Explanation and practice of std::function

1. Function pointers in C language

First look at an example test_fun.cpp of a function pointer:

#include<iostream>

//定义函数指针
typedef int (*func)();

using namespace std;

int test1(){
    cout<<"hello, test1"<<endl;
    return 0;
}

int test2(){
    cout<<"hello, test2"<<endl;
    return 0;
}

int main(int argc, char * argv[]){
    func fp = test1;
    fp();

    fp = test2;
    fp();

    return 0;
}

The execution results are as follows:

Two, the role of std::function

In fact, the function of std::function is similar to the function pointer above.

But std::function is more powerful than function pointers and has more application scenarios. std::function is a callable object wrapper, and a callable object satisfies one of the following conditions:

(1) Function pointer
(2) Class object with operator() member function (legendary functor), lambda expression
(3) Class object that can be converted into a function pointer
(4) Class member (function) pointer
( 5) bind expressions or other function objects

In actual programming, there are mainly the following scenarios:

(1) Bind a function (ordinary function or static function)
(2) Implement a callback function
(3) As a function input parameter

3. A few examples

1. Bind a function

#include <iostream>
#include <functional>

using namespace std;

int test1(){
    cout<<"hello, test1"<<endl;
    return 0;
}

int main(int argc, char * argv[]){
    std::function<int(void)> fr_test1 = test1;
	fr_test1();

    return 0;
}

2. As a callback function

#include <iostream>
#include <functional>

using namespace std;

class A{
    function<void()> callback;

public:
    A(){};
	
    A(const function<void()>& f) : callback(f) {};
	
	void notify(void){
		callback();
	}
	
	void init(const function<void()>& f){
	    callback = f;
	}
};

class Foo {
public:
    // Foo foo;
	// foo(); //此时会执行operator()方法
    void operator()(void) {
        cout << __FUNCTION__ << endl;
    }
};

void test(){
    cout << "test()" << endl;;
}

int main(void)
{
  Foo foo;
  
  A aa(foo);
  aa.notify();
  
  A bb;
  bb.init(test);
  bb.notify();
}

3. As a function input parameter

#include <iostream>
#include <functional>

using namespace std;

void call_when_even(int x, const std::function<void(int)>& f) {
    if (!(x & 1)) {
        f(x);
    }
}

void output(int x) {
    cout << x << " ";
}

int main(void){
    for (int i = 0; i < 10; ++i) {
        call_when_even(i, output);
    }
	
	cout<<endl;
	
    return 0;
}

reference:

(1) Explain the functions of C++ in simple terms

(2) std::function and lambda expressions of c++11 new features

Explanation and practice of std::function

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