如何在 DLL 中使用类？

Question

Can I put a class inside a DLL? The class i wrote is this:

    class SDLConsole
{
      public:
             SDLConsole();
             ~SDLConsole(){};
             void getInfo(int,int);
             void initConsole(char*, char*, SDL_Surface*, int, int, int);
             void sendMsg(char*,int, SDL_Surface*);
             void cls(SDL_Surface*);

      private:
              TTF_Font *font;
              SDL_Surface *consoleImg;
              int width, pos, height, line, size, ctLine;
              SDL_Surface* render(char*,int);

};

I know how to load a DLL and use the function inside a DLL, but how can I put a class inside a DLL? Thank you very much.

Answer 1

If you use run time dynamic linking (uses LoadLibrary to load the dll) you cannot access the class directly, you need to declare a interface for your class and create a function that returns a instance of this class, like this:

class ISDLConsole
{
  public:             
         virtual void getInfo(int,int) = 0;
         virtual void initConsole(char*, char*, SDL_Surface*, int, int, int) = 0;
         virtual void sendMsg(char*,int, SDL_Surface*) = 0;
         virtual void cls(SDL_Surface*) = 0;
 };

 class SDLConsole: public ISDLConsole
 {
    //rest of the code
 };

 __declspec(dllexport) ISDLConsole *Create()
 {
    return new SDLConsole();
 }

Otherwise, if you link the dll during load time, just use the information provided by icecrime: http://msdn.microsoft.com/en-us/library/a90k134d.aspx

Answer 2

Solution suggested by bcsanches ,

 __declspec(dllexport) ISDLConsole *Create()
 {
    return new SDLConsole();
 }

If you're going to use this approach as suggested by bcsanches , then make sure that you use the following function to delete your object,

 __declspec(dllexport) void Destroy(ISDLConsole *instance)
 {
       delete instance;
 }

Define such functions always in pair, as it ensures that you delete your objects from the same heap/memory-pool/etc they were created on. See this pair-functions

Answer 3

You can, and all the information you need are on this page and this page :

#ifdef _EXPORTING
   #define CLASS_DECLSPEC __declspec(dllexport)
#else
   #define CLASS_DECLSPEC __declspec(dllimport)
#endif

class CLASS_DECLSPEC SDLConsole
{
    /* ... */
};

All there is left is to define the preprocessor symbol _EXPORTING when building the DLL.

Answer 4

If you want to expose the data in a class, the above solutions won't cut it. You have to slap a __declspec(dllexport) on the class itself in the DLL compilation, and a __declspec(dllimport) in the module that links to the DLL.

A common technique is to do this (Microsoft wizards produce code like this):

#ifdef EXPORT_API
#define MY_API __declspec(dllexport)
#else
#define MY_API __declspec(dllimport)
#endif

class MY_API MyClass {
   ...
};

Then make sure EXPORT_API is defined in the DLL project, and make sure it isn't defined in the module that links to the DLL.

If you create a new DLL project in Visual C++ from scratch, and check the check box "Export symbols", some sample code will be generated using this technique.