Objects in C - Structs Syntax Related

Question

I was recently reading about ways to create something that resembles c++ objects, but in C, and I came across some pretty good examples. However, there was this single piece of code that had me thinking for hours, since it was the first time I saw this kind of syntax, and I didn't find anything like it on Google...

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The block itself is this one:

struct stack {
     struct stack_type * my_type;
     // Put the stuff that you put after private: here
};
struct stack_type {
     void (* construct)(struct stack * this); // This takes uninitialized memory
     struct stack * (* operator_new)(); // This allocates a new struct, passes it to construct, and then returns it
     void (*push)(struct stack * this, thing * t); // Pushing t onto this stack
     thing * (*pop)(struct stack * this); // Pops the top thing off the stack and returns it
     int this_is_here_as_an_example_only;
}Stack = {
    .construct = stack_construct,
    .operator_new = stack_operator_new,
    .push = stack_push,
    .pop = stack_pop
};

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Assuming all the functions being set to the pointers are defined somewhere else, my doubts are the following:

1) Why does the point ( '.' ) mean, or whats its purpose when initializing the function pointers? (example: .construct = stack_construct )

2) why is there an equal sign after 'Stack' at the end of the struct definition, and why is there something other than a ';' ( in this case the word 'Stack' ), given the fact that there is no typedef at the beginning? I assume it has something to do with initialization (like a constructor, I don't know), but it is the first time I see a struct = {...,...,...} in the definition. I've seen that when you initialize a struct like in the following example:

typedef struct s{
int a;
char b;
}struc;

void main(){
    struc my_struct={12,'z'};
}

But this is not in the main declaration of the struct, and still, there are no '=' within the {} , unlike the first example, where it showed something like...

struc my_struct={ a = 12,
                  b = 'z'};

3) This is a minor doubt, meaning, I'm much more interested in the first two. Anyway, here it goes... At the beginning of the first code, it says something like '// Put the stuff that you put after private: here'. Why is that? how would that make them private?

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That is all, I would appreciate anything, this had me thinking for hours! Thanks in advance, have a great day!

Answer 1

1) Don't worry about the members being functions, it's just this:

What does dot (.) mean in a struct initializer?

2) In

struct S {int i;}
s = {0};

the first line names a type that can be used to declare and initilize a variable.

It is really equivalent with something like

double d = 1;

where you replace double with struct S {int i;}, replace s with d, and replace 1 with the initializer for a struct, {0}. Now combine this with the dot syntax.

It just has the side effect of also defining struct S.

Update: Regarding 3), I do not think that with private they refer to an implementation of access control, but rather refer to the fact that in C++, you would usually list the data members in the private section of the class, so I understand this as instructions to add the data members after the type identifying element.