Tools.h++ Manual

15-12 104011 Tandem Computers Incorporated
15
apply() functions
virtual void apply(RWapplyCollectable ap, void* x);
An efficient method for examining the members of a Smalltalk-like collection is
member function apply() . The first argument ( RWapplyCollectable ) is a
typedef:
typedef void(*RWapplyCollectable)(RWCollectable*, void*);
that is, a pointer to a function with prototype:
void yourApplyFunction (RWCollectable* item, void* x)
where yourApplyFunction is the name of the function. You must supply this
function. It will be called for each item in the collection, in whatever order is
appropriate for the collection, and passed a pointer to the item as its first
argument. The second argument ( x ) is passed through from the call to
apply() and is available for your use. It could be used, for example, to hold
a handle to a window on which the object is to be drawn, etc.
Note – Notice the similarity to the apply() function of the generic collection
classes (see “Apply functions” on page 8 in Chapter 14, ““Generic” Collection
Classes”). The difference is in the type of the first argument of the user-
supplied function ( RWCollectable * rather than type*). As with the generic
classes, you must be careful that you cast the pointer item to the proper
derived class.
The apply functions generally employ the "most efficient method" for
examining all members of the collection. This is their great advantage. Their
disadvantage is that they are slightly clumsy to use, requiring the user to
supply a separate function.
1
1. The functional equivalent to apply() in the Smalltalk world is “do”. It takes just one argument—a piece of
code to be evaluated for each item in the collection. This keeps the method and the block to be evaluated
together in one place, resulting in cleaner code. As usual, the C++ approach is messier.