How Can C 11 Rvalue References and Move Semantics Optimize Returning Objects by Value?

C 11 rvalues and move semantics with return statement

In C 11, rvalue references and move semantics offer significant performance benefits by optimizing memory management and object ownership. This is particularly relevant when returning objects by value from functions.

Understanding Rvalue References and Move Semantics

An rvalue reference (&&) is a type that binds to a temporary object or an object that is about to be destroyed. When a temporary object is returned from a function using an rvalue reference, the compiler can optimize the return by using move semantics instead of copy semantics.

Move semantics involve transferring ownership of the object's resources from the temporary object to the target variable without creating a new copy of the object. This can significantly improve performance, especially for large objects.

Example Analysis

Let's examine the three examples you provided:

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();

In this example, the temporary object tmp is returned by value. Since rval_ref is an rvalue reference, it binds to the temporary object and extends its lifetime beyond the function call. This behavior is similar to returning a const reference to the temporary object.

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();

This example attempts to return the temporary object tmp by value using move semantics. However, it results in a runtime error because rval_ref holds a reference to the destructed tmp object.

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();

This example is equivalent to the first example. The move operation on tmp is unnecessary and can hinder performance by inhibiting return value optimization (RVO).

Best Practice

The recommended practice for returning objects by value is to simply return the object without explicitly using move semantics. In this case, the compiler will automatically optimize the return using RVO or move semantics, whichever is more efficient.

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

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

This code snippet returns the temporary object directly, allowing the compiler to decide on the optimal optimization strategy.

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