How to use lock_guard when returning protected data

Question

I have a question concerning the use of boost::lock_guard (or similar scoped locks) and using variables that should be protected by the lock in a return statement.

How is the order of destroying local objects and copying the return value? How does return value optimization affect this?

Example:

Data Class::GetData()
{
    boost::lock_guard<boost::mutex> lock(this->mMutex);
    return this->mData;
}

Would this be correct (if mData is the variable protected by mMutex)? Or would I have to use a local scope and a temporary like shown in the example below:

Data Class::GetData()
{
    Data ret;
    {
        boost::lock_guard<boost::mutex> lock(this->mMutex);
        ret = this->mData;
    }
    return ret;
}

Answer 1

Just a straight return as in your first example is correct. The return value is constructed before the local variables are destroyed, and thus before the lock is released.