C++11 Range-based for-loop efficiency “const auto

问题:

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

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

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;
}

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;
}

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?

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).

回答1:

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

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.

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

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.

回答2:

Imagine if your vector contains strings. Long strings. 5000 long strings. Copy them unnecessarily and you end up with a nicely written for loop that is awfully inefficient.

Make sure your code follows your intention. If you do not need a copy inside of the loop, do not make one.

Use a reference & as suggested above, or iterators.