Implementing a C++ (pure) interface using plain C

Question

Given a pure C++ class:

class ICppA {
public:
    virtual ~ICppA() {}
    virtual double add(double v) = 0;
    virtual double sub(double v) = 0;
    virtual double get() = 0;
};

Are there ways to create implementations of this interface in plain C in clang and gcc?

I understand the C++ standard does not cover the vtable structure, and I understand this may not be a good idea. I am looking for compiler specific, probably non-portable solutions.

Example for MSVC

The following works for MSVC, (but is not portable):

typedef struct CA CA;

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

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

void CA_DTor(CA*) {}
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; }

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

[EDIT] It seems like the code above will work on clang and gcc if the vtable contains an extra pointer before the first method, i.e.

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

[...]

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

(I am still reading to figure out what that unknown pointer is and if the solution then becomes reliable)