移动构造函数和预增量与后增量

Question

In C++, if you have a for loop that "copies" objects of a user defined type using a move constructor, does it make any difference if you use ++i or i++ as the loop counter?

I know this question seems rather vague, but I was (I believe) asked this in a phone interview. I wasn't sure if I understood the question correctly, and the interviewer took this as my not knowing the answer, and cut the interview short.

What could he have been getting at?

Answer 1

In C++, if you have a for loop that "copies" objects of a user defined type using a move constructor [...]

First of all, a move constructor is used for move-constructing, which usually means you are not "copying": you can realize moving as copying - in fact, a class which is copy-constructible is also move-constructible - but then why defining a move constructor explicitly?

[...] does it make any difference if you use ++i or i++ as the loop counter?

It depends on what i is. If it is a scalar object, like an int , then there is no difference at all.

If i is a class-type iterator, on the other hand, ++i should be more efficient (on a purely theoretical ground), because the implementation of operator ++ will not have to create a copy of the iterator to be returned before the iterator itself is incremented.

Here, for instance, is how stdlibc++ defines the increment operators for the iterator type of an std::list :

_Self&
operator++()
{
    _M_node = _M_node->_M_next;
    return *this;
}

_Self
operator++(int)
{
    _Self __tmp = *this;
    _M_node = _M_node->_M_next;
    return __tmp;
}

As you can see, the postfix version (the one accepting a dummy int ) has more work to do: it needs to create a copy of the original iterator to be refurned, then alter the iterator's internal pointer, then return the copy.

On the other hand, the prefix version just has to alter the internal pointer and return (a reference to) itself.

However, please keep in mind that when performance is concerned, all assumptions have to be backed up by measurement. In this case, I do not expect any sensible difference between these two functions.