Bill Baxter wrote:
> On Sun, Feb 22, 2009 at 5:10 AM, Denis Koroskin<2korden@gmail.com>  wrote:
>> On Sat, 21 Feb 2009 21:49:46 +0300, Bill Baxter<wbaxter@gmail.com>  wrote:
>>
>>> On Sun, Feb 22, 2009 at 1:02 AM, Weed<resume755@mail.ru>  wrote:
>>>> Bill Baxter пишет:
>>>>> 2009/2/21 Weed<resume755@mail.ru>:
>>>>>> Weed пишет:
>>>>>>> Bill Baxter пишет:
>>>>>>>
>>>>>>>> Why don't you just show us the class in the way you would like to
>>>>>>>> write it in C++, and we'll show you how to write it in D, or finally
>>>>>>>> agree with you that it's not possible.   But as long as you continue
>>>>>>>> to be hand-wavy about "common base classes" we're at a bit of an
>>>>>>>> impasse.  So far everyone thinks D can do what you want it to do
>>>>>>>> based
>>>>>>>> on your vague descriptions.
>>>>>> As I said, you can write everything using "goto" and "if".
>>>>>>
>>>>>> ...But why you do not like the original example of this thread?
>>>>> Please post again.  I don't seem to recall any detailed example.
>>>>>
>>>>> --bb
>>>>
>>>> http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=83506
>>> You should use a struct there!   Your code does not show you doing
>>> anything that would even remotely suggest using a class is worthwhile.
>>>   You're doing value operations on value types.  That's what structs
>>> are for.
>>>
>>> --bb
>> That's what *everyone* tells him!
>
> I am vaguely open to the possibility that he has a point.  FLENS uses
> inheritance, for instance.  But I don't recall what the intent of that
> inheritance was in FLENS, exactly.  I more or less ported it to D
> without that, though, so I guess I didn't find it to be essential.
> See this:
>    http://flens.cvs.sourceforge.net/viewvc/flens/FLENS/matvec/densevector.h?revision=1.36&view=markup
> Versus this:
>    http://www.dsource.org/projects/multiarray/browser/trunk/multiarray/dflat/DenseVector.d
>
> Weed, if you want to make a real argument, that would be a good place
> to start.  Investigate why and how FLENS uses inheritance.
>
> --bb

From a quick look at the flens DenseVector class:
it inherits from a Vector Interface which only declares the destructor as virtual.
My educated guess is that this interface has a similar purpose to the "struct interfaces" feature discussed here before - similar to C++ concepts.
I don't see any polymorphism here - the functions aren't virtual.

Weed пишет:
> Bill Baxter пишет:
>> On Sun, Feb 22, 2009 at 1:02 AM, Weed <resume755@mail.ru> wrote:
>>> Bill Baxter пишет:
>>>> 2009/2/21 Weed <resume755@mail.ru>:
>>>>> Weed пишет:
>>>>>> Bill Baxter пишет:
>>>>>>
>>>>>>> Why don't you just show us the class in the way you would like to write it in C++, and we'll show you how to write it in D, or finally agree with you that it's not possible.   But as long as you continue to be hand-wavy about "common base classes" we're at a bit of an impasse.  So far everyone thinks D can do what you want it to do based on your vague descriptions.
>>>>> As I said, you can write everything using "goto" and "if".
>>>>>
>>>>> ...But why you do not like the original example of this thread?
>>>> Please post again.  I don't seem to recall any detailed example.
>>>>
>>>> --bb
>>> http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=83506
>> You should use a struct there!   Your code does not show you doing
>> anything that would even remotely suggest using a class is worthwhile.
>>  You're doing value operations on value types.  That's what structs
>> are for.
> 
> Why?
> 
> What if I have not substantiated the fact that c1 is a class I should use there a structure?
> 
> Used in D a model of placement classes only in heap have a rule "if you made the class and trying to pass it by value is somewhere in your code, there is a design error?

Explains why the question is given in this form:

I am received or wrote a classes. Is it right to overload the operator opAdd and use them? I think yes.

But why not allow this operation at the same speed that allows C++?

Weed wrote:
> Weed пишет:
>> Bill Baxter пишет:
>>> On Sun, Feb 22, 2009 at 1:02 AM, Weed <resume755@mail.ru> wrote:
>>>> Bill Baxter пишет:
>>>>> 2009/2/21 Weed <resume755@mail.ru>:
>>>>>> Weed пишет:
>>>>>>> Bill Baxter пишет:
>>>>>>>
>>>>>>>> Why don't you just show us the class in the way you would like to
>>>>>>>> write it in C++, and we'll show you how to write it in D, or finally
>>>>>>>> agree with you that it's not possible.   But as long as you continue
>>>>>>>> to be hand-wavy about "common base classes" we're at a bit of an
>>>>>>>> impasse.  So far everyone thinks D can do what you want it to do based
>>>>>>>> on your vague descriptions.
>>>>>> As I said, you can write everything using "goto" and "if".
>>>>>>
>>>>>> ...But why you do not like the original example of this thread?
>>>>> Please post again.  I don't seem to recall any detailed example.
>>>>>
>>>>> --bb
>>>> http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=83506
>>> You should use a struct there!   Your code does not show you doing
>>> anything that would even remotely suggest using a class is worthwhile.
>>>  You're doing value operations on value types.  That's what structs
>>> are for.
>>  	
>> Why?
>>
>> What if I have not substantiated the fact that c1 is a class I should
>> use there a structure?
>>
>> Used in D a model of placement classes only in heap have a rule "if you
>> made the class and trying to pass it by value is somewhere in your code,
>> there is a design error?
> 
> Explains why the question is given in this form:
> 
> I am received or wrote a classes. Is it right to overload the operator
> opAdd and use them? I think yes.
> But why not allow this operation at the same speed that allows C++?

Actually, in D, it's really difficult to give a class value semantics.
If a, b are members of some class, consider
(1) a = a + b;
(2) a += b;
It is nearly impossible to efficiently make both of these have the same effect!!
You can only do it with either copy-on-write, ie, even case (2) always allocates; or by using a proxy class.
This is something which I consider to be a serious problem.
Much more serious than the speed issue.

Weed Wrote:

> Weed �����:
> > Bill Baxter �����:
> >> On Sun, Feb 22, 2009 at 1:02 AM, Weed <resume755@mail.ru> wrote:
> >>> Bill Baxter �����:
> >>>> 2009/2/21 Weed <resume755@mail.ru>:
> >>>>> Weed �����:
> >>>>>> Bill Baxter �����:
> >>>>>>
> >>>>>>> Why don't you just show us the class in the way you would like to write it in C++, and we'll show you how to write it in D, or finally agree with you that it's not possible.   But as long as you continue to be hand-wavy about "common base classes" we're at a bit of an impasse.  So far everyone thinks D can do what you want it to do based on your vague descriptions.
> >>>>> As I said, you can write everything using "goto" and "if".
> >>>>>
> >>>>> ...But why you do not like the original example of this thread?
> >>>> Please post again.  I don't seem to recall any detailed example.
> >>>>
> >>>> --bb
> >>> http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=83506
> >> You should use a struct there!   Your code does not show you doing
> >> anything that would even remotely suggest using a class is worthwhile.
> >>  You're doing value operations on value types.  That's what structs
> >> are for.
> > 
> > Why?
> > 
> > What if I have not substantiated the fact that c1 is a class I should use there a structure?
> > 
> > Used in D a model of placement classes only in heap have a rule "if you made the class and trying to pass it by value is somewhere in your code, there is a design error?
> 
> Explains why the question is given in this form:
> 
> I am received or wrote a classes. Is it right to overload the operator opAdd and use them? I think yes.
> 
> But why not allow this operation at the same speed that allows C++?

If you pass it by value you'll lose polymorphism. That must mean that you inherit that class only to avoid duplicating code. And that is easily done with template mixins.

naryl пишет:

>>>>>> --bb
>>>>> http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=83506
>>>> You should use a struct there!   Your code does not show you doing
>>>> anything that would even remotely suggest using a class is worthwhile.
>>>>  You're doing value operations on value types.  That's what structs
>>>> are for.
>>> 
>>> Why?
>>>
>>> What if I have not substantiated the fact that c1 is a class I should use there a structure?
>>>
>>> Used in D a model of placement classes only in heap have a rule "if you made the class and trying to pass it by value is somewhere in your code, there is a design error?
>> Explains why the question is given in this form:
>>
>> I am received or wrote a classes. Is it right to overload the operator opAdd and use them? I think yes.
>>
>> But why not allow this operation at the same speed that allows C++?
> 
> If you pass it by value you'll lose polymorphism.

Debatable

By the way, in my example the transfer class by value is not important(!), it is important to create a temporary class on the stack. Then we can pass a reference without problem (C++ allows this through the "&")

> That must mean that you inherit that class only to avoid duplicating code. And that is easily done with template mixins.

It is possible that this polymorphism is not needed and should be prohibited for operations by value. The class is ready, why it should not be used and have to redo it?

Can not think about changing when the class is ready - he could have enormous complexity and hierarchy.

Weed wrote:
> naryl пишет:
> 
>>>>>>> --bb
>>>>>> http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=83506
>>>>> You should use a struct there!   Your code does not show you doing
>>>>> anything that would even remotely suggest using a class is worthwhile.
>>>>>  You're doing value operations on value types.  That's what structs
>>>>> are for.
>>>>  	
>>>> Why?
>>>>
>>>> What if I have not substantiated the fact that c1 is a class I should
>>>> use there a structure?
>>>>
>>>> Used in D a model of placement classes only in heap have a rule "if you
>>>> made the class and trying to pass it by value is somewhere in your code,
>>>> there is a design error?
>>> Explains why the question is given in this form:
>>>
>>> I am received or wrote a classes. Is it right to overload the operator
>>> opAdd and use them? I think yes.
>>>
>>> But why not allow this operation at the same speed that allows C++?
>> If you pass it by value you'll lose polymorphism.
> 
> Debatable
> 
> By the way, in my example the transfer class by value is not
> important(!), it is important to create a temporary class on the stack.
> Then we can pass a reference without problem (C++ allows this through
> the "&")

Sure, but you're not using polymorphism.

> 
>> That must mean that you inherit that class only to avoid duplicating code. And that is easily done with template mixins.
> 
> It is possible that this polymorphism is not needed and should be
> prohibited for operations by value. The class is ready, why it should
> not be used and have to redo it?
> 
> Can not think about changing when the class is ready - he could have
> enormous complexity and hierarchy.

This is the fundamental tradeoff at the heart of the matter.
In D, the choice of whether an object will use polymorphism or not is considered a fundamental design decision. D gets significant benefits from this. C++ allows you to defer the decision, but it doesn't come for free.
(Generally speaking, statically typed languages do force you to make more decisions at design time). Notice that you have little gotchas in C++, such as the need to declare a virtual destructor on every struct you think you might "someday" use polymorphically. It sticks around, even if it never gets used.
One of the nice things about a D struct, compared to a C++ struct, is that you *know* it's simple, it never has that kind of baggage.

D does choose different trade-offs from C++. If it was always the same, it'd be the same language!

BTW, if you're concerned about performance, you'd do well to use compile-time polymorphism rather than run-time, when possible. D's metaprogramming support leaves C++ for dead.

Weed Wrote:

> >> But why not allow this operation at the same speed that allows C++?
> > 
> > If you pass it by value you'll lose polymorphism.
> 
> It is possible that this polymorphism is not needed and should be prohibited for operations by value. The class is ready, why it should not be used and have to redo it?
> 
> Can not think about changing when the class is ready - he could have enormous complexity and hierarchy.

It's probably already designed to be reference-type. I think, you'll have big troubles trying to change its semantic to value-type and effectively you'll have to redo it.

Kagamin wrote:
> Weed Wrote:
> 
>>>> But why not allow this operation at the same speed that allows C++?
>>> If you pass it by value you'll lose polymorphism.
>> It is possible that this polymorphism is not needed and should be
>> prohibited for operations by value. The class is ready, why it should
>> not be used and have to redo it?
>>
>> Can not think about changing when the class is ready - he could have
>> enormous complexity and hierarchy.
> 
> It's probably already designed to be reference-type. I think, you'll have big troubles trying to change its semantic to value-type and effectively you'll have to redo it.

In other words, it is not always easy to translate C++ to D. It's probably not worthwhile to translate any large codebase into another programming language, in most cases, and if you do, you will probably need to rewrite large portions of it anyway.

Don пишет:
> Weed wrote:
>> naryl пишет:
>>
>>>>>>>> --bb
>>>>>>> http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=83506
>>>>>>>
>>>>>> You should use a struct there!   Your code does not show you doing
>>>>>> anything that would even remotely suggest using a class is
>>>>>> worthwhile.
>>>>>>  You're doing value operations on value types.  That's what structs
>>>>>> are for.
>>>>> 
>>>>> Why?
>>>>>
>>>>> What if I have not substantiated the fact that c1 is a class I should use there a structure?
>>>>>
>>>>> Used in D a model of placement classes only in heap have a rule "if
>>>>> you
>>>>> made the class and trying to pass it by value is somewhere in your
>>>>> code,
>>>>> there is a design error?
>>>> Explains why the question is given in this form:
>>>>
>>>> I am received or wrote a classes. Is it right to overload the operator opAdd and use them? I think yes.
>>>>
>>>> But why not allow this operation at the same speed that allows C++?
>>> If you pass it by value you'll lose polymorphism.
>>
>> Debatable
>>
>> By the way, in my example the transfer class by value is not important(!), it is important to create a temporary class on the stack. Then we can pass a reference without problem (C++ allows this through the "&")
> 
> Sure, but you're not using polymorphism.
> 

There is no, but the same object can be used in other places, including
the polymorphism

>>
>>> That must mean that you inherit that class only to avoid duplicating code. And that is easily done with template mixins.
>>
>> It is possible that this polymorphism is not needed and should be prohibited for operations by value. The class is ready, why it should not be used and have to redo it?
>>
>> Can not think about changing when the class is ready - he could have enormous complexity and hierarchy.
> 
> This is the fundamental tradeoff at the heart of the matter.
> In D, the choice of whether an object will use polymorphism or not is
> considered a fundamental design decision.

This is awful! Much easier to give to the application programmer to decide how it will use the derived class somewhere. Such a division should be a basic principle of designing a complex program.

> D gets significant benefits
> from this.

Benefits:

1. Disappeared slicing
2. ?

> C++ allows you to defer the decision, but it doesn't come for free.

Clarify what you mean?

> (Generally speaking, statically typed languages do force you to make
> more decisions at design time). Notice that you have little gotchas in
> C++, such as the need to declare a virtual destructor on every struct
> you think you might "someday" use polymorphically. It sticks around,
> even if it never gets used.
> One of the nice things about a D struct, compared to a C++ struct, is
> that you *know* it's simple, it never has that kind of baggage.
> 
> D does choose different trade-offs from C++. If it was always the same, it'd be the same language!

There is no need to compare the structs from C++ and D. In fact, in C++ classes and structures are the same.

I like D's idea of POD structs + without them you can not ensure compatibility with C, but it is very important.

Now we are talking about classes.

In C++ classes with the same problems, such as what they do not have a common base class (Object in D). Passing classes by value is not a problem - is an advantage.

As I said earlier, the presence of pointers is also an advantage, although they are dangerous and can lead to complex bugs.

> 
> BTW, if you're concerned about performance, you'd do well to use compile-time polymorphism rather than run-time, when possible. D's metaprogramming support leaves C++ for dead.
> 

I think if there was a language with all the features as in D but with the model objects from C++, he would have won great popularity. Mainly it would have moved C++ developers who are satisfied with the model classes of C++ but not satisfied with the absence of other modern features: metaprogramming, contracts, delegates, closures etc.

Christopher Wright пишет:
> Kagamin wrote:
>> Weed Wrote:
>>
>>>>> But why not allow this operation at the same speed that allows C++?
>>>> If you pass it by value you'll lose polymorphism.
>>> It is possible that this polymorphism is not needed and should be prohibited for operations by value. The class is ready, why it should not be used and have to redo it?
>>>
>>> Can not think about changing when the class is ready - he could have enormous complexity and hierarchy.
>>
>> It's probably already designed to be reference-type. I think, you'll have big troubles trying to change its semantic to value-type and effectively you'll have to redo it.
> 
> In other words, it is not always easy to translate C++ to D. It's probably not worthwhile to translate any large codebase into another programming language, in most cases, and if you do, you will probably need to rewrite large portions of it anyway.

As a result, classes will be slow, or require more code to achieve speeds comparable to C++.

That is actually a model of classes D harder than C++.