why does allocator in c++ need a copy constructor?

Question

It is said here that it's because of exception specification. I do not understand it. Does this question have any relationship with exception specification?

Answer 1

After reading through the tutorial I was a little confused myself by the wording. But I believe it's as simple as this: the tutorial was explaining why the allocator's template header shows

allocator(const allocator&) throw();

and

template <class U> allocator(const allocator<U>&) throw();

even though the copy constructor is fairly useless for an allocator. And the answer was that the specification of an allocator does not allow the constructor to throw exceptions. Therefore the copy constructor public interface defines copy constructors with an exception specification of throw() (does not throw any exceptions) to prevent someone deriving their own allocator with copy constructors which might throw an exception.

See this link for a good description of what an exception specification is if that's what was throwing you. (No pun intended. Really.)

So, they didn't mean that when creating an allocator, you have to provide a copy constructor. They just were pointing out that the specification specifically prohibits you from defining one that throws any exceptions. `

Answer 2

The allocator requires a copy constructor because containers have a copy constructor and will need to copy their allocator in the process.