Question

I have been writing some code for a lib and experimented with a default Win32 console application to get everything running. Since I have completed all classes I wanted to extract everything into a DLL and therefore I started the adaptation with the usual macro:

#ifdef MYLIB_EXPORTS
    #define DllExport __declspec(dllexport)
#else
    #define DllExport __declspec(dllimport)
#endif

I am using one interface in my code which is defined like this:

__interface DllExport ISerializable {
    void Serialize(/* ... */);
    /* some other methods */
};

And this has worked while gaving this code in my exe. In the DLL I get an error during compilation which states

error C2039: '=' : is not a member of 'MyLib::ISerializable'
error C2664: 'MyLib::DerivedClass::operator =' : cannot convert parameter 1 from 'const MyLib::ISerializable' to 'const MyLib::DerivedClass &'

for every class which inherits ISerializable to implement the required methods. (I am using std::shared_ptr<ISerializable> a few times to have abstraction in my code.) However when I change __interface to class and make all methods pure virtual I do not get this error and compilation succeeds.

Why do I get this error? Why does my class/interface in my DLL need the assignment operator? Is there any workaround?

(Using Visual Studio 2012 RTM on Windows 8 RTM with C++11.)

---

Here is one segment where this error occurs (error point always to the last } of the class):

class DllExport Tile final : public ISerializable {
public:
    __declspec(property(get=GetIsPassable, put=SetIsPassable))
    bool IsPassable;
    __declspec(property(get=GetTileId, put=SetTileId))
    uint16_t TileId;

    bool& GetIsPassable() { return this->_IsPassable; }
    void SetIsPassable(bool val) { this->_IsPassable = val; }
    uint16_t& GetTileId() { return this->_TileId; }
    void SetTileId(uint16_t val) { this->_TileId = val; }

    bool _IsPassable;
    uint16_t _TileId;

    void Serialize(OutputFileStream& ofs);
    size_t TellSize();
    size_t Unserialize(InputFileStream& ifs, size_t metadata = 0);
};

This error also occurs in a class where I have a property like in the Tile class where I use a std::shared_ptr<ISerializable>.

Answer 1

I guess interfaces don’t have compiler-generated copy constructors or assignment operators.

One possible solution is to explicitly implement DerivedClass::operator=. That’s because the compiler-generated version will try to call ISerializable::operator=, which doesn’t exist. Same thing for copy constructors.

Another solution is to make all your classes COM classes 🙂

---

Example

Using your Tile class:

class DllExport Tile final : public ISerializable { 
public: 
    Tile(const Tile& tile) :
        _IsPassable(tile._IsPassable), _TileId(tile._TileId)
    {
    }

    /* New Code START */
    Tile& operator=(const Tile& tile)
    {
        _IsPassable = tile._IsPassable;
        _TileId = tile._TileId;
        return *this;
    }
    /* New Code END */

    __declspec(property(get=GetIsPassable, put=SetIsPassable)) 
    bool IsPassable; 
    __declspec(property(get=GetTileId, put=SetTileId)) 
    uint16_t TileId; 

    bool& GetIsPassable() { return this->_IsPassable; } 
    void SetIsPassable(bool val) { this->_IsPassable = val; } 
    uint16_t& GetTileId() { return this->_TileId; } 
    void SetTileId(uint16_t val) { this->_TileId = val; } 

    bool _IsPassable; 
    uint16_t _TileId; 

    void Serialize(OutputFileStream& ofs); 
    size_t TellSize(); 
    size_t Unserialize(InputFileStream& ifs, size_t metadata = 0); 
};