使用std::function來實現不同類的函數回調

在開發軟件過程中，經常會遇到這樣的需求，設計一個類Call來進行通用的邏輯處理，但是需要調用另外一個類A，或B中一些函數，這時就不能直接傳送類A或類B的指針進來了，如果在以往一般採用靜態函數，或者類A和類B是繼承關係採用多態來實現。目前可以採用std::function來實現函數對象的調用，也可以實現多態的方式。如下面的例子：

// ConsoleBind.cpp : This file contains the 'main' function. Program execution begins and ends there.
//

#include <iostream>
#include <functional>

using Callback_t = std::function<void(int, int)>;

class Call
{
public:
	void print(Callback_t fun)
	{
		fun(1, 2);
	}
};
class A
{
public:
	void print(int x, int y)
	{
		std::cout << "A:" << x << "," << y << std::endl;
	}

	void run(void) {
		Callback_t fun = std::bind(&A::print, this, std::placeholders::_1, std::placeholders::_2);
		m_call.print(fun);
	}

	Call m_call;
};
class B
{
public:
	void print(int x, int y)
	{
		std::cout << "B:" <<  x << "," << y << std::endl;
	}

	void run(void) {
		Callback_t fun = std::bind(&B::print, this, std::placeholders::_1, std::placeholders::_2);
		m_call.print(fun);
	}

	Call m_call;
};



int main()
{
    std::cout << "Hello World!\n";

	A a;
	B b;
	a.run();
	b.run();
}

// Run program: Ctrl + F5 or Debug > Start Without Debugging menu
// Debug program: F5 or Debug > Start Debugging menu

// Tips for Getting Started: 
//   1. Use the Solution Explorer window to add/manage files
//   2. Use the Team Explorer window to connect to source control
//   3. Use the Output window to see build output and other messages
//   4. Use the Error List window to view errors
//   5. Go to Project > Add New Item to create new code files, or Project > Add Existing Item to add existing code files to the project
//   6. In the future, to open this project again, go to File > Open > Project and select the .sln file

在這個例子裏，使用using來定義一個函數對象聲明類型，然後用std::bind動態地構造一個函數對象，這個對象就可以傳遞給調用類了。就可以實現組合方式調用的多態功能。

輸出如下：

Hello World!
A:1,2
B:1,2