Q85478: INFO: Error C2593 When Archiving an Enumerated Type

Article: Q85478
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbcode kberrmsg kbCompiler kbCPPonly kbVC
Last Modified: 22-JUL-2001

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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++, versions 1.0, 1.5, 1.51, 1.52 
   - *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, versions 1.0, 2.0, 2.1, 4.0 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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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 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : winnt:
Issue type        : kbinfo

=============================================================================

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