C++ Lambda 函数

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本文介绍C++11中引入的Lambda表达式的使用方法及其语法特点。通过一个具体的例子展示了如何利用Lambda表达式统计字符串中大写字母的数量，并解释了Lambda表达式中的变量捕获方式以及其在提高代码简洁性和安全性方面的作用。

Lambda Expressions

A lambda expression lets you define functions locally, at the place of the call, thereby eliminating much of the tedium and security risks that function objects incur. A lambda expression has the form:

[capture](parameters)->return-type {body}

The [] construct inside a function call’s argument list indicates the beginning of a lambda expression. Let’s see a lambda example.

Suppose you want to count how many uppercase letters a string contains. Using for_each() to traverses a char array, the following lambda expression determines whether each letter is in uppercase. For every uppercase letter it finds, the lambda expression increments Uppercase, a variable defined outside the lambda expression:

int main()
{
   char s[]="Hello World!";
   int Uppercase = 0; //modified by the lambda
   for_each(s, s+sizeof(s), [&Uppercase] (char c) {
    if (isupper(c))
     Uppercase++;
    });
 cout<< Uppercase<<" uppercase letters in: "<< s<<endl;
}

It’s as if you defined a function whose body is placed inside another function call. 
The ampersand in [&Uppercase] means that the lambda body gets a reference to Uppercase so it can modify it. 
Without the ampersand, Uppercase would be passed by value. C++11 lambdas include constructs for member functions as well.

[&Uppercase] 是捕获函数体内使用到的变量，变量前面加“&”是捕获引用即按引用传递，否则捕获变量即是按值传递
(char c) 是向函数传递的参数，同一般函数 f(char c)
{ } 是函数体，与一般函数体一样