Selective Member Import/Export

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).