How to create a shared_ptr in dll and export it via a factory function?

c++, dll, shared-ptr

Solution

Straight to the point, to return C++ object from C function - just returns it via output arguments:

extern "C"
{
     DLL_API void CreatreMyClassWrapper(SIZE inputSize, SomeClass* outputPtr)
     {
          *outputPtr = SomeClass(....);
     }
}

In your example `SomeClass` == `MyClassWrapperPtr`, so:

extern "C"
{
     DLL_API void CreatreMyClassWrapper(SIZE inputSize, MyClassWrapperPtr* outputPtr)
     {
          *outputPtr = make_shared<MyClassWrapper>(inputSize);
     }
}

Consider however to change your interface a little, because in current shape you need to be sure that your applications and DLLs shall use the same compiler, linker, settings, libraries...*

You might want to export `Create` and `Delete` from your DLL to be sure memory management will occur in your DLL (this is based on this answer:

DLL

extern "C"
{
     DLL_API MyClassWrapper* CreateMyClassWrapper(SIZE inputSize)
     {
          return new MyClassWrapper(inputSize);
     }
     DLL_API void DeleteMyClassWrapper(MyClassWrapper* wrapper)
     {
          delete wrapper;
     }
}

Application

     shared_ptr<MyClassWrapper> myClassWrapper(CreateMyClassWrapper(inputSize), 
                                               DeleteMyClassWrapper);

Problem

I have this pieces of code: ``` class DLL_API MyClassWrapper { private: MyClass * m_myClass; public: MyClassWrapper(SIZE inputSize); ~MyClassWrapper(); inline int OutputSize(); } typedef std::shared_ptr<MyClassWrapper> MyClassWrapperPtr; extern "C" { DLL_API MyClassWrapperPtr CreatreMyClassWrapper(SIZE inputSize) { return std::make_shared<MyClassWrapper>(inputSize); } } ``` But it doesn't work, with error: ``` Error 1 error C2526: CreatreMyClassWrapper: C linkage function cannot return C++ class 'std::shared_ptr<_Ty>' ``` I understand the problem, but how can I fix it? The options that I can see are: 1- Don't pass a shared pointer. which means that DLL user should delete the pointer after they used it. 2- Don't use extern "C" : which means that I must use mangled names. Is there any other solution?

Original source

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