struct with a pointer to a member of another struct? 1

[fyrdman]

I have a project where I want to have the two general types of structures:

struct A {
…
unsigned char *a;
…
};

struct B {
…
unsigned char b;
…
};


The general idea is that I want a member of struct A to be a pointer a member of struct A (without knowledge of the structure of B, other than B has a member unsigned char b).

My question is how do I initialize *a to the address of b? (and then properly reference b through *a within a function?)


I wish to initialize an instance of struct A as follows:

const struct A constinrom = { …, <something that extracts the addr of b> , …};


I wish to use struct A as follows (with no knowledge in anyfunction of struct B other than what is provided by struct A) :

void anyfunction (const struct A *name) {

(set variable c to equal the value in b)

(modify the value of c)

(set value in b equal the value in value of c)

}


Thank you,

Fyrdman

 

[DaveSinkula]

*That* link is very helpful, but not yet, as I will have to read it 3-4 times I think.

That one always takes me several passes, too. Maybe I should have started with this one instead.

http://www.eskimo.com/~scs/C-faq/q6.12.html

A good use for [tt]void*[/tt] is for functions like [tt]memcpy[/tt] or [tt]qsort[/tt] or its comparison routine. These are places where no particular type is specified because many types could be used.

 

[fyrdman]

DaveSinkula,

Maybe something interesting would be cautions of where you can go wrong with void * (or would that be too unbounded of a question to ask of C? )

 

[DaveSinkula]

Some notable properties of an object of type [tt]void*[/tt]:[ul]
[li]may point to any object[/li]
[li]can convert any pointer to object or incomplete type to [tt]void*[/tt] and back again and you get the same pointer value back[/li]
[li]is able to represent the values of any pointer type (except pointers to functions) such that those values can be recreated[/li]
[li]initialization and assignements do not require a cast; any necessary conversions are performed implicitly[/li]
[li]cannot perform arithmetic it[/li]
[li]cannot dereference it[/li][/ul]