What is the Difference Between Universal References and Forwarding References in C ?

Understanding the Concept of Universal and Forwarding References

In C , the distinction between universal and forwarding references can be confusing. While both terms refer to the same concept, their precise definitions have evolved over time.

In C 11, the term "universal reference" was introduced to describe the ability of a template parameter to accept both rvalue and lvalue references. This was achieved by using an rvalue reference to a cv-unqualified template parameter.

However, with the introduction of C 14, a more precise term was adopted: "forwarding reference." As defined in the C 14 standard, a forwarding reference is "an rvalue reference to a cv-unqualified template parameter."

Key Difference

The key difference between universal and forwarding references lies in their semantics. While both accept both rvalue and lvalue references, the standard clarifies that for forwarding references, the type deduced for a given argument is "lvalue reference to A" if the argument is an lvalue.

Forwarding

It's important to note that the term "forwarding reference" does not imply the use of the std::forward function within the function body. The forwarding aspect refers solely to the type deduction process. Whether or not the reference is subsequently forwarded is irrelevant.

Example

Consider the following function template:

template<typename A>
void foo(A&& a) {
  // Forwarding does not occur here
}

In this example, the type of the parameter a is a forwarding reference. If an lvalue is passed as an argument to this function, the type of a will be deduced as "lvalue reference to A." However, the function body does not perform any forwarding.

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