what is the meaing of this typedef line..

transgalactic2 · 10-26-2008, 03:53 AM

Code:

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

this is a code for the root of a tree
why do they add typedef
it makes no sense using it in the building of a data type

and there is no two names after the struct

mike_g · 10-26-2008, 04:48 AM

The typedef allows you to refer to your struct later on as:

Code:

object something;

Otherwise you would have to use:

Code:

struct object something;

everytime you want to declare an object.

transgalactic2 · 10-26-2008, 04:52 AM

thats why they should write

Code:

typedef struct  object  object;

but here typedef used in a very odd way

it makes no sense using it in the building of a data type

and there is no two names after the struct
??

mike_g · 10-26-2008, 05:01 AM

eh ???

transgalactic2 · 10-26-2008, 05:43 AM

in order that we could do

Code:

object something;

like you said

we need to write the code in this way

Code:

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

not like they did

Code:

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

why is it ok to do it in the way they write it??

matsp · 10-26-2008, 05:58 AM

Because the second form is entirely identical, but one line shorter? There is technically no difference between those two forms. It is however very much the convention to use one typedef/struct definition, rather than doing one typedef and one struct definition.

--
Mats

MK27 · 10-26-2008, 06:58 AM

Quote:

Originally Posted by transgalactic2

in order that we could do

Code:

object something;

like you said

we need to write the code in this way

In fact, you can do this:

Code:

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

odradek actual_odek;

The datatype syntax is not half so dangerous as the poiner syntax

transgalactic2 · 10-26-2008, 07:11 AM

in the simple form of typedef we use two names.

Code:

typedef struct first_name second_name;

i cant see those names in this code

Code:

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

matsp · 10-26-2008, 07:32 AM

the "second_name" can (and often is) the same as "first_name".

Code:

typdef struct object object;

is perfectly valid. There is no difference between that and defining the contents of the struct inside the typedef.

--
Mats

transgalactic2 · 10-26-2008, 07:43 AM

in this code

Code:

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

where are the locations of the first and the second names in this code?

MK27 · 10-26-2008, 08:41 AM

Quote:

Originally Posted by transgalactic2

where are the locations of the first and the second names in this code?

There is such a thing as an "anonymous" struct. It doesn't have a name. When I wrote you can do this:

Code:

typedef struct ?? { int x; char y; } odredek;
//don't really put ??
odradek actual_odek;

It could have been:

Code:

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

If you want to be able to use the two names, but usually there is no purpose to that, so you can use the first method, where the struct is anonymous.

transgalactic2 · 10-26-2008, 09:19 AM

i cant understand this code:

we create a data type using struct

Code:

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

we can create an alias for struct name1 using

Code:

typedef struct name1 name2;

but i dont know what the meaning of adding typedef to the code

Code:

typedef struct ?? {
 int x;
 char y; 
} odredek;

odradek actual_odek;

first you create some variable called odredek with type "??"(some_name)

but i cant understand why to add the word typedef because i cant see here the struct name1=name2 replacement issue

what its affect?

MK27 · 10-26-2008, 10:05 AM

Quote:

Originally Posted by transgalactic2

first you create some variable called odredek with type "??"(some_name)

but i cant understand why to add the word typedef because i cant see here the struct name1=name2 replacement issue

what its affect?

The "??" was just to indicate that you're right, there could be an object name there, but there does not have to be, which means the struct is anonymous. The only object name we need is the name of our new datatype (odradek), created with the command typedef. The base type of odradek was/is an anonymous struct.

Yes, you could have done much the same thing by just creating a struct, odradek. One advantage to creating a new datatype with typedef is that you can now have a function return it:

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); 
}

This works with a normal struct, too. Many, many library objects are actually structs of this sort, which I'm sure is part of the reason you can get around having to refer to "struct X" all the time.

The free() call demonstrates that main() did recieve a malloc'd object. Of course, you should do that anyway.

tabstop · 10-26-2008, 10:58 AM

Quote:

Originally Posted by transgalactic2

in this code

Code:

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

where are the locations of the first and the second names in this code?

The first is in red, the second is in green.

robwhit · 10-26-2008, 11:00 AM

You can define a struct like this:

Code:

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

You can then define an instance of 'struct mytype' like this:

Code:

struct mytype myinstance;

myinstance.elem1 = 1;
myinstance.elem2 = 2;

You can do both of those things at once like this:

Code:

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

myinstance.elem1 = 1;
myinstance.elem2 = 2;

You can make an instance of that struct without giving it's type a name. That's called an 'anonymous struct'.

Code:

struct {
	int elem1, elem2;
	char elem3;
} myinstance;

myinstance.elem1 = 1;
myinstance.elem2 = 2;

struct mytype anotherinstance; /* error: no type 'struct mytype' */

typedef creates an alias of a type

Code:

typedef int x;

x i;
int y;

y = 5;
i = y;
printf("%d", i);

You can use typedef to create aliases of structs

Code:

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

typedef struct mytype anothertype;

struct mytype myinstance; /* 'struct' keyword needed */
anothertype anotherinstance; /* 'struct' keyword not needed */
void f(struct mytype v1, anothertype v2);

f(myinstance, anotherinstance);
f(anotherinstance, myinstance);

struct anothertype evenanotherinstance; /* error: no type 'struct anothertype' */

You can declare the typedef and define the struct type in the same statement

Code:

typedef struct mytype {
	int elem1, elem2;
	char elem3;
} anothertype;

struct mytype myinstance;
anothertype anotherinstance;

This is equivalent to above. Note that in the context of a typedef statement, an identifier between the closing brace and the semicolon is the name of the new type, not the name of a struct instance.

You can make a typedef to a struct type that doesn't have a name

Code:

typedef struct {
	int elem1, elem2;
	char elem3;
} anothertype;

anothertype anotherinstance;
struct mytype myinstance; /* error: no type 'struct mytype' */

10-26-2008, 03:53 AM	#1
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	what is the meaing of this typedef line.. Code: typedef struct object{ int data; struct object left; struct object right; }object; this is a code for the root of a tree why do they add typedef it makes no sense using it in the building of a data type and there is no two names after the struct Last edited by transgalactic2; 10-26-2008 at 03:58 AM.
transgalactic2 is offline

10-26-2008, 04:48 AM	#2
mike_g Wheres the lesbians? Join Date: Oct 2006 Location: UK Posts: 1,219	The typedef allows you to refer to your struct later on as: Code: object something; Otherwise you would have to use: Code: struct object something; everytime you want to declare an object. __________________ Senior highbrow doctor of authority.
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10-26-2008, 04:52 AM	#3
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	thats why they should write Code: typedef struct object object; but here typedef used in a very odd way it makes no sense using it in the building of a data type and there is no two names after the struct ??
transgalactic2 is offline

10-26-2008, 05:01 AM	#4
mike_g Wheres the lesbians? Join Date: Oct 2006 Location: UK Posts: 1,219	eh ??? __________________ Senior highbrow doctor of authority.
mike_g is offline

10-26-2008, 05:43 AM	#5
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	in order that we could do Code: object something; like you said we need to write the code in this way Code: typedef struct object object; struct object{ int data; struct object left; struct object right; }object; not like they did Code: typedef struct object{ int data; struct object left; struct object right; }object; why is it ok to do it in the way they write it?? Last edited by transgalactic2; 10-26-2008 at 05:46 AM.
transgalactic2 is offline

10-26-2008, 05:58 AM	#6
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	Because the second form is entirely identical, but one line shorter? There is technically no difference between those two forms. It is however very much the convention to use one typedef/struct definition, rather than doing one typedef and one struct definition. -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
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10-26-2008, 07:11 AM	#8
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	in the simple form of typedef we use two names. Code: typedef struct first_name second_name; i cant see those names in this code Code: typedef struct object{ int data; struct object left; struct object right; }object; Last edited by transgalactic2; 10-26-2008 at 07:14 AM.
transgalactic2 is offline

10-26-2008, 07:32 AM	#9
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	the "second_name" can (and often is) the same as "first_name". Code: typdef struct object object; is perfectly valid. There is no difference between that and defining the contents of the struct inside the typedef. -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
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10-26-2008, 07:43 AM	#10
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	in this code Code: typedef struct object{ int data; struct object left; struct object right; }object; where are the locations of the first and the second names in this code? Last edited by transgalactic2; 10-26-2008 at 07:46 AM.
transgalactic2 is offline

10-26-2008, 09:19 AM	#12
transgalactic2 Banned Join Date: Oct 2008 Posts: 1,535	i cant understand this code: we create a data type using struct Code: struct object{ int data; struct object left; struct object right; }object; we can create an alias for struct name1 using Code: typedef struct name1 name2; but i dont know what the meaning of adding typedef to the code Code: typedef struct ?? { int x; char y; } odredek; odradek actual_odek; first you create some variable called odredek with type "??"(some_name) but i cant understand why to add the word typedef because i cant see here the struct name1=name2 replacement issue what its affect? Last edited by transgalactic2; 10-26-2008 at 09:23 AM.
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