How to Export Classes Containing `std::` Objects from a DLL?

Exporting Classes Containing std:: Objects (Vector, Map, etc.) from a DLL

Introduction

Exporting classes containing complex objects like vectors and strings from a DLL raises concerns regarding DLL-interface requirements. This article analyzes the issue and explores solutions for exporting such classes while addressing compiler warnings.

Compiler Warnings

When attempting to export a class that contains std:: objects, the compiler may issue warnings about members requiring a DLL-interface. These warnings indicate that the compiler cannot ensure that the exported class's methods are available to clients using the DLL.

Forward Declarations

One approach to address these warnings is to use forward declarations with DLL_EXPORT. While this may suppress the warnings, it does not actually export the required methods. To fully resolve the issue, the classes containing the std:: objects must be marked as DLL_EXPORT in their compilation unit.

Member Accessibility

Not all members of an exported class need to be DLL_EXPORT. Private members that are not accessible to clients can be ignored or have their warnings disabled. However, members that clients may interact with must export their methods.

Possible Solutions

Depending on the scenario, several approaches can be considered:

• Disable Warnings: For non-exportable members, warnings can be disabled to avoid clutter.
• DLL-Export Wrappers: Create DLL-export wrapper functions or indirection methods for members that need to be exported.
• PIMPL Idiom: Utilize the PIMPL idiom to reduce the number of externally visible members.

Forward Declarations and Instantiation

Forward declaring a template class with DLL_EXPORT does create an instantiation in the current compilation unit. However, this only applies to template specializations and not to concrete classes, which need to be marked as DLL_EXPORT explicitly.

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