"error: incomplete type is not allowed"

This is a discussion on "error: incomplete type is not allowed" within the C++ Programming forums, part of the General Programming Boards category; I've been coding for a long time, and never have I faced this error message before. I am writing a ...

  1. #1
    King of the Internet Fahrenheit's Avatar
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    "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.
    Last edited by w00tsoft; 05-08-2005 at 09:32 PM.

  2. #2
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    The problem is that std::deque<C>::deque probably requires the definition of C to be present. For instance, it might have code such as C* c = new C[45]. The compiler cannot allocate memory without knowing the size of C. And without knowing the full type of C, the compiler doesn't know what fields C has.

  3. #3
    King of the Internet Fahrenheit's Avatar
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    Everytime i've encountered something with circular references before, forward declarations have fixed it. Why not in this example?

  4. #4
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    Everytime i've encountered something with circular references before, forward declarations have fixed it. Why not in this example?
    Try this:
    Code:
    class Car;
    class Apple;
    
    class Apple
    {
    	Car aCar;
    };
    
    class Car
    {
    	Apple anApple;
    };
    
    int main()
    {
    	
    	
    
    	return 0;
    }

  5. #5
    King of the Internet Fahrenheit's Avatar
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    I don't see where i'm doing that... I assume it's in some STL container that I don't understand.

  6. #6
    Toaster Zach L.'s Avatar
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    First file... all the typedefs... then the #includes with the definitions.
    The word rap as it applies to music is the result of a peculiar phonological rule which has stripped the word of its initial voiceless velar stop.

  7. #7
    King of the Internet Fahrenheit's Avatar
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    It's obviously not the typedefs, since what I was doing there is legal. I wrote a small workaround with the typedefs still in front and it worked. I suppose it must be an oddity in Microsoft's implementation of the STL.

    Code:
    struct Type ;
      struct Class ;
      struct Function ;
      struct Variable ;
      struct Definition ;
    
      template<class T> struct Registered { 
    	T * data ; 
    	idx index ; 
    
    	Registered () { 	}
    
    	Registered (const Registered<T> & r)
    	{
    	  data = r.data ;
    	  index = r.index ;
    	}
    
    	Registered(T & d, idx i) {
    	  data = &d ;
    	  index = i ;
    	}
    
    	operator= (const Registered<T> & r) 
    	{
    	  data = r.data ;
    	  index = r.index;
    	}
    
    	T & operator* () const { return *data ; }
      } ; 
      
      typedef std::deque<std::string> StringList ;
      
      typedef StringList::const_iterator StringIter ;
      typedef Registered<std::string> RegisteredString;
    
      typedef std::deque<Type> TypeList ;
      typedef Registered<Type> RegisteredType;
    
      typedef std::deque<Class> ClassList ;
      typedef Registered<Class> RegisteredClass;
    
      typedef std::deque<Variable> VariableList ;
    
      typedef Registered<Variable> RegisteredVariable ;
    
      typedef std::deque<Function> FunctionList ;
      typedef Registered<Function> RegisteredFunction ;

  8. #8
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    Everytime i've encountered something with circular references before, forward declarations have fixed it. Why not in this example?
    The preprocessor expands #include "type.hpp" inplace in the file. The definition of Type uses a RegisteredClass. RegisteredClass is a type alias for typedef ClassList::iterator. ClassList is a type alias for std::deque<Class>. Class hasn't been defined before in your source. So, std::deque gets an incomplete type for one of it's template parameters and likely uses this incomplete type.

  9. #9
    King of the Internet Fahrenheit's Avatar
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    well since I fixed my problem and it works now i'll stop replying after this post, but forward declarations let you do that ;/ it's defined later in the source so it isn't "incomplete"

  10. #10
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    well since I fixed my problem and it works now i'll stop replying after this post, but forward declarations let you do that ;/ it's defined later in the source so it isn't "incomplete"
    The forward declaration only let's you use the type in certain ways, for instance, as a pointer. But some uses of an incomplete type aren't permited. For instance, you can't use sizeof(T) if T is incomplete. You can't define a variable of type T if T is incomplete. These rules make it possible for C++ translation unit to be compiled in one pass. By incomplete, I mena at that specific point in the source. Eventually the type gets fully defined. Other languages such as Java don't hav this restriction.

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