What are the C Reference Collapsing Rules and How Do They Enable Perfect Forwarding?

Reference Collapsing Rules in C Standard Library

Reference collapsing rules are a set of transformations that are applied to reference types in C to simplify and optimize their usage. They play a vital role in ensuring the proper functioning of the C standard library, particularly with regard to forwarding and perfect forwarding.

The reference collapsing rules are as follows:

1. A& & A& becomes A&
2. A& & A&& becomes A&
3. A&& & A& becomes A&
4. A&& & A&& becomes A&&

Rationale for the Reference Collapsing Rules

The primary purpose of the reference collapsing rules is to enable perfect forwarding. Perfect forwarding ensures that data passed to a function is forwarded exactly as if it were passed directly, regardless of whether the original data is an lvalue, xvalue, or prvalue. This is critical for maintaining the semantics of the original call.

For example, if a function takes its parameter by lvalue reference, it expects an lvalue to be passed to it. If an rvalue is passed instead, a temporary will be created and passed to the function, resulting in unnecessary copying and potential performance issues. The reference collapsing rules prevent this by ensuring that rvalues are forwarded as xvalues, which can be used interchangeably with lvalues for parameters that take lvalue references.

Utilization in C 11 Standard Library

The reference collapsing rules are utilized in C 11 standard library utilities such as std::move, std::forward, and std::remove_reference. These utilities rely on the reference collapsing rules to perform efficient and correct forwarding of data.

std::move uses the reference collapsing rules to convert rvalues to xvalues, which allows for efficient movement of data to a new location. std::forward uses the reference collapsing rules to forward incoming references as xvalues or lvalues, as appropriate, ensuring that the forwarded data retains its original semantics. std::remove_reference removes all references from a type, which can be useful when working with legacy code or when the reference collapsing rules are not sufficient.

Conclusion

The reference collapsing rules are an essential part of the C standard library and play a significant role in facilitating perfect forwarding. By understanding their rationale and utilization, programmers can leverage these rules to improve the performance and maintainability of their C code.

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