Computer Science Canada

Pointers to structures
Author:  omni [ Sat Feb 12, 2005 10:18 pm ]
Post subject:  Pointers to structures
This question seems a little newb compared to the other ones on this forum, but anyway ...

struct exmstruct{
int x;
int y;
int z;
}
...

exmstruct jeff;
exmstruct *expointer;
expointer = &jeff;

Ok so where will "expointer" be pointing to? Will it be the jeff.x? And if I do "expointer += 1" will the pointer be pointing to jeff.y after?

EDIT: Not sure if i'm thinking of structure pointers the right way. I am thinking of structure pointers like array and string pointers. Is this right?
Author:  wtd [ Sun Feb 13, 2005 12:21 am ]
Post subject: 
They're entirely the same. A pointer is a pointer is a pointer. It's simply a variable which holds the address in memory of something else.

c++:
struct example_struct
{
   int x, y, z;
};

int main()
{
   // foo is just an example_struct object
   example_struct foo;

   // bar is a pointer to an example_struct object
   example_struct * bar;

   // &foo gets the memory address for foo
   // the rest assigns that memory address to the bar pointer
   bar = &foo;

   // the above could have been better written:
   // example_struct * bar = &foo;

   return 0;
}


Now, theoretically bar points to the first member of example_struct, or x. However, there is no guarantee in C++ that members will be laid out in a given fashion, and memory may be padded in such a way that doing pointer arithmetic of this sort is evil.

The simple answer is: don't do it. Use foo.x, foo.y, foo.z.
Author:  Andy [ Sun Feb 13, 2005 2:20 pm ]
Post subject: 
c++:

struct exmstruct
{
   int x, y, z;
};

int main()
{
    exmstruct jeff;
    exmstruct *expointer;
    expointer = &jeff;
    expointer ->x=0;
    expointer ->y=0;
    expointer ->z=0;
}

u use -> to access the sub variables
Author:  wtd [ Sun Feb 13, 2005 5:32 pm ]
Post subject: 
Andy wrote:
c++:

struct exmstruct
{
   int x, y, z;
};

int main()
{
    exmstruct jeff;
    exmstruct *expointer;
    expointer = &jeff;
    expointer ->x=0;
    expointer ->y=0;
    expointer ->z=0;
}

u use -> to access the sub variables


"members"

Structs in C++ are simply classes where everything's public by default.
Author:  Andy [ Sun Feb 13, 2005 7:52 pm ]
Post subject: 
o oops.. it doesnt work on structs? Embarassed hmmmm heh use classes Rolling Eyes
Author:  md [ Sun Feb 13, 2005 8:30 pm ]
Post subject: 
no no, -> works on structs in C++, but structs are really just classes where everything is public
Author:  Mazer [ Mon Feb 14, 2005 10:13 am ]
Post subject: 
By default. You can still make members protected/private, right?
Author:  wtd [ Mon Feb 14, 2005 12:36 pm ]
Post subject: 
Coutsos wrote:
By default. You can still make members protected/private, right?


Absolutely.

You can subclass structs, too.
Author:  omni [ Mon Feb 14, 2005 7:58 pm ]
Post subject: 
If I want to create an array of strings, would I just make a 2d array?
Author:  Andy [ Mon Feb 14, 2005 8:01 pm ]
Post subject: 
didnt u learn the string class??? u guys didnt do classes???
Author:  omni [ Mon Feb 14, 2005 8:09 pm ]
Post subject: 
I'm self teaching myself this. So there is no "guys" And no I didn't learn string classes or classes in general : /
Author:  Andy [ Mon Feb 14, 2005 8:14 pm ]
Post subject: 
windsor eh? do u go to massey? anyways, you should read up on classes, they are very very useful
Author:  Mazer [ Mon Feb 14, 2005 8:18 pm ]
Post subject: 
omni: yes, a 2D char array will give you an array of strings. That said, character arrays can really be a pain. You should look up the std string class when you get a chance.
Also, find "Thinking in C++" if you can. It's floating around the internet for free and supposedly a good read. (I should read it myself when I have some time).
Author:  omni [ Mon Feb 14, 2005 8:20 pm ]
Post subject: 
@ Andy : yea i go to massey gr.11
@coustos: K i'll look it up

BTw is there any command to clear the screen? using std::cout all the time just makes the screen scroll down and that doesnt really look nice.
Author:  Mazer [ Mon Feb 14, 2005 8:33 pm ]
Post subject: 
As far as I know, yes, but they aren't very good. Ultimately you must ask, do you really need to clear the console screen?
Author:  Andy [ Mon Feb 14, 2005 10:17 pm ]
Post subject: 
system("cls")
u need to include window.h or was it windows.h w.e try both
Author:  wtd [ Tue Feb 15, 2005 12:00 am ]
Post subject: 
Coutsos wrote:
As far as I know, yes, but they aren't very good. Ultimately you must ask, do you really need to clear the console screen?


Indeed, if you do that, how does the user copy and paste output from your program? If they forget to do so and the program clears the screen, that output is gone. Or if it just scrolls the page several lines to mimic clearing the screen the user has a bunch of useless blank lines.

As for an array of strings, don't use C-strings (character arrays) or arrays. Instead use a vector of std::string objects.

code:
#include <vector>
#include <string>

int main()
{
  std::vector<std::string> foo(20);
  // instead of: std::string foo[20];

  return 0;
}


Vectors are beautiful because they permit resizing and keep track of their own size.

:


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