Creating Qt plugin and using non-virtual functions - c++

Question

First of all I use VS 2008 and dynamic build of Qt 4.7.0.
interface.h:
class PluginInterface
{
public:
virtual void foo() = 0;
};
Q_DECLARE_INTERFACE(PluginInterface, MY_PLUGIN_VERSION)
And I have and class which implements this interface:
myplugin.h
class MyPlugin: public QObject, public PluginInterface
{
public:
Q_OBJECT
Q_INTERFACES(PluginInterface)
virtual void foo(); // this functions is implemented in cpp file.
static QString goo(); // this function is also implemented in cpp file.
};
So when I use this plugin in other project(MySpecialPlugin) which will be compiled as dynamic-link library(actually also a plugin interface implementation) I can't call
MyPlugin::goo();
The header of "MyPlugin.h" is visible for MySpecialPlugin. And Ms VS2008 successfully compile the project. But I have error on link step LNK2001, undefined reference. But when change static void goo(); to virtual void goo(); it works.
I'm using Q_EXPORT_PLUGIN2 macro for creating plugins.
What I'm doing wrong? Does Qt set additional requirements on implementation of plugin interfaces?

Answer 1

First of all you need to export your class (on windows it's magic like __declspec(dllexport) after class keyword when compiling your "myplugin" and __declspec(dllimport) when compiling code using it (in this case MySpecialPlugin). You can achieve it with simple define. read more
#ifdef _WIN32
#ifdef MYPLUGIN_COMPILE
#define MYPLUGIN_EXPORT __declspec(dllexport)
#else
#define MYPLUGIN_EXPORT __declspec(dllimport)
#endif
#else
#define MYPLUGIN_EXPORT
#endif
class MYPLUGIN_EXPORT MyPlugin
{
// implementation
};
And add MYPLUGIN_COMPILE to preprocessor section of MyPlugin
You need to link against exporting library
Calling static and "normal" methods differs from calling virtual methods... static and "normal" methods are always at certain address and your code needs to know where is it... virtual call "checks" class vtable at what address your method has implementation and get that address "dynamicly" from that table. Additionaly win32 VC platform requires to define that something is "exported" (exposed) to access from outside of dll. So basicly if you want to call anything from MyPlugin directly (every member that is not virtual is called directly), you have to export it on win32 VC platform. Note that calling directly virtual method with MyPlugin::foo() will cause same error