What is the difference between auto and auto&?

Question

Here is my code. According to the print results, there is no difference, then what does the & symbol play here?

#include <stdio.h>
#include <iostream>
#include <Windows.h>

int main(void)
{
    std::cout << "Print test data\n" << std::endl;
    int a[5] = { 23,443,16,49,66 };

    for (auto &ii : a)
    {
        std::cout << "auto ii: " << ii << std::endl;
    }

    printf("\n");

    for (auto jj : a)
    {
        std::cout << "auto jj: " << jj << std::endl;
    }
    system("pause");

}

It's the same as the difference between e.g. `int` and `int&` or `MyClass` and `MyClass&`. — Some programmer dude, Aug 21 '19 at 07:08
Why are you using `` and `printf` in a C++11 program? `std::cout << "\n";` works just as well. — jww, Aug 21 '19 at 07:10

score 5 · Accepted Answer · edited Aug 21 '19 at 07:24

Auto, for(auto x : range): This usage will create a copy of each element of the range.
Auto &, for(auto& x : range): when you want to modify the elements in range (without proxy class reference processing), use auto&.

ii is a reference, so in the loop body, if ii is modified, the corresponding element in a will also be modified.

jj is not a reference. In each loop, it is a copy of the corresponding element. Its modification will not affect the corresponding element in a. Since each loop creates a copy, it will bring system overhead.

If you want to ensure that the data in a is not modified and you want to improve the efficiency, you can use the form const auto & ii : a.

What is the difference between auto and auto&?

1 Answers1