STD ::function was added to the c++ library in c++11 with a header file of

usage explain
function f F is an empty function that stores callable objects that should be called in the same form as function type T
function f(nullptr) Explicitly construct an empty function
function f(obj) Store a copy of the callable object obj in f
f Condition f: true if there is a callable in f
f(args) Call the object in f with args

Personally, I understand that function can act as a function interface in a class and receive functions outside the class, which is conducive to code isolation.

Sample code:

#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'; }}; intmain()
{
    // 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(Foo const&) >f_num = &Foo: :num_;
    std::cout << "num_: " << f_num(foo) << '\n';
 
    // store a call to a member function and object
    using 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);
}
Copy the code

Program output:

-9
42
31337
314160
314160
num_: 314159
314161
314162
18
Copy the code