When Should I Use Forwarding References (auto&&) in Range-Based For Loops?

Understanding the Advantages of Forwarding References in Range-Based For Loops

In range-based for loops, the question of when to use forwarding references (e.g., auto&&) over explicit references (auto& or const auto&) arises. While const auto& is typically suitable for read-only operations, auto&& may offer certain benefits in obscure corner cases.

Identifying the Performance Gain

One potential advantage of forwarding references lies in scenarios where the sequence iterator returns a proxy reference and non-const operations are required on that reference. An example is the modification of elements in a vector of boolean values, as illustrated below:

#include <vector>

int main()
{
    std::vector<bool> v(10);
    for (auto& e : v)  // Compiler error: cannot bind rvalue reference to non-const lvalue
        e = true;
}

This code will not compile because the iterator of a vector of booleans returns an rvalue reference, which cannot be bound to a non-const lvalue reference (auto&). Using a forwarding reference (auto&&) resolves this issue:

#include <vector>

int main()
{
    std::vector<bool> v(10);
    for (auto&& e : v)
        e = true;  // Valid: forwarding reference can bind to rvalue reference
}

Cautious Application

While forwarding references can address such corner cases, it is crucial to use them judiciously. Gratuitous use of auto&& can raise questions and warrant clear comments to explain its necessity, such as:

#include <vector>

int main()
{
    std::vector<bool> v(10);
    // using auto&& so that I can handle the rvalue reference
    //   returned for the vector<bool> case
    for (auto&& e : v)
        e = true;
}

In cases where a loop consistently handles proxy references, auto can also be used instead of auto&& without performance implications. However, auto&& becomes a more suitable solution when the loop switches between handling proxy references and other references.

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