Call C++ virtual functions through C struct of function pointers - c++

Question

I'm creating a C-wrapper for a C++ class:
class IObject {
public:
virtual int getValue() const;
virtual ~IObject() = default;
};
class Object: public IObject {
public:
virtual int getValue() const { return 123; }
};
Inspired openh264's C API, I have the following code:
// object.h
#pragma one
#ifdef __cplusplus
class IObject {
public:
virtual int getValue() const = 0;
virtual ~IObject() = default;
};
extern "C" {
#else
typedef struct IObjectVtable IObjectVtable;
typedef IObjectVtable const* IObject;
struct IObjectVtable {
int (*getValue)(IObject*);
};
#endif
int makeObject(IObject**);
int freeObject(IObject*);
#ifdef __cplusplus
} // extern "C"
#endif
// object.cpp
#include "object.h"
class Object : public IObject {
public:
virtual int getValue() const override { return 123; }
};
extern "C" {
int makeObject(IObject** obj) {
*obj = new Object;
if (*obj == nullptr) {
return 1;
}
return 0;
}
int freeObject(IObject* obj) {
delete obj;
return 0;
}
}
In main.c, I use Object as follows:
#include <stdio.h>
#include "object.h"
int main() {
IObject* obj;
makeObject(&obj);
printf("%d\n", (*obj)->getValue(obj)); /** Calling getValue */
freeObject(obj);
return 0;
}
The code compiles and correctly prints "123", but I need to pass obj as an argument of getValue(). Is it possible to avoid this, i.e., simply call obj->getValue()?
According to the documentation, openh264 can do that, but I don't understand how/why it works:
ISVCDecoder *pSvcDecoder; //similar to IObject* obj;
WelsCreateDecoder(&pSvcDecoder); //similar to makeObject(&obj);
pSvcDecoder->Initialize(&sDecParam); //similar to obj->getValue();
// not (*obj)->getValue(obj)