Derived classes in C - What is your favorite method?

Question

In my experience in object oriented C programming I have seen two ways to implement derived classes.

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First Method, have a definition of the pa

Answer 1

Second option forces you to write very long names like myobj.parent.grandparent.attribute, which is ugly. First option is better from syntax point of view, but it is a bit risky to cast child to parent - I'm not sure whether is is guaranteed by standard that different structs will have same offsets for similar members. I guess compiler may use different padding for such structs.

There is another option, if you are using GCC - anonymous struct members, which is part of MS extension, so I guess it was originated by some MS compiler and still may be supported by MS.

Declarations look like

struct shape {
    double  (*area)(struct shape *);
    const char    *name;
};

struct square {
    struct shape;           // anonymous member - MS extension
    double          side;
};

struct circle {
    struct shape;           // anonymous member - MS extension
    double          radius;
};

In your "constructor" function you need to specify correct function for calculating area and the enjoy the inheritance and polymorphism. The only problem that you always need to pass explicit this - you cannot just call shape[i]->area().

shape[0] = (struct shape *)new_square(5);
shape[1] = (struct shape *)new_circle(5);
shape[2] = (struct shape *)new_square(3);
shape[3] = (struct shape *)new_circle(3);

for (i = 0; i < 4; i++)
    printf("Shape %d (%s), area %f\n", i, shape[i]->name,
            shape[i]->area(shape[i]));    // have to pass explicit 'this'

Compile with gcc -fms-extensions. I never used it in real-life project but I tested it some time ago and it worked.