Why Does an Rvalue Reference Sometimes Act Like an Lvalue in C Function Overloading?

Understanding Rvalue Reference Variables

In C , rvalue references are a specialized type of reference that can only bind to rvalues (values with no name). However, a surprising behavior arises when a function is overloaded with both an lvalue reference and an rvalue reference overload.

Confusion: Rvalue Reference Variables Behaving Like Lvalues

Consider the following function overloads:

void f(T&&); // Overload #1 (rvalue reference)
void f(T&);  // Overload #2 (lvalue reference)

Within another function:

void g(T&& t) 
{ 
  f(t);  // Calls Overload #2 (lvalue reference)
}

The surprising result is that Overload #2 (lvalue reference) is called, even though the argument t is an rvalue reference.

Reason: Rvalue References Are Lvalues with a Special Type

Rvalue references are considered lvalues because they have a name and can be used as an lvalue for most purposes. However, their type, rvalue reference, specifies their limited binding capabilities.

How to Invoke the Rvalue Reference Overload

To invoke the rvalue reference overload, the argument must be explicitly converted to an rvalue reference using the static_cast operator:

f(static_cast<T&amp;&amp;>(t));

Other Rules Governing Rvalue References

• T&& can behave as a forwarding reference in a template context.
• T&& can bind to rvalues or references to rvalues (X const&&).
• Rvalue references have both static and dynamic binding properties.
• Reference lifetime extension occurs when rvalue references directly bind to temporary values, extending their lifetime until the lifetime of the reference expires.

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