[Info-vax] Groovy on VMS

[Arne Vajhøj]

On 3/5/2024 7:34 PM, Arne Vajhøj wrote:
> On 3/5/2024 5:53 PM, Lawrence D'Oliveiro wrote:
>> On Tue, 5 Mar 2024 17:48:21 -0500, Arne Vajhøj wrote:
>>> But I consider:
>>>
>>> o->f(a1, a2, a3) to be C++
>>
>> If you can refer to o as “this”, that’s C++. You can’t do that in C.
> 
> But this is not available by magic. It is there because
> it (at least in some implementations - I doubt that the
> standard define the "how") pass it as a hidden 1st argument.

To illustrate:

$ type m.cxx
#include <iostream>

using namespace std;

class C
{
     public:
        virtual void M1(int v) = 0; // C++ API in C++
        virtual void M2(char *s) = 0; // C++ API in C++
};

class C1 : public C
{
     public:
         void M1(int v) { cout << "C1 says: " << v << endl; }
         void M2(char *s) { cout << "C1 says: " << s << endl; }
};

class C2: public C
{
     public:
         void M1(int v) { cout << "C2 says: " << v << endl; }
         void M2(char *s) { cout << "C2 says: " << s << endl; }
};

extern "C"
{
     void f(C **o);
}

int main()
{
     C *o1 = new C1();
     f(&o1);
     delete o1;
     C *o2 = new C2();
     f(&o2);
     delete o2;
     return 0;
}

$ type s.c
#include <stdio.h>

struct C;

typedef void (*fptr1)(struct C **this, int v); // C++ API in C
typedef void (*fptr2)(struct C **this, char *s); // C++ API in C

struct C_vtable
{
     fptr1 M1;
     fptr2 M2;
};

struct C
{
     struct C_vtable *vtable;
};

void f(struct C **o)
{
     (*o)->vtable->M1(o, 123);
     (*o)->vtable->M2(o, "ABC");
}
$ cxx m
$ cc s
$ cxxlink m + s
$ run m
C1 says: 123
C1 says: ABC
C2 says: 123
C2 says: ABC

Arne