When Does an Rvalue Reference Act Like an Lvalue Inside a Function?
Dec 25, 2024 pm 12:05 PM
When Is an Rvalue Reference Behaves Like an Lvalue Within a Function?
In the code snippet below, the function foo receives an rvalue reference parameter named bar.
void foo(string&&& bar) {
string* temp = &bar;
cout << *temp << " @:" << temp << endl;
}
This code has sparked a question: is bar treated as an lvalue within the function, since the address can be taken?
Is bar an Rvalue or Lvalue?
This question is essentially answered within the question itself. Anything that has a name is an lvalue. Therefore, bar is perceived as an lvalue. However, it is crucial to note that bar has the type of "rvalue reference to string."
To treat bar as an rvalue, std::move() must be applied to it.
Why Differentiate Between Lvalue and Rvalue References?
This distinction becomes important when considering what can bind to each reference type. Lvalues can bind to lvalue references, while rvalues bind to rvalue references (and anything can bind to an lvalue reference to const).
In other words, an rvalue cannot be bound to an lvalue reference, and vice versa.
Parameter of Rvalue Reference Type
When dealing with a function parameter of rvalue reference type, it is imperative to understand what this means about the value to which the reference points. An rvalue reference indicates that the value assigned to it is an rvalue. Consequently, its resources will be freed once the expression concludes, allowing it to be managed as an rvalue.
Passing Rvalue References
When passing an rvalue reference parameter further, there are two options:
- If you are finished with the reference, use std::move(bar) to indicate that the value it references is an rvalue.
- If you intend to perform further operations on bar, hold onto the lvalue reference to prevent any unintended resource theft.
Conclusion
The distinction between lvalue and rvalue references lies not in their behavior once obtained, but rather in what can bind to them. This understanding is essential for effectively handling and managing references within functions.
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