allocating memory for an array structure member

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April 21st, 2004, 10:17 PM

gehenna_engine

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allocating memory for an array structure member

Can someone show me (if it's possible) how to appropriately allocate memory for an array structure member?

the structure would look something like this:

Code:


structure record {
    char name[32];
    char desc[80];
    char *items;

}

typedef struct record RC;

the member char *items is the one that will need to be allocated dynamically. The number of items will be determined by user input.

Thanks in advance for the help.

Last edited by gehenna_engine : April 21st, 2004 at 10:35 PM.

April 21st, 2004, 10:57 PM

murugesan

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A sample code for you

Code:

#include <stdio.h>
int main()
{
        struct record
        {
                char name[32];
                char desc[80];
                char *items;
        };
        int num=10;
        typedef struct record RC;
        RC *pp=(RC*)malloc(sizeof(RC));
        pp->items=(char*)malloc(sizeof(num));
        strcpy(pp->items,"DINESH_P_V");
        printf("%s",pp->items);
        free(pp);
}

HTH

-Murugesan

May 12th, 2004, 10:45 AM

palex

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Hi all !

I have a similar problem.
But I want to have an array of that kind of struct, something like :
RC* pp[NUMBEROF];
or RC** pp;
if it works...

When I do this :
pp[x] = new RC();
pp[x]->items=(char*)malloc(sizeof(num));
I don't have any crash, but the datas I put and retrieve inside later are invalid.

Any ideas, suggestions ?

Thanks

May 12th, 2004, 11:13 AM

Hko

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Quote:

Originally Posted by palex

pp[x] = new RC();
pp[x]->items=(char*)malloc(sizeof(num));

(We're talking C++ here, right?)
It won't solve your problem, but the second line here really belongs inside the constructor of class/struct RC. (And the "delete" for it, in the destructor)

May 12th, 2004, 11:02 PM

murugesan

Dinesh_P_V

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Quote:

Originally Posted by palex
pp[x] = new RC();
pp[x]->items=(char*)malloc(sizeof(num));

I think u need to allocate num size and not sizeof(num). Am I right ?

Or else here goes a sample.

Code:

class dinesh
{
public:
        char *items;
        dinesh()
        {
                cout<<"\ndinesh_p_v constructor\n";
        }
        ~dinesh()
        {
                cout<<"\ndinesh_p_v destructor\n";
                free(items);
        }
};
main()
{
        dinesh *dins[10];
        for(int i=0;i<10;i++)
        {
                dins[i]=new dinesh();
                dins[i]->items=(char*)malloc(10);
                sprintf(dins[i]->items,"dinesh");
                cout<<"\n"<<dins[i]->items<<"\n";
        }
        for(int i=0;i<10;i++)
                delete dins[i];
        return 0;
}

HTH

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Dinesh_P_V

May 12th, 2004, 11:30 PM

jwdonahue

Bellevue WA, USA

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Palex, this should help. If you programming in C++ quit using malloc. It's not type safe.

Code:

 // RC* pp[DIMENSION] ; // this works but it's not on the heap.
 RC **pp = new RC[DIMENSION] ;
 pp[0] = new RC() ;
 pp[0]->items = new char[ITEM_COUNT] ;

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May 13th, 2004, 04:49 AM

palex

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Thanks everybody...

My problem was (is...) what jwdonahue tried to solve.
But you can't do that :
RC **pp = new RC[DIMENSION];
(cannot convert from struct RC* to struct RC**)

-(of course, we're talking c++)-

How should I declare my pp for making it works ?

This isn't working :
------------------
structure record {
char name[32];
char desc[80];
char *items;
}
typedef struct record RC;

RC* pp[NUMBEROF];
(loop)
pp[x] = new RC();
pp[x]->items = new char[ITEM_COUNT];

My datas in items will be invalid.

This is working :
---------------
structure record {
char name[32];
char desc[80];
char items[ITEM_COUNT];
}
typedef struct record RC;

RC* pp[NUMBEROF];
(loop)
pp[x] = new RC();

My data in item are valid.

Of course, I can't use the second way, as I need lots of structs where items may be big.

So thanks and thanks in advance....

PAlex

May 13th, 2004, 04:59 AM

jwdonahue

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Woops! Let me go lick my wounds....

Well, at least my advice about not using malloc() paid off

May 13th, 2004, 05:09 AM

jwdonahue

Bellevue WA, USA

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Lets see...

Code:

  #ifdef DO_IT_THIS_WAYtypedef RC* pRC ;
  pRC pp = new pRC[NUMBEROF] ;
#else  RC *pp = new RC*[NUMBEROF] ;
 #endif

I must have had 2D arrays on my brain. I apologize for the confusion. I think it was the pp moniker that had me over-thinking the problem. An array of pointers to struct might be better named ppRC. But that's just a nit.

Last edited by jwdonahue : May 13th, 2004 at 05:12 AM. Reason: Fixed indentation.

allocating memory for an array structure member

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