Thread: Pointer inside a struct - how to access it?

I have a struct like this:

Code:

struct Person
{
  double number;
  int age, *exampoint;
}Prsn[50];

I want to acces the pointer defined inside the struct later. How can I call it? I tried this, which works with non-pointer variables. But it gives a parse error.

Code:

Prsn[1].*exampoint=&a[1]

Normally I call something inside my array of struct by writing the struct's name, a dot and the required variable. But how can I get the pointer? Please tell me if I'm making a logical mistake in thinking.

If you have a struct element that is a pointer, and you need to dereference that pointer, you need something like:

Code:

*Prsn[1].exampoint = &a[1];

though I prefer explicit parentheses so it's clear what you're trying to do:

Code:

*(Prsn[1].exampoint) = &a[1];

Originally Posted by camel-man

prsn->exampoint???

Also There is better ways to go about making an array of structs

I've tried that but my program stops running when I do that :/

Originally Posted by anduril462

If you have a struct element that is a pointer, and you need to dereference that pointer, you need something like:

Code:

*Prsn[1].exampoint = &a[1];

though I prefer explicit parentheses so it's clear what you're trying to do:

Code:

*(Prsn[1].exampoint) = &a[1];

I tried them. Compiler doesn't give an error but it says "warning:assignment makes integer from pointer without a cast" How can I prevent it? And thanks

try taking off the *
ex. Prsn[1].exampoint = &a[1];

Doh! I saw the * in your first example and thought you wanted to dereference, but I should have known because the & in &a[1] is an address. In English, do you want Prsn[1].exampoint to point to element #1 of a? If so, then you need to drop the * from my examples.

Originally Posted by camel-man

try taking off the *
ex. Prsn[1].exampoint = &a[1];

Originally Posted by anduril462

Doh! I saw the * in your first example and thought you wanted to dereference, but I should have known because the & in &a[1] is an address. In English, do you want Prsn[1].exampoint to point to element #1 of a? If so, then you need to drop the * from my examples.

Yes I removed the * .Program seems to run well at first but stops running at some time. Maybe it's not caused by the struct or the pointer. I'll check my entire syntax now.

By the way, if I remove the array part and just leave one single struct, how can i refer to the pointer? e.g. the struct is;

Code:

struct Person
{
  double number;
  int age, *exampoint;
};

Code:

struct Person
{
  double number;
  int age, *exampoint;
};

That just defines the struct, it doesn't define any variables. If you don't want an array, take the [ ] out of the definition and wherever else you use it. Something like:

Code:

// define the struct
struct Person
{
    double number;
    int age;
    int *exampoint;
};

// define a variable of type 'struct Person'
struct Person that_guy;

that_guy.number = 123.45;
that_guy.age = 42;
that_guy.exampoint = &a[1];

Originally Posted by anduril462

Code:

struct Person
{
  double number;
  int age, *exampoint;
};

That just defines the struct, it doesn't define any variables. If you don't want an array, take the [ ] out of the definition and wherever else you use it. Something like:

Code:

// define the struct
struct Person
{
    double number;
    int age;
    int *exampoint;
};

// define a variable of type 'struct Person'
struct Person that_guy;

that_guy.number = 123.45;
that_guy.age = 42;
that_guy.exampoint = &a[1];

Thanks a LOT!!!

Actually... pointers inside structs are usually some pretty bad juju....

If you aren't careful --really careful-- you're likely to end up with all your pointers pointing to the same place or, even worse, all those pointers aimed at values that have gone out of scope. As a rule it's better to simply put the data right in the struct, instead of pointing to it.

And, in this case... since an int and a pointer are the same size on a 32 bit machine, you definatly aren't saving any space. On a 64 bit machine where pointers are 8 bytes, your struct actually ends up being bigger.

Originally Posted by Salem

> Actually... pointers inside structs are usually some pretty bad juju....
Oh come on - how are you supposed to create things like linked lists then?

Pointers inside structs have the same problems (and benefits) as pointers outside of structs. Of course you've got to make sure where the pointer points to, that's no different from before.

Ok, maybe I should have said "pointers to data"... The exception being the next/prev pointers in linked lists (which I avoid like the plague)