for循环c ++中的'冒号'和'自动'？需要一些帮助来理解语法 [英] 'colon' and 'auto' in for loop c++? need some help understanding the syntax

问题描述

我需要对以下c ++语法进行一些解释：

I need some explanation for the following c++ syntax:

for(const auto& ioDev : deviceList)

鉴于：

std::vector<Device *> deviceList

具体来说，我对'：'和'auto'的用法感到困惑？

Specifically, I am confused about ':' and the usage of 'auto'?

推荐答案

如乍得的答案所述，您的for循环遍历 vector ，使用其开始和 end 迭代器。这就是冒号：语法的行为。

As described in the answer by Chad, your for-loop iterates over your vector, using its begin and end iterators. That is the behavior of the colon : syntax.

关于 const auto& 语法：您应该想象从中得到什么代码：

Regarding your const auto & syntax: you should imagine what code comes out of it:

// "i" is an iterator
const auto& ioDev = *i;

* i 的表达式是（一个引用容器中元素的类型：设备* 。这是 auto 的推导类型。因为您在 auto 后面附加了 const& ，所以变量 ioDev 是对推断类型（指针）的 const 引用，就好像是这样声明的：

The expression *i is (a reference to) the type of elements in the container: Device *. This is the deduced type of auto. Because you have const & appended to your auto, the variable ioDev is a const reference to the deduced type (a pointer), as if it were declared this way:

const Device *& ioDev = *i;

似乎不必要的复杂；如果只需要普通迭代（而不是例如操纵指针的地址，我认为这种可能性很小），请使用普通的未修改的 auto ：

It seems needlessly complicated; if you need just normal iteration (and not e.g. manipulating the address of the pointer, which I think is highly unlikely), use a plain unmodified auto:

for (auto ioDev : deviceList)

或显式类型：

for (Device* ioDev : deviceList)

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