C++11 Range-based for-loop efficiency “const auto &i” versus “auto i”

Question

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

Answer 1

Disclaimer: In general the difference between auto and auto& is subtle, partly a matter of style, but sometimes also a matter of correctness. I am not going to cover the general case here!

In a range based for loop, the difference between

for (auto element : container) {}

and

for (auto& element_ref : container) {}

is that element is a copy of the elements in the container, while element_ref is a reference to the elements in the container.

To see the difference in action, consider this example:

#include 

int main(void) {
    int a[5] = { 23,443,16,49,66 };

    for (auto i : a) i = 5;       
    for (const auto& i : a) std::cout << i << std::endl;
    for (auto& i : a) i = 5;   
    for (const auto& i : a) std::cout << i << std::endl;    
}

It will print

23
443
16
49
66
5
5
5
5
5

because the first loop works on copies of the array elements, while the second actually modifies the elements in the array.

If you dont want to modify the elements then often a const auto& is more appropriate, because it avoids copying the elements (which can be expensive).