C ++ 11基于范围的for循环效率“const auto& i”相对于“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，我可以迭代一些容器，像这样：

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

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

但我知道这不必要 - 不必要因为我只需要打印 vec 的值 - 复制一个 EDIT ： 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;
}

但是我想确保 vec 从不修改，并遵守const正确性，所以我可以做：

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;
}

因此我的问题是：如果我只需要在一些容器的值，不会是最后一个循环（ const auto& i ）总是优先的，因为没有额外的副本增加的效率（ EDIT ： vec 的每个元素

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?

我正在开发中，我正在考虑进行这种变化，因为效率是至关重要的（我在Cist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，如果你只读一个参数，你使参数 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

这些是你的默认值。当 T 是一个基本类型（内置）时，你通常只是恢复到 const T ），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.

因为效率至关重要，所以我正在开发中。

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

1. 不要盲目改变。工作程序比快速但破碎的程序更好。


2. 如何迭代循环可能不会有太大的区别;你循环的原因，不是吗？


3. 如果效率至关重要，您可以使用分析器来查找程序的哪些部分实际上慢，而不是猜测可能缓慢的部分。请参阅＃2，了解您的猜测为何会出错。

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.

这篇关于C ++ 11基于范围的for循环效率“const auto& i”相对于“自动i”的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！