What Does the Double Ampersand (&&) Mean in C 11?

Decoding the Double Ampersand in C 11

C 11 introduces a new operator, the double ampersand (T&&), which has garnered considerable attention. This enigmatic symbol has sparked curiosity and questions regarding its significance and utility.

What is the Double Ampersand?

The double ampersand operator declares an rvalue reference. This rvalue reference, as opposed to a traditional lvalue reference in C 03, can bind to both rvalues (temporaries) and lvalues without the necessity of a const qualifier. Consequently, it allows for the declaration of an rvalue reference variable that can refer to a temporary.

The Purpose of Rvalue References

Rvalue references primarily provide for the following enhancements:

Move Semantics:

Move semantics enable the definition of move constructors and move assignment operators that take an rvalue reference instead of a const lvalue reference. Move functions simulate copying behavior while avoiding the need to preserve the source. They typically modify the source to release ownership of its resources, resulting in more efficient operations and reducing unnecessary copies.

Perfect Forwarding:

Rvalue references facilitate the correct forwarding of arguments in templated functions. This allows for the creation of generic factory functions like std::forward that can accept any combination of lvalue or rvalue arguments and pass them to the appropriate constructor.

Additional Properties of Rvalue References:

• Lvalues prefer lvalue references, while rvalues prefer rvalue references in overload resolution.
• Rvalue references implicitly bind to rvalues and temporaries that arise from implicit conversions.
• Named rvalue references are lvalues, while unnamed rvalue references are rvalues.

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