Why does this class declaration not work on Visual Studio

Question

So I'm trying to get some code that is written for gcc to compile on Visual Studio 2008. I have a problem that I have narrowed down to this:

class value_t
{
public:
  typedef std::deque<value_t>         sequence_t;
  typedef sequence_t::iterator        iterator;
};

This code fails:

1>cpptest.cpp
1>c:\program files\microsoft visual studio 9.0\vc\include\deque(518) : error C2027: use of undefined type 'value_t'
1>        c:\temp\cpptest\cpptest.cpp(10) : see declaration of 'value_t'
1>        c:\temp\cpptest\cpptest.cpp(13) : see reference to class template instantiation 'std::deque<_Ty>' being compiled
1>        with
1>        [
1>            _Ty=value_t
1>        ]
1>c:\program files\microsoft visual studio 9.0\vc\include\deque(518) : error C2027: use of undefined type 'value_t'
1>        c:\temp\cpptest\cpptest.cpp(10) : see declaration of 'value_t'

However when I try this with std::vector, it compiles fine:

class value_t
{
public:
  typedef std::vector<value_t>        sequence_t;
  typedef sequence_t::iterator        iterator;
};

What's wrong? I have tried adding 'typename' everywhere I can think of, but at this point in time I'm thinking it's just a bug in the Dinkumware STL. Can anyone explain what's happening, and/or offer a solution? Thanks.

Answer 1

Its undefined behavior. See this link on c.l.c++.moderated

Snip from Daniel K's answer :-

the C++ standard (both C++03 and C++0x) says that what you are trying causes undefined behaviour, see [lib.res.on.functions]/2:

"In particular, the effects are undefined in the following cases: [..] — if an incomplete type (3.9) is used as a template argument when instantiating a template component."

Answer 2

I think the problem is that value_t is an incomplete type until you reach the end of the definition. Trying to use an incomplete type as the template parameter for a standard container isn't really supposed to work. It can/will happen to work under some circumstances, but if it failed with all standard container types, that still wouldn't signal any kind of bug. The standard requires it to be a complete type, so if it's not, you get what you get -- it probably should fail consistently, but if it happens to work, there's nothing wrong with that.

Answer 3

You are trying to use a class within itself in a template. How does it resolve this? I don't know that I have ever tried to do this, but is this even possible? I don't know why it works for std::vector, but my assumption is that it is wrong. You are defining a class, and using that definition in the definition. Seems wrong to me. Good luck on this one, I'll be interested to see some deeper answers myself...