How Can C 11 Rvalue References and Move Semantics Optimize Vector Return Values?

C 11 Rvalue References and Move Semantics: Unraveling Return Values

In the realm of C 11, understanding rvalue references and move semantics is crucial. This article aims to clarify the differences between three code examples and determine which avoids vector duplication.

First Example

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

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

Here, the function returns a local variable, tmp, by value. The reference rval_ref binds to a copy of tmp. This is akin to:

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

except that rval_ref cannot be used for non-const operations.

Second Example

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

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

This example incorrectly attempts to return a moved local variable, tmp. Since tmp is destroyed when the function exits, rval_ref holds a dangling reference. This is likely to cause a runtime error.

Third Example

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

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

This example is similar to the first, where tmp is returned by value. The std::move call is redundant and can degrade performance by disabling return value optimization (RVO).

Best Practice

For optimal performance, use the following approach:

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

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

The local variable tmp is implicitly treated as an rvalue in the return statement. The compiler will apply RVO, avoiding copies or moves, or it will use the move constructor if necessary.

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