What Standard says on declared only symbols

From:
Adam Badura <abadura@o2.pl>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 23 Oct 2007 13:04:10 CST
Message-ID:
<1193134097.746640.76550@v23g2000prn.googlegroups.com>
    Lets assume I have declared a symbol (lets say, a function "void
f()") but never defined it as well as never used it. All compilers
that I have worked with do not make any problems, they just ignore
"f". But do Standard makes any guarantee on that? Or is such program
not well formed?

    This question arose from a simple problem. I had a template
container (TreeInfo). I wanted to make it serializable (by inheriting
from my own IWritable interface). The interface introduced two (pure
virtual) functions "load" and "save". I wrote them for the TreeInfo
assuming the contained type was serializable as well. In case the
contained type was not serializable the code was incorrect (however
syntax was OK). But I thought that if the contained type was not
serializable then no one will ever serialize the container and thous
"load" and "save" would not be used and thous no error would occur
during compilation. However it didn't work this way. (Maybe it is
compiler specific, this time I used MSVC 8.0.) It seems that since
"load" and "save" are virtual they must be present in vtable and thous
must be compiled even if not used.
    I try to solve the problem somehow. I can make just a
TreeInfoSerializable which will inherit TreeInfo as well as IWritable
and expect my users to use either one as needed. I could try some
tricks with template metaprogramming (this seems most elegant however
is likely to be worst because the code is in a large project and this
could cause need for a lot of changes). I thought also on some
implementation tricks they would however relay on defined but not
declared functions. (I wrote this lengthy explanation because perhaps
someone will be able to suggest a different approach which would be a
nice bonus to the actual question stated in the first part :).)

    Adam Badura

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