Stylistic question about inheritance

Andrew Koenig ark at acm.org
Thu Mar 31 21:33:31 CEST 2005


Suppose I want to define a class hierarchy that represents expressions, for 
use in a compiler or something similar.

We might imagine various kinds of expressions, classified by their top-level 
operator (if any).  So, an expression might be a primary (which, in turn, 
might be a variable or a constant), a unary expression (i.e. the result of 
applying a unary operator to an expression), a binary expression, and so on.

If I were solving such a problem in C++, I would define a base class for all 
expressions, then derive the various kinds of expression classes from that 
base class.  However, I would not anticipate ever creating objects of the 
base class, so I would make it abstract.

In Python, I can imagine doing the same thing:

    class Expr(object):
        pass

    class UnaryExpr(Expr):
        # ...

    class BinaryExpr(Expr):
        # ...

and so on.  However, although I don't have a choice in C++ about having a 
base class--you can't use dynamic binding without it--in Python I do have 
that choice.  That is, I don't need to have the base class at all unless I 
want to have some operations that are common to all derived classes.

Of course, there are reasons to have a base class anyway.  For example, I 
might want it so that type queries such as isinstance(foo, Expr) work.  My 
question is: Are there other reasons to create a base class when I don't 
really need it right now?





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