gistfile1.txt

#include "TaggedPointer.h"

#include <stdlib.h>
#include <stdio.h>

typedef struct Header {
    const void* tag;
} Header;

Header* base(void *tagged) {
    return tagged - sizeof(Header);
}

void* payload(void* this) {
    return this + sizeof(Header);
}

const void* const TaggedPointer_GetTag(void *tagged) {
    if(tagged && base(tagged)) {
        return base(tagged)->tag;
    }
    return NULL;
}

void* TaggedPointer_Create(size_t size, const void* const tag) {
    if(tag) {
        Header *this = calloc(1, sizeof(Header) + size);
        this->tag = tag;
        return payload(this);
    }
    return NULL;
}

void TaggedPointer_Destroy(void* tagged) {
    free(base(tagged));
}
#ifndef UNTITLED_TAGGEDPOINTER_H
#define UNTITLED_TAGGEDPOINTER_H

#include <stdlib.h>

const void* const TaggedPointer_GetTag(void* tagged);
void*             TaggedPointer_Create(size_t size, const void* const tag);
void              TaggedPointer_Destroy(void* tagged);

#endif

TaggedPointer.c

#include "TaggedPointer.h"

#include <stdlib.h>
#include <stdio.h>

typedef struct Header {
    const void* tag;
} Header;

Header* base(void *tagged) {
    return tagged - sizeof(Header);
}

void* payload(void* this) {
    return this + sizeof(Header);
}

const void* const TaggedPointer_GetTag(void *tagged) {
    if(tagged && base(tagged)) {
        return base(tagged)->tag;
    }
    return NULL;
}

void* TaggedPointer_Create(size_t size, const void* const tag) {
    if(tag) {
        Header *this = calloc(1, sizeof(Header) + size);
        this->tag = tag;
        return payload(this);
    }
    return NULL;
}

void TaggedPointer_Destroy(void* tagged) {
    free(base(tagged));
}

TaggedPointer.h

#ifndef UNTITLED_TAGGEDPOINTER_H
#define UNTITLED_TAGGEDPOINTER_H

#include <stdlib.h>

const void* const TaggedPointer_GetTag(void* tagged);
void*             TaggedPointer_Create(size_t size, const void* const tag);
void              TaggedPointer_Destroy(void* tagged);

#endif