Question

I’m trying to learn about new feature of C++ namely move constructor and assignment X::operator=(X&&) and I found interesting example but the only thing I quite not even understand but more dissagree is one line in move ctor and assignment operator (marked in the code below):

MemoryBlock(MemoryBlock&& other)
   : _data(NULL)
   , _length(0)
{
   std::cout << "In MemoryBlock(MemoryBlock&&). length = " 
             << other._length << ". Moving resource." << std::endl;

   // Copy the data pointer and its length from the 
   // source object.
   _data = other._data;
   _length = other._length;

   // Release the data pointer from the source object so that
   // the destructor does not free the memory multiple times.
   other._data = NULL;
   other._length = 0;//WHY WOULD I EVEN BOTHER TO SET IT TO ZERO? IT DOESN'T MATTER IF IT'S ZERO OR ANYTHING ELSE IT IS JUST A VALUE.
}

So my question is: do I have to set the value of lenght_ to zero or can I leave it untouched? There won’t be any memory-leak and one expression less afaics.

Answer 1

Because the “moved from” object is still going to be destructed eventually, so you have to leave it in a consistent state. Exactly how you do this depends on your object, of course, and in this case it apparently means nulling out the data pointer and setting the length to zero.