Problems with inheritance by composition

I'm continuing my discussion of how to implement and use virtual functions in C. The basic approach is to implement each class as a C structure and implement each associated method as a function whose first parameter is a pointer to the structure. For each polymorphic class (a class with at least one virtual function), the corresponding C structure has a member called a vptr (VEE-pointer), which points to a table of function pointers called a vtbl (VEE-table).¹

Virtual functions are useful only in inheritance hierarchies–collections of classes where some are derived from others. A few of months ago, I showed two distinct ways to mimic inheritance in C, which I call “inheritance by composition” and “inheritance by copying and editing.”² Although the mechanics of inheritance by composition might be the simpler of the two, I generally prefer inheritance by copying and editing because it yields a better user interface. This month, I'll explain some problems with inheritance by composition that make this technique undesirable.

As in prior articles, my sample classes represent an assortment of two-dimensional geometric shapes such as circle , rectangle , and triangle , all derived from a common base class called shape . In C, the shape “class” implementation requires two structures: one for the shape data members and one for the corresponding shape vtbl. The C declarations for the shape class look like:

// shape.h - a C base class for shapes~~~typedef struct shape shape;typedef struct shape_vtbl shape_vtbl;struct shape_vtbl {    double (*area)(shape const *me);    double (*perimeter)(shape const *me);};struct shape {    shape_vtbl *vptr;    color outline, fill;};void shape_construct(shape *me, color o, color f);double shape_area(shape const *me);double shape_perimeter(shape const *me); 

The function declarations at the end of the header declare all of the shape member functions, even the virtual ones, as non-virtual functions. Each call to a member function (whether virtual or non-virtual) acts upon a specific “target” object. Each member function has a parameter named me , which the function uses to access its target.

Using “inheritance by composition,” you declare the circle structures in the circle.h header as:

typedef struct circle circle;struct circle {    shape base;           // the base class subobject    double radius;};typedef struct circle_vtbl circle_vtbl;struct circle_vtbl {    shape_vtbl base;      // the base class vtbl    // virtual functions introduced in circle};

In the circle.c source file, you define the_circle_vtbl (the circle vtbl object) and initialize its members. Unfortunately, given the declarations thus far, this straightforward initialization won't compile:

circle_vtbl the_circle_vtbl = {    circle_area,        // initializes base.area    circle_perimeter    // initializes base.perimeter};

Let me explain why.

As its name suggests, the circle_area operates on a circle object:

double circle_area(circle const *me);

However, using inheritance by composition, the base member of the circle_vtbl declares its area member as:

double (*area)(shape   const *me);

That is, it operates on a shape object.

When you try to initialize the_circle_vtbl.base.area with (the address of) the circle_area function, the compiler complains that the pointers types are incompatible, which indeed they are. The compiler also complains aboutinitializing the perimeter member with circle_perimeter for the same reason. You have to use casts to get the initializers to compile, as in:

circle_vtbl the_circle_vtbl = {   (double (*)(shape const *))  circle_area,   (double (*)(shape const *))  circle_perimeter};

By the way, one could argue that the initializer really should have another set of nested braces, as in:

circle_vtbl the_circle_vtbl = { {       (double (*)(shape const *))circle_area,       (double (*)(shape const *))circle_perimeter} };

However, C lets you get by with only one set of braces.

You can avoid these casts by declaring the circle functions so that they match the vtbl members exactly, namely as:

double circle_area(shape const *me);double circle_perimeter(shape const *me);

But then you need casts to implement the functions, as in:

double circle_area(shape const *me) {   circle const *pc = (circle const *)me;     return PI * pc->radius * pc->radius;}

I don't know a way to avoid these casts when using inheritance by composition. If you do, please speak up.

These casts are a nuisance, but the real problem with inheritance bycomposition is that it doesn't scale up very well to handle deep classhierarchies. For example, suppose the shape hierarchy looks like:

In this hierarchy, circle is derived directly from shape , but rectangle and triangle are derived from polygon , which is derived from shape .

In C++, the notation for a virtual function call is the same for a pointer to an object anywhere in the hierarchy. For example, given:

double a;shape *ps;polygon *pp;

then you can compute the area for either the shape or the polygon using a call of the same form, namely:

a = ps->area();

for the shape and:

a = pp->area();

for the polygon .Using inheritance by composition in C, the form of the call changesdepending on how deep the target is in the hierarchy. For example, tocompute the area of a shape you write:

a = ps->vptr->area(ps);

but to compute the area of a polygon , you write:

a = pp->base.vptr->area((shape *)pp);

It greats worse the deeper the target type is in the hierarchy.

Inheritance by copying and editing doesn't have this problem. Withinheritance by copying and editing, the notation for a virtual call isthe same for every type in a hierarchy. Unfortunately, inheritance bycopying and editing has other problems.

I'll look at the pros and cons of inheritance by copying and editing in my next column.

Dan Saks is president of Saks & Associates, a C/C++ training and consulting company. For more information about Dan Saks, visit his website at www.dansaks.com. Dan also welcomes your feedback: e-mail him at . .