How Do C 11 Reference Collapsing Rules Enable Perfect Forwarding?

Role of Reference Collapsing Rules in C 11

Reference collapsing rules play a crucial role in enabling perfect forwarding in C 11. Perfect forwarding allows functions to seamlessly accept and pass on parameters by value, lvalue reference, or rvalue reference.

Reference Collapsing Rules

The reference collapsing rules for C 11 are:

• A& & & -> A&
• A& & & -> A&
• A& & & -> A&
• A& & & & & -> A& &

Rationale for Reference Collapsing

These rules ensure that, regardless of how a parameter is passed (by lvalue, rvalue, or rvalue reference), it will always be "seen" by the receiving function as the desired type for perfect forwarding.

• To Lvalue Reference: When passing an lvalue by lvalue reference (e.g., T&), the collapsing rule converts "& & &" to "&", providing the correct lvalue reference for binding to the receiver's non-const lvalue reference.
• To Const Lvalue Reference: When passing a const lvalue by const lvalue reference (e.g., const T&), the collapsing rule ensures that the constness is preserved and the parameter is bound to the correct const lvalue reference in the receiver.
• To Rvalue Reference: Passing an rvalue reference (e.g., T& &), regardless of the incoming value type, results in a collapse to "& &", which allows perfect forwarding of xvalues (temporary objects) and prvalues (function return values of type T& &).

Utilization in C 11 STL Utility Functions

STL utility functions such as std::move() and std::forward() utilize reference collapsing internally to facilitate perfect forwarding. For instance:

std::forward<> collapses "& &" to "&" for lvalue references and "& &" to "& &" for rvalue references, enabling Call to receive its parameter in the appropriate format.

Relationship with std::remove_reference

Utilities like std::remove_reference can be used to remove references from a type deduction (e.g., std::remove_reference returns T). However, it does not replace the need for reference collapsing rules in C 11. std::remove_reference simply removes the reference layers, while reference collapsing ensures that the forwarded parameter is received in the correct format for perfect forwarding.

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