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Using dllimport and dllexport in C++ Classes

You can declare C++ classes with the dllimport or dllexport attribute. These forms imply that the entire class is imported or exported. Classes exported this way are called exportable classes.

The following example defines an exportable class. All its member functions and static data are exported:

#define DllExport   __declspec( dllexport )

class DllExport C {
   int i;
   virtual int func( void ) { return 1; }
};

Note that explicit use of the dllimport and dllexport attributes on members of an exportable class is prohibited.

When you declare a class dllexport, all its member functions and static data members are exported. You must provide the definitions of all such members in the same program. Otherwise, a linker error is generated. The one exception to this rule applies to pure virtual functions, for which you need not provide explicit definitions. However, because a destructor for an abstract class is always called by the destructor for the base class, pure virtual destructors must always provide a definition. Note that these rules are the same for nonexportable classes.

If you export data of class type or functions that return classes, be sure to export the class.

When you declare a class dllimport, all its member functions and static data members are imported. Unlike the behavior of dllimport and dllexport on nonclass types, static data members cannot specify a definition in the same program in which a dllimport class is defined.

All base classes of an exportable class must be exportable. If not, a compiler warning is generated. Moreover, all accessible members that are also classes must be exportable. This rule permits a dllexport class to inherit from a dllimport class, and a dllimport class to inherit from a dllexport class (though the latter is not recommended). As a rule, everything that is accessible to the DLL's client (according to C++ access rules) should be part of the exportable interface. This includes private data members referenced in inline functions.

Because member functions and static data within a class implicitly have external linkage, you can declare them with the dllimport or dllexport attribute, unless the entire class is exported. If the entire class is imported or exported, the explicit declaration of member functions and data as dllimport or dllexport is prohibited. If you declare a static data member within a class definition as dllexport, a definition must occur somewhere within the same program (as with nonclass external linkage).

Similarly, you can declare member functions with the dllimport or dllexport attributes. In this case, you must provide a dllexport definition somewhere within the same program.

It is worthwhile to note several important points regarding selective member import and export:

  • Selective member import/export is best used for providing a version of the exported class interface that is more restrictive; that is, one for which you can design a DLL that exposes fewer public and private features than the language would otherwise allow. It is also useful for fine-tuning the exportable interface: when you know that the client, by definition, is unable to access some private data, you need not export the entire class.

  • If you export one virtual function in a class, you must export all of them, or at least provide versions that the client can use directly.

  • If you have a class in which you are using selective member import/export with virtual functions, the functions must be in the exportable interface or defined inline (visible to the client).

  • If you define a member as dllexport but do not include it in the class definition, a compiler error is generated. You must define the member in the class header.

  • Although the definition of class members as dllimport or dllexport is permitted, you cannot override the interface specified in the class definition.

  • If you define a member function in a place other than the body of the class definition in which you declared it, a warning is generated if the function is defined as dllexport or dllimport (if this definition differs from that specified in the class declaration).

END Microsoft Specific

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