Why Can\'t I Use `auto&` with `std::vector` in a Range-for-Loop?

Range-for-Loops with std::vector

In C , range-for-loops are commonly used to iterate over STL containers. However, one might encounter an error when attempting to use this approach with a std::vector.

Consider the following code snippet:

<code class="cpp">std::vector<int> intVector(10);
for(auto& i : intVector)
    std::cout << i;</code>

This code works because std::vector is a container with iterators that dereference to an int&. This means that we can bind the dereferenced value of the iterator to our own lvalue reference auto&.

However, if we try to perform the same operation with a std::vector, we will encounter an error similar to:

<code class="cpp">std::vector<bool> boolVector(10);
for(auto& i : boolVector)
    std::cout << i;</code>

This is because std::vector is not a container in the traditional sense but rather a bitset. It stores boolean values in a packed manner, using integers to represent multiple booleans. As a result, its iterators return a Proxy object instead of a bool&.

Proxies are temporary objects that cannot be bound to an lvalue reference. This is why we cannot use auto& in the range-for-loop above. Instead, we need to use auto&&, which will correctly bind to an lvalue reference if given one or maintain the temporary Proxy alive if it's given a proxy. Here's the corrected code:

<code class="cpp">for(auto&& i : boolVector)
    std::cout << i;</code>

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