括号在C ++ 11 lambda表达式的结尾 [英] Parentheses at the end of a C++11 lambda expression

问题描述

我困惑了一些例子，我遇到了与C ++ 11 lambdas。
例如：

I am confused with some of the examples that I have come across with C++11 lambdas. For eg:

#include <iostream>
#include <string>

using namespace std;

int main()
{ 
        cout << []()->string{return "Hello World 1!";}() << endl;

        []{cout << "Hello World 2!" << endl;}();

        string result = [](const string& str)->string {return "Hello World " + str;}("2!");
        cout << "Result: " << result << endl;

        result = [](const string& str){return "Hello World " + str;}("3!");
        cout << "Result: " << result << endl;

        string s;
        [&s](){s = "Hello World 4!";};   // does not work
        cout << s << endl; 
        [&s](){s = "Hello World 4!";}(); // works!
        cout << s << endl;

        return 0;
}

我无法弄清楚最后的括号是什么。他们实例化，作为一个构造函数，lambda？假定lambda的模板是：

I am unable to figure out what the parentheses at the end are doing. Are they instantiating, as a constructor, a lambda? Given that the template for a lambda is:

[capture_block](parameters) mutable exception_specification -> return_type {body}

这让我很担心，那些实例需要那些括号。

it baffles me that those parentheses are required for those instances to work. Can someone explain what they are why they are required?

推荐答案

很好，由于lambda表达式基本上是一个匿名函数，圆括号在结尾只做调用此函数。

Well, given that a lambda expression is basically an anonymous function, the parentheses at the end do nothing more than just call this function. So

result = [](const string& str){return "Hello World " + str;}("3!");

等效于

auto lambda = [](const string& str){return "Hello World " + str;};
string result = lambda("3!");

对于没有参数的lambda，同样的方式也是如此，例如

This holds in the same way for a lambda with no arguments, like in

cout << []()->string{return "Hello World 1!";}() << endl;

否则（如果未调用）尝试输出一个lambda表达式，工作。通过调用它只是输出结果 std :: string 。

which would otherwise (if not called) try to output a lambda expression, which in itself doesn't work. By calling it it just puts out the resulting std::string.

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