How do You Export Classes Containing std:: Objects from a DLL?

Exporting Classes Containing std:: Objects from a DLL

When attempting to export classes containing std:: objects, such as vectors or strings, from a DLL, you may encounter warnings about the need for a dll-interface. This warning is triggered when the compiler detects that members of complex types within the class are not explicitly marked for DLL export.

Forward declaring the member types with DLL_EXPORT can partially address the issue. However, to ensure proper export of member functions, the corresponding classes in their compilation unit must also be marked as DLL_EXPORT.

This requirement stems from the fact that the compiler cannot determine which methods of the class are utilized by clients of the DLL_EXPORTed class. Therefore, it enforces that all accessible methods must be exported. Ignoring the warning and disabling it is not advised in this context.

To address the warning for non-dll-exportable members, the following approaches can be considered:

• Disable the warning: This is appropriate for private members or members that cannot be accessed by clients.
• Create wrappers or indirection methods: This involves creating DLL_EXPORT wrapper methods or creating indirection methods, effectively hiding the internal implementation from clients.
• Use PIMPL: This idiom helps reduce the number of externally visible members, minimizing the issue.

In the example provided, forward declaring the template specialization std::allocator with DLL_EXPORT generates an instantiation of the template specialization within the current compilation unit. This creates the methods for std::allocator in the DLL, including their exports. However, this approach is not appropriate for concrete classes, as it only supports instantiation of template classes.

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