Thread: casting from pointer to void

Originally Posted by django

I have a function that produces several messages which are implemented as different structs. The only thing the messages have in common is a member called 'type' to indicate which type of message it is.

Is that really the only thing these message structs have in common? Do you perhaps perform some operation on them for which the precise details varies with each type, but nonetheless is something in common?

Instead of returning a void pointer, why not return a struct like this;

Code:

typedef struct
{
     TypeDescriptor type;
     void *data;
} Thing;

Then just check the type member, and convert data appropriately.

You could also do the same thing, if you know all you possible types in advance, using a union.

Originally Posted by django

Depends. There are for example order messages which are all orders (like: I want to buy this and that) but who vary a little with respect to the member fields. They get processed in comparable ways. Why do you ask?

I ask because the more information you provide, the more likely someone might suggest a design that best fits your problem. I was contemplating between just suggesting a solution along the lines of what grumpy proposed in post #3, or going for something that perhaps is more like polymorphism in C++.

Originally Posted by django

I would also be interested in how you would approach polymorphism like designs in C.

One way is to provide a function pointer member instead of a type (descriptor) member. This function pointer would point to a function that takes say, an object or pointer of the Thing struct as an argument. It would cast the data void pointer appropriately and then operate on the resulting object. This way, you can iterate over the array and just call the correct function with each element of the array.

Originally Posted by django

I just want to make sure: if you cast something from a pointer to void, there is no way to detect the type of struct?

Yes, type information is lost unless you save it elsewhere.