Why Can I Pass Rvalues by Const Reference in C ?

Preserving Rvalues with Const References: A C Puzzle

In C , passing rvalues (temporary objects) by const reference is allowed, unlike normal references. Consider the following program:

<code class="cpp">void display(const int& a) { cout << a; }

int main() {
  int a = 5;
  display(a);      // Works with an lvalue
  display(5);      // Also works with an rvalue
  return 0;
}</code>

The program allows passing both lvalues and rvalues to the display function, even though the reference is marked as const. This behavior is puzzling, as const references are typically associated with the preservation of lvalues.

The Const Reference Lifetime Extension

The key to understanding this behavior lies in the semantics of const references in C . A const reference prolongs the lifetime of the referred object until the end of the containing scope. In the case of an rvalue, this effectively prevents the destruction of the temporary object until the const reference goes out of scope.

Example: Prolonging Rvalue Lifetime

In our example, the following occurs when display(5) is called:

• An rvalue of type int is created with the value 5.
• A const reference a is bound to this rvalue.
• The const reference prolongs the lifetime of the rvalue until the end of the display function.
• The rvalue is used within the function to print the value 5.
• When the display function returns, the const reference goes out of scope, and the rvalue is finally destroyed.

This demonstrates how a const reference can keep pointing to an rvalue, allowing it to remain in existence even though it would otherwise be immediately destroyed.

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