How do I cast these void pointers?

Question

Im doing exam review and one of the questions says there is something wrong with this code and im supposed to fix it. I know it has something to do with the void pointer but cant figure it out. Does anyone know what i would do?

void whatAmI(void *vp, int n) {
    if (n == 1) {
        printf(“Integer: %d\n”, vp);
    } else if (n == 2) {
        printf(“Double: %.2fl\n”, vp);
    } else {
        printf(“Unknown type!”);
    }
}

no, this is just sample code I was given. I have to cast or something like that but I dont know how. — user3672889, Jun 26 '14 at 21:52
Well, `vp` is a pointer and is neither an integer nor a float. And those aren't real quotes. — David Heffernan, Jun 26 '14 at 21:55
it is really a bad example of code as integer and float cannot be represented with the same raw data! — daouzli, Jun 26 '14 at 21:58
Okay if you considere n as determining the type of data, that could make sense but it's finally not a good approach! — daouzli, Jun 26 '14 at 22:00
@daouzli; A good approach could be using `union` but there you also need a "tag field"/ "discriminant". — haccks, Jun 26 '14 at 22:17
@haccks that's indeed a kind of situation where an union can make sense. But it can be considered as overkill! — daouzli, Jun 26 '14 at 22:32

haccks · Accepted Answer · 2014-06-26T22:21:13.533

6

You need to dereference the pointer vp to print the value stored at the location pointed by vp. But a void pointer can't be dereferenced (doing so invokes undefined behavior), so you need to cast it:

void whatAmI(void *vp, int n) {
    if (n == 1) {
        printf("Integer: %d\n", *(int *)vp);  
    } else if (n == 2) {
        printf("Double: %.2fl\n", *(double *)vp);  
    } else {
        printf("Unknown type!");
}

edited Jun 26 '14 at 22:21

answered Jun 26 '14 at 21:53

haccks

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What about strict aliasing rules? Won't dereferencing `(int*)vp` and `(float*)vp` break them here? – Daniel Kamil Kozar Jun 26 '14 at 21:59
1

@DanielKamilKozar; *Aliasing refers to the situation where the same memory location can be accessed using different names.* But here memory is referenced using same name `vp` :) – haccks Jun 26 '14 at 22:05
@haccks : Thanks. I always get nervous when I see `*(int*)something` in the code. – Daniel Kamil Kozar Jun 26 '14 at 22:07
1

@DanielKamilKozar; Haven't you tried `qsort` and its `compare` function ? You should try that and then you will know more abot generic pointer `void *`. By the way you can read more about [Strict aliasing](http://stackoverflow.com/a/99010/2455888) here. – haccks Jun 26 '14 at 22:11
2

The `"Double: %.2fl\n"` suggests the pointer points to a `double` object and in that case you have to use `*(double *) vp`. – ouah Jun 26 '14 at 22:19
@ouah; Yes. I missed that. Its late night here. Too tired now. – haccks Jun 26 '14 at 22:21

How do I cast these void pointers?

1 Answers1