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From: clyde@hitech.ht.oz (Clyde Smith-Stubbs)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Can you nest subroutines in C?
Summary: pointers to functions are the real problem
Message-ID: <212@hitech.ht.oz>
Date: 3 Jul 89 06:48:45 GMT
References: <4501@crash.cts.com> <603@laic.UUCP>
Organization: HI-TECH Software, Brisbane, QLD, Australia
Lines: 25

In article <603@laic.UUCP>, darin@nova.laic.uucp (Darin Johnson) writes:
> >        I do not understand why C does not support Nested routines.  Perhaps
> >this will be added to the ANSI standard at some point in the future.

I looked into this in depth recently - there is no real problem with
managing a display to allow access to the local variables of outer
functions (except for the messiness of locating register variables) but
the problem arises since in C you can pass a pointer to a function to
an unrelated piece of code. Imagine if you passed a pointer to a a function
at block level 4 to a global function which stored it, then the level 4
function got executed after its own enclosing global function had terminated.
It could attempt to access variables outer to it which did not exist!

The only way around it I could think of was to enforce some compile-time
constraints on what you could do with pointers. I suspect that such
constraints could be devised which would reduce the problem to something
no worse than the existing C problem that arises when e.g. you return
a pointer to an automatic variable from a function (that one catches every
C programmer sooner or later).

In short, I see nested functions as practical to implement in C; there are
certain cases where nested functions can make a solution to a problem much
more elegant than is currently possible.

Clyde Smith-Stubbs, HI-TECH Software, Brisbane, QLD, Australia.