C ++中奇怪的括号括号表示法，每个循环看起来都像一个 [英] Strange bracket-parentheses notation in C++, looking somewhat like a for each loop

问题描述

这就是代码的样子:

auto generateHash = [](std::vector<File> &files) -> std::shared_ptr<std::string> {
    // Other code here
}

这是什么意思?每个循环都可以吗?一开始的括号是做什么的?括号是做什么的?箭头是什么意思?由于没有C ++ 11编译器，我无法编译它，并且在C ++参考中找不到它.

What does this mean? Is it a for each loop? What do the brackets in the beginning do? What do the parentheses do? What does the arrow mean? I can't compile it because of no C++11 compiler, and I can't find it in the C++ reference.

推荐答案

这是什么意思?

What does this mean?

这是一个 lambda -一个函数对象.您可以像调用带有文件向量的函数一样调用它(由于某些奇怪的原因而通过可变引用传递)，并且它返回一个字符串(由于某些奇怪的原因而由共享指针进行管理).

It's a lambda - a function object. You can call it like a function with a a vector of files (passed by mutable reference, for some weird reason), and it a returns string (managed by a shared pointer, for some weird reason).

std::vector<File> files = get_some_files();
std::shared_ptr<std::string> hash = generateHash(files); // call the lambda

每个循环都是一个吗?

Is it a for each loop?

不.看起来像

for (auto thing : sequence) {/* code */}

开头的括号是做什么的?

What do the brackets in the beginning do?

他们表示这是lambda.它们可以包含要在lambda内部的代码中使用的局部变量的捕获列表.

They signify that it's a lambda. They can contain the capture list of local variables that you want to make available to the code inside the lambda.

箭头是什么意思?

What does the arrow mean?

这是跟踪返回类型.在C ++ 11中，如果需要，可以将该语法与普通函数一起使用.但这是指定lambda的返回类型的唯一方法.

That's a trailing return type. In C++11, you can use that syntax with normal functions if you want; but it's the only way to specify a lambda's return type.

我在C ++参考中找不到它.

I can't find it in the C++ reference.

就在这里: http://en.cppreference.com/w/cpp/语言/lambda

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