How to access structure member using address of structure

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Hi,

I know the way to access the members of structure using . or -> operator.
I can access members of array using base pointer for example:

C

int array[8];
printf("%d", array+1); // Will print element at 1 st place in array

How can I access element of a structure using base pointer, maybe something like this:

C

struct st
{
    int i;
    int j;
};
struct st strcture;
*(&structure+1) = 100;          // This gives some error
printf("%d", &structure+1);     // Should print the value of i of st that is 100 but some garbage is returned

Am I clear with my question? How can I do it?

Thanks
Vicky

Posted 2-Jun-10 23:05pm

vikasvds

Updated 3-Jun-10 0:33am

Moak

v2

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CPallini · Accepted Answer · 2010-06-02T23:27:00

Solution 1

You should properly cast the pointer (you know, pointer arithmetic...).
For instance

C

#include <stdio.h>
struct st
{
  int i;
  int j;
};
int main()
{
  st a = {5,11};
  st * pa = &a;
  int first, second;
  first = *((int*)pa);
  second = *((int*)pa +1);
  printf ("first=%d,second=%d\n", first, second);
  return 0;
}

:)

Posted 2-Jun-10 23:27pm

CPallini

Updated 2-Jun-10 23:33pm

v2

Comments

vikasvds 3-Jun-10 10:21am

Thank you very much!!
Your explanation cleared my doubt.
Thanks again.

r.ps · Accepted Answer · 2010-06-03T04:22:00

Solution 2

CPallini has given you the answer. But be aware of something called 'structure padding'. Look it up.

Posted 3-Jun-10 4:22am

r.ps

Aescleal · Accepted Answer · 2010-06-03T08:36:00

At the risk of sounding like the daft paper clip that used to give you advice in Microsoft Office...

"Hey, it looks like you want to access a structure field through a pointer and offset..."

In C you're forced to use something that's got loads of casts in it - there was an example of how to do that in a previous answer. As another answer mentioned you might have stucture alignment issues.

If you're using C++ you can use pointers to members which gives you all the goodness of offsets into a structure without running into problems with alignment.

Say you've got a structure B:

struct B
{
    int i;
    int j;
};

You can declare a pointer to either of the members using the following syntax:

int B::*p = &B::i;

Which says you can access one of the integers in the structure relative to the starting address of one of the structures. e.g:

B b;

B *pb = &b;

pb->*p = 100;

The last statement sets the i member of b to be 100, indirectly.

This is particularly handy when you want to iterate through an array of structures modifying a variable member of the structure...

e.g.

int B::*members[2] = { &B::i, &B::j };

You can then switch between members of the structure depending on which one of the members you access:

pb->*members[0] = 50;

It gets even cooler when you start using member function pointers, but that's another story.

Cheers,

Ash