#include <functional>#include <iostream> 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(){// store a free function    std::function<void(int)> f_display= print_num;    f_display(-9); // store a lambda    std::function<void()> f_display_42=[](){ print_num(42);};    f_display_42(); // store the result of a call to std::bind    std::function<void()> f_display_31337=std::bind(print_num,31337);    f_display_31337(); // store a call to a member function    std::function<void(const Foo&,int)> f_add_display=&Foo::print_add;const Foo foo(314159);    f_add_display(foo,1);    f_add_display(314159,1); // store a call to a data member accessor    std::function<int(Fooconst&)> f_num=&Foo::num_;std::cout<<"num_: "<< f_num(foo)<<'\n'; // store a call to a member function and objectusing std::placeholders::_1;    std::function<void(int)> f_add_display2=std::bind(&Foo::print_add, foo, _1);    f_add_display2(2); // store a call to a member function and object ptr    std::function<void(int)> f_add_display3=std::bind(&Foo::print_add,&foo, _1);    f_add_display3(3); // store a call to a function object    std::function<void(int)> f_display_obj= PrintNum();    f_display_obj(18); auto factorial=[](int n){// store a lambda object to emulate "recursive lambda"; aware of extra overhead        std::function<int(int)> fac=[&](int n){return(n<2)?1: n* fac(n-1);};// note that "auto fac = [&](int n) {...};" does not work in recursive callsreturn fac(n);};for(int i{5}; i!=8;++i)std::cout<< i<<"! = "<< factorial(i)<<";  ";std::cout<<'\n';}

-94231337314160314160num_: 314159314161314162185! = 120;  6! = 720;  7! = 5040;