Elegantly call C++ from C

Question

We develop some project in plain C (C99). But, we have one library as source codes (math library) in C++. We need this library so I would like to a

Answer 1

The only reason to want to inherit from the duck struct would be to expose some to its attributes in the C API, which is generally considered bad style anyway. Without inheritance, your C header would look like this:

struct Duck;

struct Duck* new_Duck(int feet);
void delete_Duck(struct Duck* d);
void Duck_quack(struct Duck* d, float volume);

And this would be the corresponding implementation, with no need for type casts:

extern "C" {
#include "Duck.h"
}

class Duck {
public:
    Duck(int feet) : {}
    ~Duck() {}

    void quack(float volume) {}
};

struct Duck* new_Duck(int feet) { return new Duck(feet); }
void delete_Duck(struct Duck* d) { delete d; }
void Duck_quack(struct Duck* d, float volume) { d->quack(volume); }

In the same way, a C API can be created for a C++ interface (pure virtual class) and its implementations. In that case, only the constructor need to be based on the concrete implementation (e.g. new_RubberDuck(2)). The destructor and all other functions will automatically operate on the correct implementation, same as in C++.