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Is Returning by Rvalue Reference Always More Efficient Than Returning by Lvalue Reference?
Is Returning by Rvalue Reference Always More Efficient Than Returning by Lvalue Reference?
Dec 24, 2024 am 12:15 AM
Returning by Rvalue Reference: Is It More Efficient?
Returning an object by rvalue reference may not always be more efficient than returning by lvalue reference. It can lead to dangling references if the returned object is a temporary that gets destructed after the function returns.
The Original Code:
The provided code snippet attempts to return a moved rvalue reference to a temporary Beta_ab object:
Beta_ab&& Beta::toAB() const {
return move(Beta_ab(1, 1));
}
This is not recommended because it returns a dangling reference. To properly move a temporary into the return value, the function should return a value rather than an rvalue reference:
Beta_ab Beta::toAB() const {
return Beta_ab(1, 1);
}
Using Rvalue References with Other Functions:
Returning an rvalue reference can be beneficial in certain situations. For example, if there is a getAB() function that is frequently called on temporary objects, it can be more efficient to return an rvalue reference:
struct Beta {
Beta_ab ab;
Beta_ab const& getAB() const& { return ab; }
Beta_ab &&getAB() && { return move(ab); }
};
In this example, move is necessary because ab is not a local automatic or a temporary rvalue. The ref-qualifier && ensures that the second getAB() function is invoked on rvalue temporaries, resulting in a move instead of a copy.
Conclusion:
While returning by rvalue reference can be efficient in some cases, it should be used cautiously to avoid dangling references. Returning a value is generally preferred for functions that return temporary objects.
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