When Should You Use Forwarding References in Range-Based For Loops?

Why Consider Forwarding References in Range-Based For Loops?

When traversing a range using range-based for loops, it's common to declare the loop variable as auto& or const auto&. However, there are specific scenarios where employing forwarding references (auto&&) may offer an advantage.

One such scenario arises when the sequence iterator returns a proxy reference, and you intend to modify that reference directly. Consider the following example:

#include <vector>

int main()
{
    std::vector<bool> v(10);
    for (auto& e : v)
        e = true; // Compilation error with rvalue reference
}

Attempting to assign a value to a non-const lvalue reference using an rvalue reference returned from std::vector::reference results in a compilation error. Utilizing a forwarding reference, however, resolves this issue:

#include <vector>

int main()
{
    std::vector<bool> v(10);
    for (auto&& e : v)
        e = true;
}

It's important to note that using forwarding references gratuitously can be confusing and should only be employed when a specific use case necessitates it. In such cases, providing a brief comment explaining the rationale can enhance code clarity.

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