Objects

The object system allows to do object oriented programming in C99. The object model has the following features:

  • Simple inheritance. All object types must “inherit” from w_obj_t. Of course, composition of objects is also possible.
  • Objects are typically allocated in the heap, but it is possible to allocate them statically, or in the stack with the aid of the W_OBJ_STATIC macro.
  • Objects keep a reference counter, which can be manipulated using w_obj_ref(), and w_obj_unref(). Objects are deallocated when their reference counter drops to zero.
  • It is possible to assign a “destructor function” to any object using w_obj_dtor().
  • Minimal overhead: objects do not have a vtable by default, and dynamic method dispatching is not done unless explicitly added by the user.
  • Uses only C99 constructs, and it does not require any special compiler support.
  • Optionally, when using GCC or Clang, the reference count for an object can be automatically decreased when a pointer to it goes out of scope, by marking it with the w_lobj macro.

A number of features included in libwheel make use of the object system (for example, the Input/Output Streams), or includes support to seamlessly integrate with the object system (for example, the List Container can update the reference counter when objects are added to it).

Usage

This example shows how to define a base “shape” object type: shape_t; and two derived types for squares (square_t) and rectangles (rectangle_t).

In order to have methods which work on any object derived from the shape type, a “vtable” is added manually to perform dynamic dispatch using a shared struct which contains function pointers to the actual implementations for each shape. Another valid approach would be to add the function pointers directly in shape_t to avoid the extra indirection. This second approach would be better if the function pointers to method implementations could change at runtime, at the cost of each instance of a shape occupying some extra bytes of memory.

Header:

// Objects have no vtable by default, so one is defined manually.
typedef struct {
    double (*calc_area)      (void*);
    double (*calc_perimeter) (void*);
} shape_vtable_t;

// Base object type for shapes.
W_OBJ (shape_t) {
    w_obj_t         parent;  // Base object type.
    shape_vtable_t *vtable;  // Pointer to vtable.
};

// A square shape.
W_OBJ (square_t) {
    shape_t parent;  // Inherits both base object and vtable.
    double  side_length;
};

// A rectangular shape.
W_OBJ (rectangle_t) {
    shape_t parent;  // Inherits both base object and vtable.
    double  width;
    double  height;
};

// Functions used to create new objects.
extern shape_t* square_new (double side_length);
extern shape_t* rectangle_new (double width, double height);

// Convenience functions to avoid having to manually make the
// dynamic dispatch through the vtable manually in client code.
static inline double shape_calc_area (shape_t *shape) {
    return (*shape->vtable->calc_area) (shape);
}
static inline double shape_calc_perimeter (shape_t *shape) {
    return (*shape->vtable->calc_perimeter) (shape);
}

Implementation:

// Methods and vtable for squares.
static double square_calc_area (void *obj) {
    double side_length = ((square_t*) obj)->side_length;
    return side_length * side_length;
}
static double square_calc_perimeter (void *obj) {
    return 4 * ((square_t*) obj)->side_length;
}
static const shape_vtable_t square_vtable = {
    .calc_area      = square_calc_area,
    .calc_perimeter = square_calc_perimeter,
};

shape_t* square_new (double side_length) {
    square_t *square = w_obj_new (square_t);  // Make object.
    square->parent.vtable = &square_vtable;   // Set vtable.
    square->side_length = side_length;
    return (shape_t*) square;
}

// Methods and vtable for rectangles.
static double rectangle_calc_area (void *obj) {
    rectangle_t *rect = (rectangle_t*) obj;
    return rect->width * rect->height;
}
static double rectangle_calc_perimeter (void *obj) {
    rectangle_t *rect = (rectangle_t*) obj;
    return 2 * (rect->width + rect->height);
}
static const shape_vtable_t rectangle_vtable = {
    .calc_area      = rectangle_calc_area,
    .calc_perimeter = rectangle_calc_perimeter,
};

shape_t*
rectangle_new (double width, double height) {
    rectangle_t *rect = w_obj_new (rectangle_t);  // Make object.
    rect->parent.vtable = &rectangle_vtable;      // Set vtable.
    rect->width = width;
    rect->height = height;
    return (shape_t*) rect;
}

Using shapes:

// Uses the generic shape_* functions.
static void print_shape_infos (shape_t *shape) {
    w_print ("Shape area: $F\n", shape_calc_area (shape));
    w_print ("Shape perimeter: $F\n", shape_calc_perimeter (shape));
}

int main (void) {
    w_lobj shape_t *s = square_new (10);
    w_lobj shape_t *r = rectangle_new (10, 20);
    print_shape_infos (s);  // Works on any object derived from shape_t.
    print_shape_infos (r);  // Ditto.
    return 0;
}

Types

w_obj_t

Base type for objects.

All other object types must “derive” from this type for the objects system to work properly. This is achieved by having a member of this type as first member of object types — either explicitly or by “inheriting” it from another object type:

W_OBJ (my_type) {
    // Explicitly make the first member be an "w_obj_t"
    w_obj_t parent;
};

W_OBJ (my_subtype) {
    // The first member itself has an "w_obj_t" as first member.
    my_type parent;
};

Macros

W_OBJ_DECL(type)

Makes a forward declaration of a object class of a certain type.

See also W_OBJ_DEF.

W_OBJ_DEF(type)

Defines the structure for an object class of a certain type.

This macro should be used after the type has been declared using the W_OBJ_DECL macro.

Typical usage involves declaring the type in a header, and the actual layout of it in an implementation file, to make the internals opaque to third party code:

// In "my_type.h"
W_OBJ_DECL (my_type);

// In "my_type.c"
W_OBJ_DEF (my_type) {
    w_obj_t parent;
    int     value;
    // ...
};
W_OBJ(type)

Declares and defines the structure for an object class of a certain type. This is equivalent to using W_OBJ_DECL immediately followed by W_OBJ_DEF.

For example:

W_OBJ (my_type) {
    w_obj_t parent;
    int     value;
    // ...
};

This is used instead of a combination of W_OBJ_DECL and W_OBJ_DEF when a forward declaration is not needed, and it does not matter that the internals of how an object class is implemented are visible in headers:

W_OBJ_STATIC(destructor)

Initializes a statically-allocated object, and sets destructor to be called before the object is deallocated by w_obj_destroy().

Similarly to w_obj_mark_static(), this macro allows to initialize objects for which the memory they occupy will not be deallocated.

Typical usage involves initializing static global objects, or objects allocated in the stack, e.g.:

W_OBJ (my_type) {
    w_obj_t parent;
    int     value;
};

static my_type static_object = {
    .parent = W_OBJ_STATIC (NULL),
    .value  = 42,
};

void do_foo (void) {
    my_type stack_object = {
        .parent = W_OBJ_STATIC (NULL),
        .value  = 32,
    };

    use_object (&stack_object);
}

Functions

void* w_obj_ref(void *object)

Increases the reference counter of an object.

The object itself is returned, to allow easy chaining of other function calls.

void* w_obj_unref(void *object)

Decreases the reference counter of an object.

Once the reference count for an object reaches zero, it is destroyed using w_obj_destroy().

The object itself is returned, to allow easy chaining of other function calls.

void w_obj_destroy(void *object)

Destroys an object.

If a destructor function was set for the object using w_obj_dtor(), then it will be called before the memory used by the object being freed.

void* w_obj_dtor(void *object, void (*destructor)(void*))

Registers a destructor function to be called when an object is destroyed using w_obj_destroy().

The object itself is returned, to allow easy chaining of other function calls.

void w_obj_mark_static(void *object)

Marks an object as being statically allocated.

When the last reference to an object marked as static is lost, its destructor will be called, but the area of memory occupied by the object will not be freed. This is the same behaviour as for objects initialized with the W_OBJ_STATIC macro. The typical use-case for this function to mark objects that are allocated as part of others, and the function is called during their initialization, like in the following example:

W_OBJ (my_type) {
    w_obj_t     parent;
    w_io_unix_t unix_io;
};

void my_type_free (void *objptr) {
    w_obj_destroy (&self->unix_io);
}

my_type* my_type_new (void) {
    my_type *self = w_obj_new (my_type);
    w_io_unix_init_fd (&self->unix_io, 0);
    w_obj_mark_static (&self->unix_io);
    return w_obj_dtor (self, _my_type_free);
}
type* w_obj_new(type)

Creates a new instance of an object of a given type.

Freshly created objects always have a reference count of 1.

type* w_obj_new_with_priv_sized(type, size_t size)

Creates a new instance of an object of a given type, with additional space of size bytes to be used as instance private data.

A pointer to the private data of an object can be obtained using w_obj_priv().

type* w_obj_new_with_priv(type)

Creates a new instance of an object of a given type, with additional space to be used as instance private data. The size of the private data will be that of a type named after the gicen type with a _p suffix added to it.

A pointer to the private data of an object can be obtained using w_obj_priv().

Typical usage:

// In "my_type.h"
W_OBJ (my_type) {
    w_obj_t parent;
};

extern my_type* my_type_new ();


// In "my_type.c"
typedef struct {
    int private_value;
} my_type_p;


my_type* my_type_new (void) {
    my_type *obj = w_obj_new_with_priv (my_type);
    my_type_p *p = w_obj_priv (obj, my_type);
    p->private_value = 42;
    return obj;
}
void* w_obj_priv(void *object, type)

Obtains a pointer to the private instance data area of an object of a given type.

Note that only objects created using w_obj_new_with_priv_sized() or w_obj_new_with_priv() have a private data area. The results of using this function on objects which do not have a private data area is undefined.