I know that you can do objects in C by having a struct with the data and functions that take the struct as their first parameter. I was wondering if it is possible somehow to do struct inheritance in C?

I thought about the derived struct having a pointer to it's parent, but that won't work because for example we have the struct Shape and a function that takes Shape as it's parameter. Now what if we create a derived struct for Shape called Rectangle. How can you pas the Rectangle to the Shape function?

You can give the Shape function the Rectangle's pointer, but what if you do not know the exact hierarchy? The code will soon be a big, hard-to-understand mess.

exactly, with pointer, you get one important problem you just said. poorly
documented mess. i don't see an other way to do.

you can do by hand hertigace by pointing a function on an other struct's function. but that sounds silly thing to do.

Reading the Wikipedia article on "C++ classes" said that most C++ compilers implement inheritance by concatenating the derived class's members to the base class's, so the relative index in memory is the same wether you get the base or derived class.

Here's a diagram:

So if you pass an instance of struct B to a function that takes a struct A, you still get the right variable because the index is the same.

But I don't know how to make an elegant implementation of this in C, especially when the instance variables are of different sizes.

EDIT: And I read in another article that it is a bad idea for the struct to have function pointers because you get huge amounts of redundant data if you have a lot if instances.

In C++ you can pass a pointer (or reference) to class B to a function that takes a pointer to class A. That can be replicated in C, except that you will have to manually cast or the compiler will complain about type mismatches.

You just need one function table for each class, and every instance needs only the one pointer to this table (this is called the vtable in C++).

Review the plain-C function family for fopen( ) fclose( ) fread( ) and fwrite( ). All of these hang off a common struct{ }.

In your stated case with struct A{ } and then struct B{ struct A; ...}. I believe this is an example of a "uses" relationship -- specifically,
B uses A.

I have included a void * as a struct member and then loaded the items with a pointer to various A1, ..., An structs. A second member would indicate which of the various A-items was in place. Passing B around this way, when I needed the contents of the A-item, I would cast the void * to be the explicit A-item type that I needed.

Written plain-C since the early seventies.
Used C++ as "a better C compiler" since the late nineties.

~~~ 0;-Dan