From: sbnaran AT localhost DOT localdomain (Siemel Naran)
Newsgroups: comp.os.msdos.djgpp,comp.lang.c++
Subject: Re: Casting a class as a function pointer.
Date: 14 Jul 1999 02:08:41 GMT
Organization: University of Illinois at Urbana-Champaign
Lines: 39
Message-ID: <slrn7ons3o.gkc.sbnaran@localhost.localdomain>
References: <378A2FFE DOT D9A4F8A0 AT americasm01 DOT nt DOT com> <slrn7ol5nn DOT 7fp DOT sbnaran AT localhost DOT localdomain> <378AB489 DOT 75F19B1E AT unb DOT ca> <slrn7omnrb DOT c2d DOT sbnaran AT localhost DOT localdomain> <378B7784 DOT E718C436 AT americasm01 DOT nt DOT com> <slrn7omvch DOT c2d DOT sbnaran AT localhost DOT localdomain> <Pine DOT SUN DOT 3 DOT 96 DOT 990713141829 DOT 8556C-100000 AT silver DOT cs DOT umanitoba DOT ca> <3795989d DOT 81933298 AT nntp DOT ix DOT netcom DOT com>
NNTP-Posting-Host: arboria-73.slip.uiuc.edu
X-Newsreader: slrn (0.9.4.3 UNIX)
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

On Tue, 13 Jul 1999 19:56:53 GMT, David Harmon <source AT netcom DOT com> wrote:
>On Tue, 13 Jul 1999 14:18:58 -0500 in comp.lang.c++, Mark Phillips

>>What's wrong with operator functions?  Not that I disbelieve you, it's
>>just that I'm very unenlightened...
>
>It's not what's wrong with operator functions; it's what's wrong with
>implicit conversion operators.  Unless you are the kind of person who
>never makes mistakes, the compiler will apply them in places where you
>may not expect it, where otherwise the compiler would have caught a type
>mismatch error for you.

Right.  Some of these errors will be caught at compile time, but the
error messages will be very obscure and difficult to wade through.
Some errors will only show themselves at runtime.  For example, suppose
that class String has a conversion to "const char *".  Now consider
this typo
   void f(const String& in) { String out=in+3; }
Maybe we meant to concatenate 'in' and "3".  Or maybe we meant to
add atoi(in) and 3, then convert the result into a String.  However,
there is no operator+(String,int).  So the compiler converts 'in'
to "const char *" using the implicit conversion operator, then
converts 3 from int to size_t and calls operator+(pointer,size_t).
The effect is the same as this
   void f(const String& in) { String out=&in[3]; }
This basically extracts a right substring of 'in'.  And if 'in' is
3 chars or less, then we have a memory access violation.  Because
the above code passes compilation, it's going to be very hard to
detect the error.  The program is going to behave strange, and
we're not going to know where to look for the bug.

Accordingly, class std::string has no operator conversion.  Instead
it has an explicit get function with a rather bizarre name
std::string::c_str() const.

--
----------------------------------
Siemel B. Naran (sbnaran AT uiuc DOT edu)
----------------------------------