[C++知识库]闭包的实现

方法一、仿函数

#include "stdafx.h"
#include <iostream>

using namespace std;

//仿函数，
class myFunctor
{
public:
	
	myFunctor(int i):r(i){}//构造函数
	//重载operator()
	int operator() (int tmp)
	{
		return tmp + r;
	}
private:
	int r;
};


int main()
{
	myFunctor obj(10);//调用构造函数
	std::cout << obj(1) << std::endl;//调用仿函数
	system("pause");
	return 0;
}

编译输出

11
请按任意键继续. . .

方法二、std::bind绑定器

#include "stdafx.h"
#include <iostream>
#include <functional> //std::function

using namespace std;
using namespace placeholders;

//普通函数
void func()
{
	cout << "func  " << endl;
}
//类
class Test
{
public:
	//静态函数
	static int funcStatic(int tmp)
	{
		cout << "Test::funcStatic  ";
		return tmp;
	}

	//普通成员函数
	void func(int x, int y)
	{
		cout << "Test::fun  " << x << " " << y << endl;
	}

	int a;

};

//仿函数
class MyFunctor
{
public:

	MyFunctor() {}//构造函数
				  //重载operator()
	int operator() (int tmp)
	{
		cout << "myFunctor::operator  ";
		return tmp;
	}

};


int main()
{
	//1、绑定普通函数
	function<void(void)> f1 = func;
	f1();

	//2、绑定类中的静态函数
	function<int(int)> f2 = Test::funcStatic;
	cout << f2(1) << endl;

	//3、绑定类中的仿函数，绑定对象
	MyFunctor obj;
	function<int(int)> f3 = obj;
	cout << f3(2) << endl;

	//4、绑定类中成员函数
	Test testObj;
	function<void(int, int)> f4 = bind(&Test::func, &testObj, _1, _2);
	f4(1, 2); //等价于，testObj.func(11,22)

	//5、绑定成员变量
	function<int &()> f5 = bind(&Test::a, &testObj);
	f5() = 11;//等价于testObj.a = 11
	cout << "Test.a=" << testObj.a << endl;

	system("pause");
	return 0;
}

编译输出

func
Test::funcStatic  1
myFunctor::operator  2
Test::fun  1 2
Test.a=11
请按任意键继续. . .

#include "stdafx.h"
#include <iostream>
#include <functional> //std::bind

using namespace std;

//普通函数
void func(int x,int y)
{
	cout << "func  "<<x << " "<<y << endl;
}


int main()
{
	//下面三种形式都作用都是一样的
	cout << "-----------------------------" << endl;
	bind(func, 11, 22)();

	cout << "-----------------------------" << endl;
	//使用占位符
	//std::placeholders::_1,函数调用时，被第一个参数替换
	//std::placeholders::_2，函数调用时，被第二个参数替换
	bind(func, std::placeholders::_1, std::placeholders::_2)(11, 22);

	cout << "-----------------------------" << endl;
	//引入命名空间
	using namespace std::placeholders;
	bind(func, _1, _2)(11, 22);
	
	system("pause");
	return 0;
}

编译输出

-----------------------------
func  11 22
-----------------------------
func  11 22
-----------------------------
func  11 22
请按任意键继续. . .

方法三 lambda表达式

用于定义并创建匿名的函数的对象，以简化编程工作。

语法形式

[函数对象参数] （操作符重载函数参数）mutable或exception声明 ->返回值类型{函数体}

[] (int x, int y) -> int { int z = x + y; return z; }

#include "stdafx.h"
#include <iostream>
#include <functional> //std::function

using namespace std;
using namespace placeholders;


int g_tmp = 1;
class MyClass
{
public:
	MyClass();
	~MyClass();
	int i = 0;
	void func()
	{
		auto f1 = [=]() {
			cout << i << endl;
			cout << g_tmp << endl;
		};
		auto f2 = [&]() {
			cout << i << endl;
		};

		//只是捕获成员变量，全局变量
		auto f3 = [this]() {
			cout << i << endl;
			cout << g_tmp << endl;
		};
	}
};

MyClass::MyClass()
{
}

MyClass::~MyClass()
{
}


int main()
{
	int a = 0;
	int b = 0;
	auto f1 = []() {};
	auto f2 = [a, b]() {cout << a << "," << b << endl; };
	auto f3 = [a, b](int x, int y) {
		cout << a << "," << b << endl;
		cout << "x=" << x << endl;
		cout << "y=" << y << endl;
	};
	f3(10, 20);

	//以值传递方式传给lambda表达式
	auto f4 = [=] {cout << a << "," << b << endl; };

	//以引用方式捕获外部变量
	auto f5 = [&] {cout << a << "," << b << endl; };
	
	//a以值传递，其它以引用方式传递
	auto f6 = [&,a]{cout << a << "," << b << endl; };

	//a以引用传递，其它以值方式传递
	auto f7 = [=, &a] {cout << a << "," << b << endl; };

	//默认情况下，lambda函数，以const修饰函数体，值传递无法修改，想修改加mutable
	//值传递是，lambda新建一个变量，外部变量给这个变量赋值，
	auto f8 = [=]()mutable
	{
		a++;
		{cout << a << "," << b << endl; };
	};
	f8();
	cout << a << endl;

	//引用传递
	auto f9 = [&]()mutable
	{
		a++;
		{cout << a << "," << b << endl; };
	};
	f9();
	cout << a << endl;
	cout << "..........................." << endl;
	//lambda表达式有返回值
	function<int(int)> func1= [](int a) { return a + 1; };
	cout << "func1(10)=" << func1(10) << endl;

	function<int(int)> func2 = bind([](int a) {return a + 1; }, placeholders::_1);
	cout << "func2(10)=" << func2(10) << endl;

	
	auto func3 = [](int x, int y) -> int { return x + y; };
	
	//没有引用外部变量的lambda表达式可以转换为函数指针，反之任何形式的函数指针都不能转换为lambda表达式
	//定义一个函数指针
	typedef int(*PFUNC)(int, int);
	PFUNC p1 = func3;//将func3转换为函数指针
	cout <<"测试函数指针："<< p1(10, 10) << endl;
	cout << "..........................." << endl;

	system("pause");
	return 0;
}

编译输出：

0,0
x=10
y=20
1,0
0
1,0
1
...........................
func1(10)=11
func2(10)=11
测试函数指针：20
...........................
请按任意键继续. . .

番外：

lamda的优势

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;


//选择大于5的证书存入容器，并打印
vector<int> numList;
vector<int> largeNumList;



int tmp = 5;
void FunFind(int n)
{
	if ( n > tmp )
	{
		largeNumList.push_back(n);
	}
}

void FunPrint(int n)
{
	cout << n << " ";
}

int main()
{
	for (int i =0; i <=10; i++)
	{
		numList.push_back(i);
	}
	
	cout << "---------------------------------" << endl;
	for (auto p1 = numList.begin();p1 != numList.end(); p1++)
	{
		if (*p1 > tmp)
		{
			largeNumList.push_back(*p1);
		}
	}

	for (auto p1 = largeNumList.begin(); p1 !=largeNumList.end(); p1++)
	{
		cout << *p1 << " ";
	}
	cout << endl;

	largeNumList.clear();
	cout << "---------------------------------" << endl;
	//传统回调函数的方式
	for_each(numList.begin(), numList.end(), FunFind);
	for_each(largeNumList.begin(), largeNumList.end(), FunPrint);
	cout << endl;
	largeNumList.clear();
	cout << "---------------------------------" << endl;
	//lambda表达式的方式
	for_each(numList.begin(), numList.end(), 
		[](int n) {
		if (n > tmp)
		{
			largeNumList.push_back(n);
		}
	});

	for_each(largeNumList.begin(), largeNumList.end(), 
		[](int n) {
		cout << n << " ";
	});
	cout << endl;
	system("pause");
	return 0;
}