Article ID: 107501 - Last Review: April 28, 2005 - Revision: 3.0

You receive a C4226 or a C4236 error message when you compile code to contain the keyword in 32-bit Visual C++

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SUMMARY

The __export keyword provided with the Visual C++ for Windows compiler is obsolete with the Microsoft Visual C++ 32-bit compiler. The equivalent functionality for WIN32 can be achieved by using the keyword __declspec with the dllexport attribute. With Visual C++ 32-bit Edition, version 4.0, compiling code containing the __export (or _export) keyword results in either of the following warnings:

C4236: nonstandard extension used : '__export' is an obsolete keyword, see documentation for __declspec(dllexport)

- or -

C4226: nonstandard extension used : '__export' is an obsolete keyword

Compiling code containing the __export (or _export) keyword with 32-bit Visual C++, versions 1.0 and 2.x, results in the C4226 warning.

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MORE INFORMATION

The __declspec construct is not supported by the tools supplied with Microsoft Win32 Software Development Kit (SDK).

When porting DLL source code from Windows to Win32, replace each instance of __export with __declspec( dllexport ). The __declspec(dllexport) keyword can be used to export data, functions, classes, or class member functions. For example:

   /* exported function */ 
   __declspec( dllexport ) void func();

   /* exported data */ 
   __declspec( dllexport ) int i;

   // exported class
   class __declspec( dllexport ) DLLClass
   {
      ...
   };

   class DLLClass
   {
   public:
       // exported member function
       __declspec( dllexport ) void MemberFunction( void );
   };

The sample code below demonstrates exporting classes and class member functions from a DLL using the __declspec( dllexport ) and __declspec( dllimport ) storage class attributes in the DLL and EXE, respectively.

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Sample Code

   /* DLL Sample: TESTDLL.CPP
   /* Compile options needed: /D"_X86" /MT TESTDLL.CPP /link
   /*                  /DLL /OUT:testdll.dll /implib:testdll.lib
   */ 

   #include <stdio.h>

   class DLLClass
       {
       public:
        // exported member function
        __declspec( dllexport ) void functionA( void ) {
            printf("\nIn Function A of the exported function");
        }
       };

   // exported class
   class __declspec( dllexport) ExportDLLClass
       {
       public:
        void functionB(void) {
           printf("\nIn Function B of the exported class");
        }
       };

   // exported instance of the DLLClass
   __declspec(dllexport) DLLClass test;


   /* Source that calls the DLL Sample: CALLDLL.CPP
   /* Compile options needed: /D"_X86" /D"_CONSOLE" /ML CALLDLL.CPP
   /*                  TESTDLL.LIB
   */ 

   #include <stdio.h>

   class DLLClass
       {
       public:
        // imported member function
        __declspec( dllimport ) void functionA( void );
       };

   class __declspec( dllimport) ExportDLLClass
       {
       public:
        void functionB(void);
       };

   __declspec( dllimport ) DLLClass test;

   void main(void)
   {
      ExportDLLClass TestClass;

      test.functionA();
      TestClass.functionB();
   }

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REFERENCES

For more information on exporting, query on the following words in the Microsoft Knowledge Base:

__export and def and prolog and dllexport

For more information creating DLLs for WIN32 or on the dllexport and dllimport storage class attributes, refer to Chapter 4 of the "Programming Techniques" manual that ships with Visual C++ 32-bit Edition, or search for dllexport or dllimport or export in the Visual C++ Books Online. >From within the Visual Workbench, select Help and choose Keyword Search.

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