
bar(6)
bar(6)