Objects

C has no built-in "object" concept, but you can build your own object system using exactly the tools you already have: structs, tagged unions, and function pointers. This is the same foundation real language runtimes (Python's CPython, Lua's VM) are built on.

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

typedef enum {
    OBJ_INT,
    OBJ_STRING
} ObjType;

typedef union {
    int intValue;
    char *stringValue;
} ObjValue;

typedef struct Obj {
    ObjType type;
    int refCount;      // we'll use this in the next lesson
    ObjValue as;
} Obj;

struct Obj *objNewInt(int value) {
    Obj *obj = malloc(sizeof(Obj));
    obj->type = OBJ_INT;
    obj->refCount = 1;
    obj->as.intValue = value;
    return obj;
}

struct Obj *objNewString(char *value) {
    Obj *obj = malloc(sizeof(Obj));
    obj->type = OBJ_STRING;
    obj->refCount = 1;
    obj->as.stringValue = strdup(value); // heap-allocate a copy
    return obj;
}

void objPrint(Obj *obj) {
    switch (obj->type) {
        case OBJ_INT:    printf("%d\n", obj->as.intValue); break;
        case OBJ_STRING:  printf("%s\n", obj->as.stringValue); break;
    }
}

int main(void) {
    Obj *a = objNewInt(42);
    Obj *b = objNewString("hello");
    objPrint(a);
    objPrint(b);
    return 0;
}

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

typedef enum {
    OBJ_INT,
    OBJ_STRING
} ObjType;

typedef union {
    int intValue;
    char *stringValue;
} ObjValue;

typedef struct Obj {
    ObjType type;
    int refCount;      // we'll use this in the next lesson
    ObjValue as;
} Obj;

struct Obj *objNewInt(int value) {
    Obj *obj = malloc(sizeof(Obj));
    obj->type = OBJ_INT;
    obj->refCount = 1;
    obj->as.intValue = value;
    return obj;
}

struct Obj *objNewString(char *value) {
    Obj *obj = malloc(sizeof(Obj));
    obj->type = OBJ_STRING;
    obj->refCount = 1;
    obj->as.stringValue = strdup(value); // heap-allocate a copy
    return obj;
}

void objPrint(Obj *obj) {
    switch (obj->type) {
        case OBJ_INT:    printf("%d\n", obj->as.intValue); break;
        case OBJ_STRING:  printf("%s\n", obj->as.stringValue); break;
    }
}

int main(void) {
    Obj *a = objNewInt(42);
    Obj *b = objNewString("hello");
    objPrint(a);
    objPrint(b);
    return 0;
}

This is the tagged union pattern from a few lessons ago, put to real use: Obj can represent any value your future language or program needs, and type tells every function which union member is actually valid right now.

Why refCount is already here

Every Obj is heap-allocated (so it can outlive the function that created it and be shared around freely), which means something has to decide when it's safe to free. That's exactly the problem the next two lessons solve, this refCount field is the first piece of a reference-counting garbage collector, and this Obj struct is what it will manage.

Objects

Why refCount is already here

📝 Objects Quiz