Re: Opt-out polymorphism?
Stewart Gordon: > Then you're not overriding at all. You're just declaring a function in the > derived class > that happens to have the same name. I think Sean refers to the second usage of "new" in C#: http://msdn.microsoft.com/en-us/library/51y09td4%28v=vs.71%29.aspx Bye, bearophile
Re: Opt-out polymorphism?
On 13/02/2011 21:34, Sean Eskapp wrote: Is there a way to specify that a function is nonvirtual, but can still be "overriden" in base classes? e.g. Then you're not overriding at all. You're just declaring a function in the derived class that happens to have the same name. As such, it seems to me to have little use besides writing confusing code. Stewart.
Re: Opt-out polymorphism?
On Sun, 13 Feb 2011 16:34:04 -0500, Sean Eskapp
wrote:
Is there a way to specify that a function is nonvirtual, but can still be
"overriden" in base classes? e.g.
class A
{
void foo()
{
writeln("A");
}
}
class B : A
{
void foo()
{
writeln("B");
}
}
void main()
{
(new A).foo();
(new B).foo();
}
Should output:
A
B
Is there a way to do this?
You can make them templates. Templates are not final, and are not virtual.
e.g.: (untested)
class A
{
void foo()()
{
writeln("A");
}
}
class B : A
{
void foo()()
{
writeln("B");
}
}
The huge *huge* drawback is this:
A a = new B;
a.foo(); // outputs "A"
So I don't see a very common use case for this.
-Steve
Re: Opt-out polymorphism?
Generalizing the original question to *all* member functions, it can be
desirable to to have non-polymorphic inheritance, at least not *runtime*
polymorphic. I get the impression that it wouldn't be used much by most
people who post on these newsgroups, but there are application areas it
can be very useful.
For example, in tightly embedded applications it can be important to
minimize code-bloat in the use of templates in C++, so presumably also
applicable to parametized classes/structs in D. In C++ the idiom I'm
refering to has the template inherit the
template-parameter-type-independent (common) code from a non-template
base. An example plucked out of the air could be a linked list, the
link logic with no payload could be encapsulated in a non-template base
and the payload logic added in a template class that derives from it. I
expect that it would also be useful in D if embedded application
programming were an area that D were serious about catering for.
The criterion that determines whether it needs to be runtime polymorphic
or not is whether the resulting object is intended to be used through
its base class (polymorphic) interface or its derived class
(non-polymorphic) interface.
Aside: this distinction is the reason for Herb Sutter's C++ guideline
"Always make base class destructors virtual and public or nonvirtual and
protected" from his book "More Exceptional C++".
Steve
On 14/02/11 11:59, Jonathan M Davis wrote:
On Sunday 13 February 2011 13:34:04 Sean Eskapp wrote:
Is there a way to specify that a function is nonvirtual, but can still be
"overriden" in base classes? e.g.
class A
{
void foo()
{
writeln("A");
}
}
class B : A
{
void foo()
{
writeln("B");
}
}
void main()
{
(new A).foo();
(new B).foo();
}
Should output:
A
B
Is there a way to do this?
No. It's potentially error-prone to do that, and allowing the programmer to
specificy that sort of thing complicates things. C# allows you to choose whether
a function is derived or not in a fairly clear manner, and C++ allows you to do
it in a somwhat poor manner, but in D, the compiler makes all of the decisions
about whether a function is virtual or not. That definitely simplifies things,
and
since in virtually all cases, you want virtual functions, it's not generally an
issue.
The only ways that you're likely to be able to make a function non-virtual in a
class are by making it private (which may or may not continue to make functions
non-virtual, since that contradicts TDPL) and if you make a function final. But
D
takes the tact of either it's overridable and virtual, or it's non-overridable
and non-virtual. So, you can't do what you're trying to do.
And honestly, in most cases, I think that what you're trying to do is just plain
begging for bugs. It is kind of cool that C# found a relatively clean way to
deal with it, but I honestly don't know what it's useful for. I'd be worried
about a program which overrode non-virtual functions. It's highly likely to
cause bugs.
Jonathan M Davis
Re: Opt-out polymorphism?
On Sunday 13 February 2011 17:40:39 bearophile wrote: > Jonathan M Davis: > > And honestly, in most cases, I think that what you're trying to do is > > just plain begging for bugs. It is kind of cool that C# found a > > relatively clean way to deal with it, but I honestly don't know what > > it's useful for. I'd be worried about a program which overrode > > non-virtual functions. It's highly likely to cause bugs. > > C# designers are _not_ stupid or ignorant people :-) I never claimed that they were. They cleaned up how overriding works in comparison to C++, and they given essentially the same options for overriding as C++ only they definitely do it better. However, just because it's possible to do something in a particular language doesn't mean that it's a good idea to do it. I seriously question that it's _ever_ a good idea to have a function which isn't virtual but can be overridden. What reasonable use case does that have. I've never seen it, and I've definitely seen bugs caused by having functions not be virtual and yet still overridden. C# definitely improves over C++ in this regard, but the ability to override without polymorphism is of questionable value. The C# designers chose to have it in the language just like C++ did but do it better. The Java and D designers chose to make overriding and virtual functions intrinsically linked so that you can't override anything that isn't virtual. No, the C# designers aren't stupid - far from it - but that doesn't mean that allowing non-virtual functions to be overridden is a good idea. - Jonathan M Davis
Re: Opt-out polymorphism?
Jonathan M Davis: > And honestly, in most cases, I think that what you're trying to do is just > plain > begging for bugs. It is kind of cool that C# found a relatively clean way to > deal with it, but I honestly don't know what it's useful for. I'd be worried > about a program which overrode non-virtual functions. It's highly likely to > cause bugs. C# designers are _not_ stupid or ignorant people :-) Bye, bearophile
Re: Opt-out polymorphism?
On Sunday 13 February 2011 13:34:04 Sean Eskapp wrote:
> Is there a way to specify that a function is nonvirtual, but can still be
> "overriden" in base classes? e.g.
>
> class A
> {
> void foo()
> {
> writeln("A");
> }
> }
>
> class B : A
> {
> void foo()
> {
> writeln("B");
> }
> }
>
> void main()
> {
> (new A).foo();
> (new B).foo();
> }
>
> Should output:
> A
> B
>
> Is there a way to do this?
No. It's potentially error-prone to do that, and allowing the programmer to
specificy that sort of thing complicates things. C# allows you to choose
whether
a function is derived or not in a fairly clear manner, and C++ allows you to do
it in a somwhat poor manner, but in D, the compiler makes all of the decisions
about whether a function is virtual or not. That definitely simplifies things,
and
since in virtually all cases, you want virtual functions, it's not generally an
issue.
The only ways that you're likely to be able to make a function non-virtual in a
class are by making it private (which may or may not continue to make functions
non-virtual, since that contradicts TDPL) and if you make a function final. But
D
takes the tact of either it's overridable and virtual, or it's non-overridable
and non-virtual. So, you can't do what you're trying to do.
And honestly, in most cases, I think that what you're trying to do is just
plain
begging for bugs. It is kind of cool that C# found a relatively clean way to
deal with it, but I honestly don't know what it's useful for. I'd be worried
about a program which overrode non-virtual functions. It's highly likely to
cause bugs.
Jonathan M Davis
Opt-out polymorphism?
Is there a way to specify that a function is nonvirtual, but can still be
"overriden" in base classes? e.g.
class A
{
void foo()
{
writeln("A");
}
}
class B : A
{
void foo()
{
writeln("B");
}
}
void main()
{
(new A).foo();
(new B).foo();
}
Should output:
A
B
Is there a way to do this?
