How Do C Reference Collapsing Rules Enable Perfect Forwarding?

Understanding Reference Collapsing Rules in C

C employs reference collapsing rules to facilitate "perfect forwarding," enabling functions to receive parameters as if they were called directly. These rules collapse various combinations of references to streamline parameter passing, supporting efficient handling of different parameter types.

Reference Collapsing Rules for Perfect Forwarding

The four reference collapsing rules are:

• A&& && collapses to A&
• A&& &&& collapses to A&
• A& && collapses to A&
• A& &&& collapses to A&&

Purpose of Reference Collapsing

These rules serve the following purposes:

• Lvalue References: When the receiving function expects an lvalue (non-temporary), reference collapsing ensures that lvalues are forwarded as non-temporary references. This forces a copy operation, mimicking the behavior of direct function calls.
• Rvalue References: When the receiving function expects an rvalue (temporary or otherwise), reference collapsing allows rvalues to be forwarded as rvalues, triggering move semantics. This closely simulates the behavior of direct function calls with rvalue arguments (except for elision).

C 11 STL Utilities and Reference Collapsing

STL utilities such as std::move() and std::forward() utilize these rules to implement perfect forwarding. These utilities ensure that parameters are passed in a manner that preserves their original temporality, promoting both efficiency and code brevity.

Relationship with std::remove_reference

std::remove_reference is not routinely used to avoid the need for reference collapsing rules. Rather, it is used to remove reference qualifiers from a type, regardless of whether reference collapsing has occurred. In conjunction with reference collapsing, std::remove_reference can be used to manipulate the type and temporality of parameters.

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