Why does C use the same syntax for rvalue references and universal references?

Why Rvalue and Universal References Share Syntax

In C , both rvalue and universal references share the same syntax, denoted by T&&. This choice has raised questions about potential confusion and the possibility of alternative syntaxes.

Rvalue References

Rvalue references are used to denote a non-modifiable reference to an rvalue (right-value) expression. They prevent lvalues (left-values) from being bound to the reference, as illustrated below:

<code class="cpp">void foo(int&amp;&amp; a);

int i = 42;
foo(i); // error</code>

Universal References

In contrast, universal references can bind to both rvalues and lvalues. This is achieved through template argument deduction and reference collapsing rules. Consider the following examples:

<code class="cpp">template<typename T>
void bar(T&amp;&amp; b);

bar(i); // okay</code>

In the bar function, the template parameter T can be deduced as int when passed an rvalue, resulting in a function parameter of int&&. Similarly, when passed an lvalue, T can be deduced as int&, resulting in a function parameter of int&.

Reasons for Shared Syntax

The rationale behind sharing the same syntax for rvalue and universal references lies in the fact that they follow the same rules for argument deduction and reference collapsing. This consistency allows for seamless integration of forwarding references (perfect forwarding) in templates.

If alternative syntaxes were adopted, such as T&&&, T&*, or T@, it would introduce additional complexity into the language. Moreover, these alternative notations would require the introduction of new grammar rules that present their own challenges and potential for ambiguity.

Rejection of Alternative Syntaxes

The C committee considered alternative syntaxes but ultimately rejected them for the following reasons:

• They would create inconsistency within the type system.
• They would introduce unnecessary complexity and potential for confusion.
• They would violate the principle of least surprise, as the chosen syntax aligns well with the deduction and collapsing rules.

In summary, the shared syntax between rvalue and universal references provides a clean and consistent approach for handling forwarding references and rvalue semantics in C . The consistency with the reference collapsing rules eliminates the need for separate syntax and maintains the integrity of the language's type system.

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