RVO vs. std::move: When Should I Explicitly Use Move Semantics for Return Values in C 11?

How to Utilize C 11 Return Value Optimization and Move Semantics Effectively

When dealing with local variables in return statements, you may encounter the choice between leveraging the compiler's Return Value Optimization (RVO) and explicitly using std::move. This article aims to shed light on the appropriate usage of these techniques.

Understanding Return Value Optimization

RVO is a compiler optimization that allows the compiler to avoid unnecessary copying of objects returned by functions. Instead of copying the object, the compiler directly moves its contents to the return location. This can significantly improve performance by reducing unnecessary memory allocation and copying operations.

In the provided example:

SerialBuffer read( size_t size ) const
{
    SerialBuffer buffer( size );
    read( begin( buffer ), end( buffer ) );
    return buffer;
}

RVO can be applied to this function, as the buffer object is declared locally within the function. The compiler can optimize the return statement to directly move the contents of buffer to the return location, avoiding unnecessary copying.

When to Use std::move

In C , the move constructor is used to transfer the ownership of an object's resources to another object without copying. This process is known as moving. Explicitly calling std::move can be beneficial in certain scenarios:

• When you want to explicitly transfer ownership of the object's resources to the return value.
• When you need to prevent RVO from occurring to avoid potential errors or side effects caused by double-freeing memory.

In the example provided:

SerialBuffer read( size_t size ) const
{
    SerialBuffer buffer( size );
    read( begin( buffer ), end( buffer ) );
    return std::move( buffer );
}

std::move is called in this function to explicitly transfer the ownership of buffer's resources to the return value. This can be useful in cases where you want to guarantee that the object's resources are not owned by multiple objects simultaneously.

Recommendation

As a general rule, it is recommended to let the compiler perform RVO whenever possible. RVO is an efficient technique that can significantly improve code performance without requiring any additional coding effort. Only explicitly use std::move when necessary, such as when you need to explicitly transfer ownership or prevent RVO from occurring.

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