Object-Oriented Programming With ANSI-C (pdf)

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10 Delegates — Callback Functions
10.7 Another application — sort
Let us implement a small text sorting program to check if Filter really is reusable, to
see how command line options are handled, and to appreciate that a delegate can
belong to an arbitrary class.
A sort filter must collect all text lines, sort the complete set, and finally write
them out. Section 7.7 introduced a List based on a dynamic ring buffer which we
can use to collect the lines as long as we add a sorting method. In section 2.5 we
implemented a simple String class; if we integrate it with our class hierarchy we
can use it to store each line in the List.
Let us start with the main program which merely creates the filter with its
delegate:
int main (int argc, char * argv [])
{ void * filter = new(Filter(), new(Sort(), 0));
return mainLoop(filter, argv);
}
Because we can attach the callback methods to any class, we can create the
delegate directly in a subclass of List:
% SortClass: ListClass Sort: List {
char rflag;
%—
void flags (_self, Object @ filter, char flag);
int line (_self, const Object @ filter, const char * fnm, \
char * buf);
int quit (_self, const Object @ filter);
%}
To demonstrate option handling we recognize −r as a request to sort in reverse
order. All other flags are rejected by the flags() method which has flag as a tag for
respondsTo():
% flag: Sort flags {
%casts
assert((flagM) flags == flags);
if (flag == ’r’)
self —> rflag = 1;
else
fprintf(stderr, "usage: %s [—r] [file...]\n",
progname(filter)),
exit(1);
}
Given String and List, collecting lines is trivial:
% Sort line {
%casts
assert((lineM) line == line);
addLast(self, new(String(), buf));
return 0;
}