INFO: Error C2593 When Archiving an Enumerated Type

ID: Q85478

The information in this article applies to:

The C/C++ Compiler (CL.EXE), included with:
- Microsoft C/C++ for MS-DOS, version 7.0
- Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
- Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

In ANSI C, enumerated types are synonyms for "int"; however, in C++, enumerated types are distinct integral types but are not integers themselves. Therefore, if e is an enumerated type and archive is of type CArchive, compiling these statements


   archive << e;
   archive >> e;

results in the following error messages from Microsoft C/C++ versions 7.0 and 8.0:

error C2593: 'operator <<' is ambiguous

error C2593: 'operator >>' is ambiguous

The compiler cannot choose which operator to use, because the enumerated type could be promoted to match any integral type. The programmer must specify which integral type the operator should use.

MORE INFORMATION

The following are the prototypes taken from AFX.H. These are the functions that the compiler chooses from when it compiles the statement "archive << e;"


  CArchive& operator<<( BYTE by );
   CArchive& operator<<( WORD w );
   CArchive& operator<<( LONG l );
   CArchive& operator<<( DWORD dw );

where BYTE, WORD, LONG, and DWORD are defined in AFX.H as follows:


   typedef unsigned char    BYTE;   // 8-bit unsigned entity
   typedef unsigned short   WORD;   // 16-bit unsigned number
   typedef long             LONG;   // 32-bit signed number
   typedef unsigned long    DWORD;  // 32-bit unsigned number

Therefore, the following statements compile without error:


   archive << (WORD) e;
   archive >> (WORD&) e;

-or-


   archive << (unsigned short) e;
   archive >> (unsigned short&) e;

NOTE: The following statements do not compile because they are also ambiguous:


   archive << (int) e;
   archive >> (int&) e;

In these cases, the compiler cannot choose which operator to use, because the int could be promoted to match any integral type. Again, the programmer must specify which integral type the compiler should use.

The following is a complete example demonstrating how to store and load an enumerated type:

Sample Code


   /* Compile options needed: none
   */ 

   #define _DOS
   #include <afx.h>

   enum DAY
   {
      sunday, monday, tuesday, wednesday, thursday, friday, saturday
   };

   void main( void );

   void main( )
   {
      CFile myFile;

   // Open the file for writing and associate an archive object with it.

      myFile.Open( "testfile", CFile::modeWrite, NULL );
      CArchive archiveOut( &myFile, CArchive::store );

      enum DAY today = tuesday;

   // Use the insertion operator to store the enumerated value
   //    archiveOut << today;
   // will generate an error

      archiveOut << (WORD) today;

      archiveOut.Close();
      myFile.Close();

   // Open the file for reading and associate an archive object with it.

      myFile.Open( "testfile", CFile::modeRead, NULL );
      CArchive archiveIn( &myFile, CArchive::load );

   // Use the extraction operator to load the enumerated value
   // "archiveIn >> today;" would generate an error

      archiveIn >> (WORD&) today;

      archiveIn.Close();
   }

Additional query words: 8.00 8.00c 9.00 9.10

Keywords : kbcode kberrmsg kbCompiler kbCPPonly kbVC
Version : winnt:
Platform : winnt
Issue type : kbinfo