Is there some way to use pointers inside C switch statements?

Question

Usually in a dynamically typed programming language, the object struct has a tag field used to identify the object type. For example:

struct myObject {
    int tag;
    ...
}

So it is easy to perform different actions using a switch statement based on the tag field. For example:

switch (obj->tag) {
    case OBJ_INTEGER: ...
    case OBJ_STRING: ...
    case OBJ_FUNC:...
}

In my case instead of the int tag field I used a void *isa pointer that points to the class that represents that object. Everything worked fine expect that instead of using an elegant switch statement I am forced to use a series of if/else statements. For example:

if (obj->isa == class_integer) {
    ...
} else if (obj->isa == class_string) {
    ...
} else if (obj->isa == class_func) {
    ...
}

I know that I can't use pointers inside a C switch statements but I wondering if I can use some clever trick in order to speedup the series of if statements.

Answer 1

The beauty of having isa pointer in the first place is an ability to dispense with switch statements completely.

Consider a switch:

switch (obj->tag) {
    case OBJ_INTEGER: do_something_int(obj); break;
    case OBJ_STRING: do_something_str(obj); break;
    case OBJ_FUNC: do_something_func(obj); break;
}

Let's say that isa pointer points to some struct which you own - say, struct dyn_type. Add a function pointer to your struct, like this:

typedef void (*ProcessPtr)(dyn_obj * obj);

struct dyn_type {
    ... // fields of your current struct
    ProcessPtr process;
};

Set process field to do_something_int for OBJ_INTEGER, do_something_str for OBJ_STRING, etc. Now you can replace switch with

((struct dyn_type*)obj->isa)->process(obj)