How do I properly use a range-based for loop for an std::array

Question

I can iterate through the array with a traditional for loop and a traditional for loop with iterators, but when I try to use a ranged-based for loop, I do not get the same result.

#include <iostream>
#include <array>

int main() {
    std::array<int, 5> ar = {1, 2, 3, 4, 5};
    for(int i = 0; i < 5; i++) {
        std::cout << ar[i] << " ";
    }
    std::cout << std::endl;

    for(std::array<int, 5>::const_iterator it = ar.begin(); it != ar.end(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;

    for(const int& i: ar) {
        std::cout << ar[i] << " ";
    }
    std::cout << std::endl;
}

1 2 3 4 5 
1 2 3 4 5 
2 3 4 5 0

songyuanyao · Accepted Answer · 2021-08-20 01:09:55Z

3

In range-based for loop for(const int& i: ar), i refers to element but not index. So

for(const int& i: ar) {
    std::cout << i << " ";
}

answered Aug 20, 2021 at 1:09

songyuanyao

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3 Comments

catnip Over a year ago

And there's not much point using for(const int& i: ... here. for(int i: ... is more efficient (passing by value is more efficient than passing by reference for primitive types, since it avoids an extra dereference and the copy is cheap).

prehistoricpenguin Over a year ago

@PaulSanders Does the compiler optimized out the unnecessary reference here?

catnip Over a year ago

@prehistoricpenguin Most likely, yes. But why take the chance? Coding the loop with a reference here is just silly.

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