Sample code to demonstrate how to implement a C++ object in plan C; Works only in Microsoft VS - where the vtable starts with the function pointers, as used by COM objects

PureFromPlainC.cpp

#include <new>

///////
// Sample code to demonstrate how to implement a C++ object in plan C
// Works only in Microsoft VS - where the vtable starts with the function
// pointers, as used by COM objects
///////


// The C++ interface we wish to implement
class ICppA {
public:
    virtual ~ICppA() {}
    virtual double add(double v) = 0;
    virtual double sub(double v) = 0;
    virtual double get() = 0;
};

// A real C++ implementation, used for comparison
class CppA : public ICppA {
private:
    double total = 0;
public:
    virtual ~CppA() override {}
    virtual double add(double v) override { return (total += v); }
    virtual double sub(double v) override { return (total -= v); }
    virtual double get() override { return total ; }
};

// The plain C implementation of the C++ interface
extern "C" {

typedef struct CA CA;

// The plain C vtable
struct CA_VTable {
    void (*dtor)(CA* self);
    double (*add)(CA* self, double v);
    double (*sub)(CA* self, double v);
    double (*get)(CA* self);
};

// The plain C implementation
struct CA {
    CA_VTable* _vtable;
    double total;
};

void CA_DTor(CA* o) {}
double CA_Add(CA* o, double v) { return (o->total += v); }
double CA_Sub(CA* o, double v) { return (o->total -= v); }
double CA_Get(CA* o) { return o->total; }

// Create a static vtable that all instances can use
static CA_VTable _CA_VTable = {
    CA_DTor,
    CA_Add,
    CA_Sub,
    CA_Get
};

void CA_CTor(CA* ca) { ca->_vtable = &_CA_VTable; ca->total = 0; }

} // extern "C"


// our objects should have a vtable pointer and a double
static_assert(sizeof(CppA) == sizeof(void*) + sizeof(double), "false");
static_assert(sizeof(CA) == sizeof(void*) + sizeof(double), "false");

// demonstrate that both the real C++ implementation and C implementation behave the same way
#include <iostream>
void testA(ICppA* a)
{
    std::cout << "initial value: " << a->get() << std::endl;
    std::cout << "add 5: " << a->add(5) << std::endl;
    std::cout << "sub 2: " << a->sub(2) << std::endl;
}

int main()
{
    CppA cppa;
    testA(&cppa);

    CA ca;
    CA_CTor(&ca);
    testA((ICppA*)&ca);
    CA_DTor(&ca);
    return 0;
}