C++11基于范围的for循环效率“const auto &i"与“自动 i" [英] C++11 Range-based for-loop efficiency "const auto &i" versus "auto i"

问题描述

在 C++11 中，我可以像这样迭代某个容器:

In C++11, I can iterate over some container like so:

for(auto i : vec){
   std::cout << i << std::endl;
}

但我知道这是不必要的 - 不必要，因为我只需要打印 vec 的值 - 复制 (EDIT:每个元素)vec，所以我可以这样做:

But I know that this needlessly - needlessly, since I only need to print the values of vec - makes a copy of (EDIT: each element of) vec, so instead I could do:

for(auto &i : vec){
   std::cout << i << std::endl;
}

但我想确保 vec 的值永远不会被修改并遵守常量正确性，所以我可以这样做:

But I want to make sure that the values of vec are never modified and abide by const-correctness, so I can do:

for(const auto &i : vec){
   std::cout << i << std::endl;
}

所以我的问题是:如果我只需要查看某个容器的值，那么最后一个循环 (const auto &i) 不会总是由于没有额外副本(编辑:vec的每个元素)vec?

So my question is: If I only need to look at the values of some container, wouldn't the very last loop (const auto &i) always be preferred due to the increased effieciency of not having an extra copy of (EDIT: each element of) vec?

我正在开发一个程序，我正在考虑在其中进行此更改，因为效率在其中至关重要(这是我首先使用 C++ 的原因).

I have a program that I'm developing in which I'm considering making this change throughout, since efficiency is critical in it (the reason I'm using C++ in the fist place).

推荐答案

是的.同样的原因，如果你只读过一个参数，你就会将参数设为 const&.

Yes. The same reason if you only ever read an argument you make the parameter const&.

T        // I'm copying this
T&       // I'm modifying this
const T& // I'm reading this

那些是您的默认设置".但是，当 T 是基本类型(内置)时，您通常只需恢复到 const T(无引用)进行读取，因为副本比别名更便宜.

Those are your "defaults". When T is a fundamental type (built-in), though, you generally just revert to const T (no reference) for reading, because a copy is cheaper than aliasing.

我正在开发一个程序，我正在考虑在其中进行此更改，因为效率在其中至关重要

I have a program that I'm developing in which I'm considering making this change throughout, since efficiency is critical in it

1. 不要盲目地进行彻底的改变.一个有效的程序比一个快速但损坏的程序要好.
2. 你如何迭代你的循环可能不会有太大的不同；你循环是有原因的，不是吗?循环体更有可能是罪魁祸首.
3. 如果效率至关重要，您希望使用分析器来查找程序的哪些部分实际上很慢，而不是猜测可能 慢一点.请参阅 #2，了解为什么您的猜测可能是错误的.

1. Don't make blind sweeping changes. A working program is better than a fast but broken program.
2. How you iterate through your loops probably won't make much of a difference; you're looping for a reason, aren't you? The body of your loop will much more likely be the culprit.
3. If efficiency is critical, you want to use a profiler to find which parts of your program are actually slow, rather than guess at parts that might be slow. See #2 for why your guess may be wrong.

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