c++ iterating with auto reference

Question

Although having read a bunch of posts here, I still often make mistakes in using auto& reference in for (auto& x : some_container) . Usually, to avoid constructing temporary copy and speed up for-loop iteration for string types, I use auto& as a default option.

Could someone please guide me on when and how to correctly use auto&?

EDIT:

Below may be a bad example. I know we shall avoid vector from the book effective c++. However, this question also asks for general guidelines for using auto& iterating containers

Today, I fall into another error trap of using auto&

#include <vector>
#include <iostream>

using namespace std;

int main() {
    int n = 3;
    vector<bool> vec(n, false);
    for(auto& x : vec) cout << x << " ";
    cout << endl;

    return 0;
}

The error message is show as below. What does it mean?

vector_bool.cpp:9:15: error: non-const lvalue reference to type '__bit_reference<[2 * ...]>' cannot bind to
      a temporary of type '__bit_reference<[2 * ...]>'
    for(auto& x : vec) cout << x << " ";
              ^ ~

The trap is using `vector`. If you can, you should avoid it. It is not like a regular vector. — NathanOliver, Jul 10 '19 at 15:14
Thanks, I know that from effective c++. However, this question also asks for general guidelines for using auto& iterating containers. — thinkdeep, Jul 10 '19 at 15:19
Take a read of [this](https://en.cppreference.com/w/cpp/container/vector_bool) — Alex Hodges, Jul 10 '19 at 15:21
In hindsight, they should have made a space-optimized std::vector_bool ... but too late now, that ship has sailed. — Eljay, Jul 10 '19 at 15:21
`auto &` = I want to modify and not copy. `const auto &` = I want to not copy. `auto` = I want to copy and modify. `const auto` = I want to copy. — Alex Hodges, Jul 10 '19 at 15:24
For your question, it is answered here: https://stackoverflow.com/questions/29859796/c-auto-vs-auto — Eljay, Jul 10 '19 at 15:25

c++ iterating with auto reference

0 Answers0