When exporting classes that contain complex types like std::vector or std::map, it's essential to provide a DLL interface to ensure that clients can access these members. This means declaring the class as DLL_EXPORT and marking member functions and objects as DLL_EXPORT to allow external access.
The warning C4251 indicates that a complex type member doesn't have a DLL interface. This can occur when using std:: container types because the compiler cannot fully determine which methods will be accessed by clients.
Although forward declaring template classes with DLL_EXPORT may seem to suppress the warning, it doesn't actually inject DLL_EXPORT for member functions during compilation. It merely triggers its instantiation within the current compilation unit.
Forward declaration with DLL_EXPORT does not completely address the warning. For members that must be accessed by clients, such indirection can be problematic, potentially preventing inline optimization.
In certain scenarios, it may be reasonable to disable the warning for standard containers if they are available through shared library headers to both clients and the DLL. However, this requires careful consideration of potential issues with assignment operators and copy constructors.
Using standard containers in a DLL interface can be problematic due to the need for DLL interfaces for every potential client-accessible method. As an alternative, it may be better to encapsulate container usage within non-exported helper classes or consider using a static library instead.
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