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

Advantages of Using Forwarding References in Range-Based For Loops

In range-based for loops, the default reference type is auto&, which is suitable for read-only operations. However, in scenarios where the iteration involves modifying the elements, it may be necessary to use forwarding references (auto&&) to handle certain corner cases.

One such scenario arises when the sequence iterator returns a proxy reference. Proxy references are returned in cases where the underlying container is a non-const container with elements of type bool. In such cases, using auto& to bind the iterator's rvalue reference to a non-const lvalue reference will result in a compilation error.

To resolve this issue and enable non-const operations on the iterator's reference, it becomes necessary to use auto&&. By using auto&&, the compiler can correctly bind the iterator's rvalue proxy reference to the non-const lvalue reference.

Consider the following example:

std::vector<bool> v(10);
for (auto& e : v)  // Error
   e = true;

This code will not compile because vector::reference returned from the iterator won't bind to a non-const lvalue reference. By using auto&& instead, we can resolve this issue:

for (auto&& e : v)  // Works
    e = true;

It's important to note that using auto&& should not be done without a specific need. Gratuitous use of auto&& can lead to confusion and should be avoided unless it solves a technical challenge. If used, it's advisable to include a comment to explain the reasoning behind its use.

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