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

C 11 rvalues and move semantics with return statement

Understanding rvalue references and move semantics

In C 11, rvalue references and move semantics were introduced to improve performance and efficiency when working with temporary values. An rvalue reference (&&) refers to a temporary object, while move semantics allows us to move the contents of one object into another without copying.

Example 1

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>

In this example, the return_vector function returns a temporary std::vector object, which is caught by the rvalue reference rval_ref. Since the temporary object is moved into rval_ref, it effectively prevents the copy.

Example 2

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>

In this case, the return_vector function returns the temporary std::vector object by value and uses the std::move function to explicitly move the contents of the temporary object into the returned object. This creates a runtime error because rval_ref now holds a reference to the destructed temporary object.

Example 3

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 Example 1, where the return_vector function returns the temporary std::vector object by value and uses std::move to move the contents. However, in this case, the std::move is unnecessary and can hinder performance by preventing return value optimization.

Best practice

The best approach is to use Example 4, where the temporary object is implicitly moved into the returned object without explicitly using std::move. The compiler will perform return value optimization (RVO) to avoid unnecessary copying.

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>

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