How Do Rvalue References and Move Semantics Impact C 11 Return Statements?

Rvalue References and Move Semantics in C 11 with Return Statements

C 11 introduced rvalue references and move semantics to enhance performance by avoiding unnecessary copies and optimizing object creation. Understanding the differences between these techniques is crucial for writing efficient code.

Example Analysis

Consider the following C 11 code examples:

First Example:

std::vector<int> return_vector() {
    std::vector<int> tmp {1, 2, 3, 4, 5};
    return tmp;
}

std::vector<int> &&rval_ref = return_vector();

Second Example:

std::vector<int>&& return_vector() {
    std::vector<int> tmp {1, 2, 3, 4, 5};
    return std::move(tmp);
}

std::vector<int> &&rval_ref = return_vector();

Third Example:

std::vector<int> return_vector() {
    std::vector<int> tmp {1, 2, 3, 4, 5};
    return std::move(tmp);
}

std::vector<int> &&rval_ref = return_vector();

Explanation

First Example:

• The return type is a lvalue reference std::vector&.
• tmp is a local variable and hence a temporary rvalue.
• The temporary tmp is returned by copy, so no move semantics are involved.
• rval_ref becomes a non-const lvalue reference to the returned vector.

Second Example:

• The return type is an rvalue reference std::vector&&.
• tmp is still a temporary rvalue.
• std::move(tmp) forces the function to return the temporary rvalue by move, instead of copy.
• This means that tmp is moved into the return value, and rval_ref now holds a valid rvalue reference.

Third Example:

• This example is similar to the first one.
• The return type is a lvalue reference std::vector&, despite the std::move(tmp).
• std::move(tmp) is unnecessary and serves no purpose in this context.
• The temporary tmp is returned by copy, and rval_ref becomes a non-const lvalue reference to the returned vector.

Best Practice

The recommended way to return a temporary object as a move-only lvalue reference (e.g., std::vector&&) is by simply omitting the reference and letting the compiler decide whether to move or copy the object based on return value optimization (RVO).

std::vector<int> return_vector() {
    std::vector<int> tmp {1, 2, 3, 4, 5};
    return tmp;
}

std::vector<int> rval_ref = return_vector();

This approach provides the best combination of performance and correctness.

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