C++中的lambda表达式简单介绍

auto add = [](int x, int y){ return x + y; };

auto func = []
	{
		cout << "hello world" << endl;
	};

auto swap = [](int& a, int& b)
	{
		int tmp = a;
		a = b;
		b = tmp;
	};

int x = 0;
// 捕捉列表必须为空，因为全局变量不⽤捕捉就可以⽤，没有可被捕捉的变量
auto func1 = []()
	{
		x++;
	};
int main()
{
	// 只能⽤当前lambda局部域和捕捉的对象和全局对象
	int a = 0, b = 1, c = 2, d = 3;
	auto func1 = [a, &b]
		{
			// 值捕捉的变量不能修改，引⽤捕捉的变量可以修改
			//a++;
			b++;
			int ret = a + b;
			return ret;
		};
	cout << func1() << endl;
	// 隐式值捕捉
	// ⽤了哪些变量就捕捉哪些变量
	auto func2 = [=]
		{
			int ret = a + b + c;
			return ret;
		};
	cout << func2() << endl;
	// 隐式引⽤捕捉
	// ⽤了哪些变量就捕捉哪些变量
	auto func3 = [&]
		{
			a++;
			c++;
			d++;
		};
	func3();
	cout << a << " " << b << " " << c << " " << d << endl;
	// 混合捕捉1
	auto func4 = [&, a, b]
		{
			//a++;
			//b++;
			c++;
			d++;
			return a + b + c + d;
		};
	func4();
	cout << a << " " << b << " " << c << " " << d << endl;
	// 混合捕捉1
	auto func5 = [=, &a, &b]
		{
			a++;
			b++;
			/*c++;
			d++;*/
			return a + b + c + d;
		};
		func5();
		cout << a << " " << b << " " << c << " " << d << endl;
	// 局部的静态和全局变量不能捕捉，也不需要捕捉
	static int m = 0;
	auto func6 = []
		{
			int ret = x + m;
			return ret;
		};
	// 传值捕捉本质是⼀种拷⻉,并且被const修饰了
	// mutable相当于去掉const属性，可以修改了
	// 但是修改了不会影响外⾯被捕捉的值，因为是⼀种拷⻉
	auto func7 = [=]()mutable
		{
			a++;
			b++;
			c++;
			d++;
			return a + b + c + d;
		};
	cout << func7() << endl;
	cout << a << " " << b << " " << c << " " << d << endl;
	return 0;
}

vector<pair<int, int>> v = { {1,2}, {3,4}, {8,3}, {5,1} };
//遇到这样的场景，假如我们想要用多种排序规则进行排序，以前是需要写不同的仿函数传给sort
/*struct Compare_first
{
	bool operator()(const pair<int, int>& p1, const pair<int, int>& p2)
	{
		return p1.first < p2.first;
	}
}; 
struct Compare_second
{
	bool operator()(const pair<int, int>& p1, const pair<int, int>& p2)
	{
		return p1.second < p2.second;
	}
};
sort(v.begin(), v.end(), Compare_first());
sort(v.begin(), v.end(), Compare_second());*/
//有了lambda表达式后，就方便许多了
sort(v.begin(), v.end(), [](const pair<int, int>& p1, const pair<int, int>& p2) {return p1.first < p2.first; });
for (auto pa : v)
{
	cout << pa.first << ":" << pa.second << " ";
}
cout << endl;
sort(v.begin(), v.end(), [](const pair<int, int>& p1, const pair<int, int>& p2) {return p1.second < p2.second; });
for (auto pa : v)
{
	cout << pa.first << ":" << pa.second << " ";
}
cout << endl;