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Alf P. Steinbach
Guest
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 Posted: Mon Jul 19, 2004 2:06 pm Post subject: Re: Achieving virtualness from base class constructors |
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* Alexander Terekhov:
| Quote: |
"Alf P. Steinbach" wrote:
[...]
One thing I don't see, immediately at least, is how to implement
"nonpoly" behavior,
?? "A class that declares or inherits a virtual function is called a
polymorphic class." Nonpoly thing would simply "disable" polymorphic
behavior (because it just can't be done right [current "cheating"
aside for a moment] during object construction and destruction).
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Yes, how to do that (no matter where) is one thing I don't see about
how this proposal should work.
| Quote: | and another, how that could be made to not break
assumptions in base classes (when their constructor have executed
successfully),
?? What assumptions?
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For example, that in any Base member function a Base pure virtual
function can be called without UB.
| Quote: | and a third, how to provide that in post-pre calls; i.e.,
what calls can be made, and how can they be be ensured to be non-poly?
struct T {
/* ... */
virtual void f() = 0;
void f1() nonpoly {
// (*this) is "T nonpoly"
f(); // ill-formed
}
void f2() {
// (*this) is "T poly"
}
T() {
// (*this) is "T nonpoly"
f1(); // OK
f2(); // ill-formed
}
post T() {
// (*this) is "T poly"
f1(); // OK
f2(); // OK
}
~T() throw() {
// (*this) is "T nonpoly"
f1(); // OK
f2(); // ill-formed
}
pre ~T() throw() {
// (*this) is "T poly"
// ...
}
};
or something like that ("poly_cast<>" aside for a moment).
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I'm almost sure I'm now interpreting this incorrectly, but it seems that
by default _all_ calls a constructor can now make would in the new
scheme be ill-formed, thereby breaking all existing C++ code?
Btw., I suggest continuing this discussion in comp.std.c++, which I have
cross-posted this response to (i.e., I suggest removing the
comp.lang.c++.moderated group in any response).
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Alexander Terekhov
Guest
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| Posted: Mon Jul 19, 2004 5:03 pm Post subject: Re: Achieving virtualness from base class constructors |
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"Alf P. Steinbach" wrote:
[...]
| Quote: | and a third, how to provide that in post-pre calls; i.e.,
what calls can be made, and how can they be be ensured to be non-poly?
struct T {
/* ... */
virtual void f() = 0;
void f1() nonpoly {
// (*this) is "T nonpoly"
f(); // ill-formed
}
void f2() {
// (*this) is "T poly"
}
T() {
// (*this) is "T nonpoly"
f1(); // OK
f2(); // ill-formed
}
post T() {
// (*this) is "T poly"
f1(); // OK
f2(); // OK
}
~T() throw() {
// (*this) is "T nonpoly"
f1(); // OK
f2(); // ill-formed
}
pre ~T() throw() {
// (*this) is "T poly"
// ...
}
};
or something like that ("poly_cast<>" aside for a moment).
I'm almost sure I'm now interpreting this incorrectly, but it seems that
by default _all_ calls a constructor can now make would in the new
scheme be ill-formed, thereby breaking all existing C++ code?
|
Almost (in the ctors and dtors of the polymorhic classes;
"compatibility" mode aside for a moment). How else would you close
that hole in the C++ type system? I'd suggest that people should
better ASAP start using POLY and NONPOLY empty defines.  ;-)
regards,
alexander.
---
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