Feature request: Deploying a class instance in the default data segment

[Weed]

Bill Baxter пишет:
> On Thu, Dec 18, 2008 at 2:05 PM, Weed <resume755 at mail.ru> wrote:
>> Feature request: Deploying a class instance in the default data segment and
>> creation of a class instance at compile time.
>>
>> In this post I'll try to prove the necessity of this feature.
>>
>> I have found that in D it is impossible to create a polymorphic object
>> (class or struct) which can be created at compile-time.
>>
>> Good specific example of such an object is a matrix class.
>>
>> For some matrices it is good to create and initialize them at compile
>> time, not dynamically at run time.
>>
>> Also for matrices of predefined size it is wrong to duplicate their
>> dimensions (their width and height should be declared invariant and,
>> consequently occupy memory only once)
>>
>> At the same time dynamically created matrices of arbitrary size which
>> can be changed at run time are also needed. For example to make a slice of
>> giant matrix it's more efficient to modify it in-place than to copy it.
>> These matrices have variable width and height.
>>
>> So we have minimum two types of matrices: dynamic and constant sized with
>> ability of compile time initialization. It is necessary to provide
>> interaction between them (for example overload arithmetic operators
>> for them). For this purpose it is necessary for them to be inherited
>> from common parent.
> 
> This point I don't think is true.  DynamicMatrix can implement methods
> like opMult(StaticMatrix M), and vice versa.

I'm not sure (see below)

> The only thing I can think of you can't do is make an array of
> pointers to Matrix, the elements of which might be a DynamicMatrix, or
> might be StaticMatrix.

Yes, or suppose I want to add a class matrix, which is part of an 
existing large matrix (which is a kind of frame).

> But even that, if you really feel you need it for some reason, could
> be done using a struct/class with a union inside.  Most cases where
> you might want to accept either/or can be done with templates.

I think this is impossible. If I create struct for compile-time matrix 
and class for all other I will not be able to create a methods for 
interaction between these objects through the templates because some of 
them will be announced before the other and it will not be able to get 
another type of object in a template.

(I do not know, understand this long phrase on my strange English or 
not. tell if the phrase is unclear)


>> So it can not be structures, only classes.
>>
>> But classes can not be created at compile time.
>>
>> So I propose to add possibility of deployment of class instance in the
>> default data segment and creation of class instance at compile time.
> 
> This might be a useful feature, but I'm not finding your argument for
> it quite convincing enough yet.

Yes, perhaps matrix is not good example