Thread: "error: incomplete type is not allowed"

I've been coding for a long time, and never have I faced this error message before. I am writing a compiler for a language that dynamically compiles itself on other platforms.. while the specifications for that are not the problem, it's an odd problem I got when I tried "modularizing" my applications with C++ and classes.

Code:

struct Type ;
  struct Class ;
  struct Function ;
  struct Variable ;
  struct Definition ;
  
  typedef std::deque<std::string> StringList ;
  typedef StringList::iterator RegisteredString;

  typedef std::deque<Type> TypeList ;
  typedef TypeList::iterator RegisteredType;

  typedef std::deque<Class> ClassList ;
  typedef ClassList::iterator RegisteredClass;

  typedef std::deque<Variable> VariableList ;
  typedef VariableList::iterator RegisteredVariable ;

  typedef std::deque<Function> FunctionList ;
  typedef FunctionList::iterator RegisteredFunction ;

  enum Level {
	Private, Public, Protected
  } ;

#include <Type.hpp> // requires class
#include <Variable.hpp> // requires type
#include <Function.hpp> // requires type / variable
#include <Class.hpp> // requires variable / function
#include <Definition.hpp> // requires everything

Type.hpp

Code:

struct TETRA Type 
{
  RegisteredClass rc ;
  dword array_depth ;
  dword flags ;

  operator == (const Type & t) ;
  operator != (const Type & t) ;
} ;

Variable.hpp

Code:

struct TETRA Variable 
{
  RegisteredString name ;
  RegisteredType type ;

  Level level ;

  operator== (const Variable & v) ;
  operator!= (const Variable & v) ;

  operator== (const Function & f) ;
  operator!= (const Function & f) ;

  operator== (const Class & c) ;
  operator!= (const Class & c) ; 

  operator std::string () ;
} ;

Function.hpp

Code:

struct TETRA Function 
{
  RegisteredString name ;
  RegisteredType type ;
  Level level ;

  VariableList arguments ;
  VariableList variables ;

  RegisteredVariable AddVariable (const Variable & v) ;
  RegisteredVariable AddArgument (const Variable & v) ;

  operator== (const Variable & v) ;
  operator!= (const Variable & v) ;

  operator== (const Function & f) ;
  operator!= (const Function & f) ;

  operator== (const Class & c) ;
  operator!= (const Class & c) ;

  operator std::string () ;
} ;

Class.hpp

Code:

struct TETRA Class 
{
  RegisteredString name ;

  VariableList members ;
  FunctionList functions ;

  operator== (const Variable & v) ;
  operator!= (const Variable & v) ;

  operator== (const Function & f) ;
  operator!= (const Function & f) ;

  operator== (const Class & c) ;
  operator!= (const Class & c) ;

  operator std::string() ;

  RegisteredVariable AddMember (const Variable & v) ;
  RegisteredFunction AddFunction (const Function & f) ;
}
;

Definition.hpp ( I don't think it matters, but whatever. )

Code:

struct TETRA Definition
{
  StringList strings ;
  TypeList types ;
  ClassList classes ;

  Definition () ;

  RegisteredType AddType (const Type & t) ;

  inline idx GetTypeIndex (const RegisteredType rt) {
	return (idx) (rt - types.begin()) ;
  }
  
  void ReadTypes (std::istream & is, dword count) ;
  void WriteTypes (std::ostream & os) ;

  RegisteredString AddString (const std::string & s) ;
  
  inline idx GetStringIndex (const RegisteredString rs) {
	return (idx) ( rs - strings.begin () ) ;
  }

  void WriteStrings (std::ostream & os) ;
  void ReadStrings (std::istream & is, dword count) ;

  RegisteredClass AddClass (const Class & c) ;

  RegisteredClass FindClass (const std::string & s) ;
  RegisteredClass FindClass (const RegisteredString s) ;

  void Read (std::istream & is, dword flags) ;
  void Write (std::ostream & os, dword flags) ;

  ~Definition () ;
} ;

I get about ~60 error messages from the headers, usually something like

Compiling with Intel C++ 8.0
String.cpp
c:\Program Files\Microsoft Visual Studio .NET\Vc7/include/deque(59): error: incomplete type is not allowed
_DEQUESIZ = sizeof (_Ty) <= 1 ? 16
^
detected during instantiation of class "std::deque<_Ty, _Ax> [with _Ty=Tetra::Type, _Ax=std::allocator<Tetra::Type>]" at line 49 of "D:\tetra\runtime\Tetra.hpp"

I can paste them all if necessary, I don't want the flood this topic however. Any help is appreciated, and if you have questions about what I am doing in the code just ask. My forte is C and lowish level stuff, I'm really just now starting to dabble in STL and C++.

Thanks in advance.