Why Do Range-Based For Loops Behave Differently with `std::vector`?

Range-for-Loops and std::vector

When using range-based for loops with standard library containers, the data type of the iterator often dictates the data type of the counter variable. In the case of std::vector, however, a unique behavior arises due to its underlying storage mechanism.

In the first example:

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

The std::vector contains integers, so the iterator type is an std::vector::iterator. This iterator dereferences to a T&, which in this case is int&, making the counter variable of type int&.

Now, let's consider the second example:

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

Here, the std::vector contains bools, which are stored in an integer-packed format. The iterator type is std::vector::iterator, which dereferences to a std::vector::reference, also known as a std::_Bit_reference. This reference type is an rvalue (temporary) and cannot be bound to a non-const reference. This results in the compilation error:

<code class="text">invalid initialization of non-const reference of type ‘std::_Bit_reference&amp;’ from an rvalue of type ‘std::_Bit_iterator::reference {aka std::_Bit_reference}’</code>

The solution is to use auto&&, which will bind to an lvalue reference if it's an lvalue reference, or create a temporary copy of the rvalue if it's a temporary:

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

With this modification, the code will output the contents of boolVector as expected.

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