Why Does Using Range-Based Loops with std::vector Result in a Compiler Error?

Range-for-Loops and std::vector

In C , range-based loops have become widespread for their simplicity and readability. They iteratively iterate over elements of sequences or container objects.

However, a curious inconsistency arises when attempting to use range-based loops with std::vector. While the following code will function as expected:

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

using range-for-loops with std::vector results in a compiler error:

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

The error message "invalid initialization of non-const reference of type 'std::_Bit_reference&' from an rvalue of type 'std::_Bit_iterator::reference {aka std::_Bit_reference}'" indicates that a non-const reference is being initialized with an rvalue.

Understanding the Issue

The reason for this discrepancy lies in the distinct implementation of std::vector. Unlike std::vector, which stores each integer separately, std::vector employs bit packing to conserve memory. Consequently, its iterators do not directly access individual bools but instead return references to std::_Bit_reference, which serves as a proxy to access the underlying bits.

Solution: Using auto&&&

To resolve this issue, it is necessary to use auto&&& instead of auto&. This idiom will correctly collapse the reference to an lvalue reference when provided with an lvalue (such as intVector in the first example), while simultaneously binding and maintaining the temporary alive if a proxy is provided (as in the case of boolVector).

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

By employing this technique, you can effectively iterate over std::vector and access its elements using range-based loops.

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