Requirements for elements in std::unordered_set

Question

std::unordered_set<my_type> my_set;

Which requirements must my_type fulfill here? (Besides a specialization for std::hash)

Chaffee

You can just look this up in n3337 [unord.req] 23.2.5. Find n3337 on this page. – Fionnula

Answer 1

You need a specialization for std::hash, and you need an operator == defined to handle hash collisions.

EDIT: You should also make sure your type has a copy constructor (or let the compiler generate one for you) because STL containers have value semantics.

EDIT2: as an example of how to do this, you can check out this other SO answer.

Exsect

What about move/copy constructors and assignment operators? – Chaffee

Yeah you'll probably need those too. But more directly, the type must be hashable and equality comparable. – Exsect

So move doesn't do the job? (I kind of hoped it would, since vector etc. go fine with it..) – Chaffee

Moving would do the job if you are moving the object into the container. However, moving isn't a requirement because if the object cannot be moved, but can still be copied, then you can use it in the container. – Exsect

I meant no copy but only move. ;) – Chaffee

Oh! Well it looks like unordered_set has an insert( ) overload for rvalues. So moving instead of copying should be okay. Test it! – Exsect

Well, I could've tested it before, I'm asking here to be sure it is compliant. – Chaffee

@Chaffee : If your type is not copyable then the std::unordered_set<> instance will also not be copyable. I.e., move-only is fine as long as you're okay with the std::unordered_set<> instance also being move-only. – Lamm

-1: "You should also make sure your type has a copy constructor" C++11 containers are moveable as well. And containers don't have to be copyable; they're only copyable if the contents are copyable. – Debi

Also -1: Exactly which of move constructor, copy constructor, move assignment, and copy assignment are needed are exactly what I was looking for! – Conceal