what is the meaing of this typedef line..

transgalactic2 · 10-26-2008, 11:06 AM

i understood your anonymous struct
creating a data type from an anonymous name

but it not the case here:

Code:

typedef struct object{
  int data;
  struct object *left;
 struct object  *right;
}object;

i have a variable called "object"
but there is no anonimus struct
on the contrary i have the first name but not the alias second name
??

transgalactic2 · 10-26-2008, 11:12 AM

so the "object" in the last line unlike normal structs does not represent a variable but a variable type
an alias to
struct object

in a regular struct like this:

Code:

struct mytype {
	int elem1, elem2;
	char elem3;
} object;

object.elem1 = 1;
object.elem2 = 2;       //object represents a variable here unlike my example

tabstop · 10-26-2008, 11:15 AM

Quote:

Originally Posted by transgalactic2

so the "object" in the last line unlike normal structs does not represent a variable but an alias to
struct object

Well, that's what a typedef does. Makes the second thing an alias for the first thing.

transgalactic2 · 10-26-2008, 11:25 AM

regarding this code:

Code:

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

typedef struct { int x; char *y; } odradek;

odradek Example (int x, char *y) {
	odradek a_odek;
	a_odek.x=x;
	a_odek.y=malloc(strlen(y)+1);
	strcpy(a_odek.y, y);
	return a_odek;
}


int main() {
	int x=10;
	char this[]="that would be";
	odradek example=Example(x,this);
	printf("%d: %s\n",example.x,example.y);
	free(example.y); 
}

i know that variable "this" is of type orbarek
but in order to assign values to "this" (in java i used a costructor)
where is it in this code?

itCbitC · 10-26-2008, 11:30 AM

this is an array of characters while a_odek is of the type odradek which is an alias of the complex type you have defined.

transgalactic2 · 10-26-2008, 11:32 AM

aahhh sorryy i thought there is "this" and constructors in C

tabstop · 10-26-2008, 11:32 AM

The variable "this" is of type char[], and it is initialized (remember initialization?) right where it is declared, one line above the line you highlighted. To change its value, you should use strcpy, just as you did in the Example function.

transgalactic2 · 10-26-2008, 11:37 AM

thanks

**Salem** · 10-26-2008, 11:49 AM

> i have a variable called "object"
You have two things, and neither of them are objects.

Code:

typedef struct object{
  int data;
  struct object *left;
 struct object  *right;
}object;

The names in red are the structure tag name, which serves to identify the struct by name. You NEED a tag name if you're creating a self-referencing structure like you're doing here.
Much of the time, the tag name is omitted if the struct is embedded in a typedef declaration (such as this).

The blue name is the name of the typedef name for the struct.
It's a completely different name to the red one.

BOTH can be used to create objects, like so.

Code:

object foo;
struct object bar;

Elysia · 10-26-2008, 12:21 PM

Quote:

Originally Posted by transgalactic2

i understood your anonymous struct
creating a data type from an anonymous name

but it not the case here:

Code:

typedef struct object{
  int data;
  struct object *left;
 struct object  *right;
}object;

i have a variable called "object"
but there is no anonimus struct
on the contrary i have the first name but not the alias second name
??

You seem to misunderstand.
The "object" part at the end is part of the typedef, and does not create an instance of the struct.
The actual definition or declaration of the struct is ignored, so the above code would ultimately look like:

Code:

typedef struct object object;

(Note that I just cut out the contents of the struct type.)
If, on the other hand, we don't put typedef before the struct, so it becomes:

Code:

struct object{
  int data;
  struct object *left;
 struct object  *right;
}object;

Then it does indeed create an instance of the struct called object.

10-26-2008, 11:06 AM	#16
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	i understood your anonymous struct creating a data type from an anonymous name but it not the case here: Code: typedef struct object{ int data; struct object left; struct object right; }object; i have a variable called "object" but there is no anonimus struct on the contrary i have the first name but not the alias second name ??
transgalactic2 is offline

10-26-2008, 11:12 AM	#17
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	so the "object" in the last line unlike normal structs does not represent a variable but a variable type an alias to struct object in a regular struct like this: Code: struct mytype { int elem1, elem2; char elem3; } object; object.elem1 = 1; object.elem2 = 2; //object represents a variable here unlike my example Last edited by transgalactic2; 10-26-2008 at 11:17 AM.
transgalactic2 is offline

10-26-2008, 11:30 AM	#20
itCbitC Registered User Join Date: Oct 2008 Location: TX Posts: 1,262	this is an array of characters while a_odek is of the type odradek which is an alias of the complex type you have defined.
itCbitC is offline

10-26-2008, 11:32 AM	#21
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	aahhh sorryy i thought there is "this" and constructors in C
transgalactic2 is offline

10-26-2008, 11:32 AM	#22
tabstop and the Hat of Guessing Join Date: Nov 2007 Posts: 8,740	The variable "this" is of type char[], and it is initialized (remember initialization?) right where it is declared, one line above the line you highlighted. To change its value, you should use strcpy, just as you did in the Example function.
tabstop is offline

10-26-2008, 11:37 AM	#23
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	thanks
transgalactic2 is offline

10-26-2008, 11:49 AM	#24
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	> i have a variable called "object" You have two things, and neither of them are objects. Code: typedef struct object{ int data; struct object left; struct object right; }object; The names in red are the structure tag name, which serves to identify the struct by name. You NEED a tag name if you're creating a self-referencing structure like you're doing here. Much of the time, the tag name is omitted if the struct is embedded in a typedef declaration (such as this). The blue name is the name of the typedef name for the struct. It's a completely different name to the red one. BOTH can be used to create objects, like so. Code: object foo; struct object bar; __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
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