C++ 0X学习：Lambda
C++0x 终于支持 Lambda 了，以后使用STL算法的时候，不必那么麻烦编写函数对象了。
C++0x 的语法还是比较简单。下面通过几个例子来介绍下。
先是通过 std::for_each 演示一个简单的例子：
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> my_vector({1, 2, 3, 4 ,5});
std::for_each(my_vector.begin(), my_vector.end(),
[](int value) { std::cout << value << std::endl; });
return 0;
}
首先，是不是发现 my_vector 的构造函数怪怪的呢。因为C++0x支持一种从C带来的初始化列表概念，
语法类似于 C 的 int array[] = {1, 2, 3, 4 ,5}。关于这个详细介绍，有机会下次再介绍
。
1.Lambda 的格式
对于这里的 lambda：[](int value) { std::cout << value << std::endl; }
从这里看出，lambda 是以 [] 开头的匿名函数。这个lambda 仅仅把参数打印出来，是一个 void 的函数。
对于需要返回值的 lambad，格式如下：[](int a, int b) { return a + b;}
在这里，返回值的类型是 decltype(a + b)，decltype是一个新关键词，用于在编译期推导出类型。
如果想强制设定返回值类型，可以这样编写：[](int a, int b) -> int { return a + b;}
这里把返回值设置为 int 型。
2. [] 的表示方式
C++ 的 lambad 是支持闭包的。那么，是通过 [] 来表示 lambda 的变量集合，即闭包。
[] //没有定义变量，使用未定义的变量会导致错误。 [a, &b] //a以传值方式传入，b以引用方式传入 [&] // 表示在 lambda所使用到的变量，皆以引用方式传入 [=] // 表示在 lambda所使用到的变量，皆以传值方式传入 [&, a] //表示 a 以传值方式传入，其他都是以引用方式传入 [=, &a] // 表示 a 以引用方式传入，其他都是以传值方式传入 如果企图修改以值传入的变量，则编译的时候会报错(gcc 4.5)
下面来看看一个闭包的lambda例子：
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<int> my_vector({1, 2, 3, 4 ,5});
int total = 0;
std::for_each(my_vector.begin(), my_vector.end(),
[&total](int value) {
total++;
});
std::cout << "total:" << total << std::endl;
return 0;
}
这里的输出结果是： total:5
3.涉及 this 指针的使用
在成员函数中涉及到对象的 this 指针，必须显式地传入lambda函数，否则成员函数的lambda函数无法使用任何对象的变量或者函数，格式如下：
[this]() { this->call_function(); }
2018年5月22日 14:32
可以这样编写：[](int a, int b) -> int { return a + b;}
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