C++ 11 —— lambda表达式（匿名函数）

[capture](parameters) -> return_type { /* ... */ }

[]        //no variables defined. Attempting to use any external variables in the lambda is an error.
[x, &y]   //x is captured by value, y is captured by reference
[&]       //any external variable is implicitly captured by reference if used
[=]       //any external variable is implicitly captured by value if used
[&, x]    //x is explicitly captured by value. Other variables will be captured by reference
[=, &z]   //z is explicitly captured by reference. Other variables will be captured by value

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

#include <iostream>
#include <vector>
#include <string>
#include <functional> /** function */
#include <algorithm> /** for_each */

/** 建立一个类似创建线程函数的用法的函数来完成执行匿名函数 */
int my_eval(std::function <int(int, int)> f, int x, int y) {
	return f(x, y);
}

int main() {
	/**用法1: 通过将匿名函数存储在变量中，并作为参数传递*/
	std::function<int(int, int)> f0 = [](int x, int y) -> int {return x + y; };//lambda 表达式
	auto                         f1 = [](int x, int y) -> int {return x * y; };//没有使用任何外部变量
	std::cout << "f0: " << my_eval(f0, 3, 4) << std::endl;
	std::cout << "f1: " << my_eval(f1, 2, 3) << std::endl;

	/**用法2: 保存到vector中*/
	std::vector<decltype(f0)> func_s{ f0, f1 };
	func_s.push_back([](int x, int y) {return x - y; });
	for (auto f : func_s)//遍历匿名函数
		std::cout << "f: " << f(3, 1) << std::endl;

	/**用法3: 使用for_each传入匿名函数*/
	std::vector<int> nums{ 1,2,3,4,5 };
	int sum = 0;

	//代码中使用了sum 因而通过[&]引用隐式捕获，如果代码没有使用任何外部变量，则不传递参数
	std::for_each(begin(nums), end(nums), [&](int x) {sum += x; });
	//x为遍历nums传递的值
	std::cout << "f3: " << sum << std::endl;

	//同上用法 但匿名函数参数有不同的地方
	std::string str{ "hello world" };
	std::string s = "";
	std::string c = " ";

	//指明外部变量s通过引用捕获，c通过传递值捕获
	std::for_each(begin(str), end(str), [&s, c](char x) {s += (x + c); });

	std::cout << "f4: " << s << std::endl;

	/**用法4: 保存到函数指针*/
	auto my_lambda_func = [](int x) {std::cout << "f5: " << x << std::endl; };
	void(*func_ptr)(int) = my_lambda_func;
	func_ptr(4);//回调

	return 0;
}