FIX: Abstract Class Instantiated as a Temporary Object

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE), included with: Microsoft Visual C++, 32-bit Edition, version 4.0

SYMPTOMS

In certain cases, Visual C++ allows abstract classes to be instantiated as temporary objects. The sample code below demonstrates this.

CAUSE

An abstract class should not be allowed to be instantiated as an object. An abstract class is any class with at least one pure virtual function. This is part of the definition of the C++ language.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Microsoft Visual C++, 32-bit Edition, version 4.1.

MORE INFORMATION

Sample Code

   /* Compile options needed: /c /GX
   */

   class Exception    //abstract class
   {
       int m_nCause;
       char * m_pszMsg;
   public:
       Exception();
       Exception( int, const char * = 0);
       Exception( const Exception &);
       ~Exception();
       inline virtual int Cause() = 0;           //pure virtual function
       inline virtual const char * Msg() = 0;    //pure virtual function
   };

   int Exception::Cause()
   {
       return m_nCause;
   }

   const char * Exception::Msg()
   {
       return m_pszMsg;
   }

   void main()
   {
       try
       {
           int r = Exception(55,"no error").Cause();    //should be an
   ERROR
           throw Exception(-1,"error");                 //should be an
   ERROR
       }
       catch (Exception&e)
       {
       }
   }