我可以为非const参考参数赋值吗？

Question

In the following (legacy) code block, is the assignment to popped_value valid or should there be a memcpy ?

bool peek(value_type& popped_value)
    {
    bool retval=false;
    RWLock::ScopedReadLock lock(queueLock);
    if ( ! m_queue.empty())
        {
        popped_value=m_queue.front(); //question...
        retval=true;
        }
    return retval;
    }

I don't want to change the parameter to a pointer.

Answer 1

The only place you can set the object that a reference variable references is when the reference variable is created:

• In the case of a global or local variable, the Type & ref = value; statement that defines the variable,
• In the case of a non-static data member of some class, the initialization list in a constructor for that class, or
• In the case of an argument to some function that is a reference, in the statement that calls that function.

At any other time, you are not changing the thing that reference variable references when you assign a value to that reference, eg, reference_variable = value; . You are instead changing the value of the thing to which that reference variable refers.

That almost certainly is exactly the desired behavior in the referenced block of code.

Answer 2

That's fine, and it's how I would expect that to be written.

If value_type is a custom class, then some value_type operator=(...) will be called. Presumably the implementation of this method is sane.