LinuxQuestions.org - calling global structs

- Programming (https://www.linuxquestions.org/questions/programming-9/)

- - calling global structs (https://www.linuxquestions.org/questions/programming-9/calling-global-structs-215803/)

calling global structs

Im the progress still of porting C++ to C, yes yes yes I know, but ran across another problem.

I'm making each C++ class into a C struct,
and I need to know how to inherit another struct like one would inherit a class in C++.

Thanks guys! (and gals)

You can't meld structs together, but you can nest them:

Code:

struct base_struct

{

  int some_stuff;

  char *some_more_stuff;

};



struct lesser_struct

{

  int yet_more_stuff;

  struct base_struct base_stuff;

};

Is that what you were looking for?

Yea I believe that it is! Let me give it a try. thanks

mmmmm let me see if I got this

FILE 1

Code:

struct THEA A

{

    int blah ;

    int more crap;

};

FILE2

Code:

struct THEB B

{

    int look;

    struct THEA A;

}



............ later function the struct THEB B, was passed in



A->blah = the_result;

        OR

B->blah = the_result

neither work so ...... mmmmmmmmmmm what am I doing wrong

You need to declare your structures like:

Code:

struct THEA

{

  int blah;

  int more_crap;

};



struct THEB

{

  int look;

  struct THEA A;

};



int main(void)

{

  struct THEB B;



  B.look = 1;

  B.A.blah = 2;

  B.A.more_crap = 86;



  return 0;

}

You can't use one of THEA's members directly using B. You have to step through the nesting like that. You could of course pass &B.A to a function then use A->blah or A->more_crap in that function. But you can't do something like B.my_crap in my example above. You have to do B.A.more_crap.

Here's a simple example. Say you're tracking book sales. Every book has a name and a number of pages and so forth. So you set up a book struct like so:

Code:

struct book_base

{

  char *name;

  int num_pages;

};

Now, normally you'd set up array of struct book_base, one element for each book you're tracking. So say you had 3 books. You might do something like:

Code:

{

  struct book_base books[3] =

  {

    { "Book 1", 213 }, { "Book 2", 99 }, { "Book 3", 792 }

  };

}

Now let's say you have 50 of each book and you want to track where they are. You could do something like this:

Code:

struct indiv_book

{

  int sold;              // 0 = not sold, still in store; 1 = sold

  time_t sell_date;//  0 if not yet sold otherwise time_t value of when sold

  struct book_base *book; // Pointer to book information

};

Then you might do something like:

Code:

{

  struct indiv_book book[150];  // 50 each of 3 books



  book[0].sold = 0;  // Not sold yet

  book[0].sell_date = 0; // Set to time(0) when sold

  book[0].book = &books[1]; // This particular book is a "Book 2".

}

Then you could see the name of book[0] that you're tracking by doing: printf("%s\n", book[0].book->name);

Or you could check how many pages book[0] has by looking at book[0].book->num_pages.

Clear as mud? :)

almost but Im trying to set a variable from the first struct not something from the second struct.

Then you would do like: book[0].book->name = "new name";

EDIT: Or you could do books[1].name = "new name"; or course if you wanted to change it directly without going through an indiv_book struct instance.

hi!

ever tried something like this:

Code:

struct base_struct

{

  int some_stuff;

  char *some_more_stuff;

};



struct lesser_struct : public base_struct

{

  int yet_more_stuff;

};

(Thanks itsme86 for the example)
I have learned that in C++, structs are EXACTLY like classes except that the default visibility is public instead of private.
So, you also could have used classes instead.

So long...
bruce