ostringstream is an undefined type?

Question

In my C++ project I'm trying to do this:

 std::ostringstream stream(std::ostringstream::out);

But I'm getting an error:

error C2027: use of undefined type 'std::basic_ostringstream<_Elem,_Traits,_Alloc>'

I'm already including iostream and ostream libraries so I'm not sure why it's treating ostringstream as undefined...