TITLE: Function declarations with no definitions 


PROBLEM: jbuck@forney.berkeley.edu (Joe Buck)

If you advertise the existence of a function you don't provide, you deserve
to lose, even if the ARM (which, by the way, isn't the final ANSI/ISO
standard) doesn't explicitly say so.


RESPONSE: sdm@cs.brown.edu (Scott Meyers), 1992Feb20

No no no no no no no.  This thinking makes no sense for C++ programs,
because you don't have control over which functions are declared and
defined.  Remember assignment, copy construction, and address-of?  If you
don't declare them, the compiler generates them for you.  So what do you do
if you explicitly want to disallow copy construction?  Answer:  you declare
the copy constructor private AND you explicitly do NOT define it.  By
making it private, you limit its accessibility, but by not defining it, you
plan on getting a link error if you accidently call it yourself in one of
your member functions.  I haven't checked recently, but this is precisely
the strategy that was used in the iostream library to make sure nobody ever
passed a stream by value.

Somewhere in the ARM, by the way, I'm almost certain it expressly allows
you to declare a function and then not define it iff you never call it.

The original question, by the way, concerned virtual functions, which is
certainly a tricky business.  In the general case, the compiler can't
possibly hope to figure out which derived class virtuals might be called,
so it assumes that if a base class virtual is called, then it might really
resolve to a derived class virtual, and it demands a definition for each
derived class implementation.  Better static analysis of a program can
eliminate some of these demands (including the one in the original posting,
as I recall), but they can't make them go away in general.  

It would probably be a good idea for the ANCInators to explicitly address
this issue so compiler writers would know what kinds of analysis they are
required to do.  My feeling is that they should be required to do none, but
may do as much as they please.