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

[Weed]

Bill Baxter пишет:
> 2008/12/18 Weed <resume755 at mail.ru>:
>> 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 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.
> 
> You mean there will be this circular dependency?
> 

Yes

>    struct DynamicMatrix {
>        DynamicMatrix opMul(StaticMatrix m) {...}
>        ...
>    }
> 
>    struct StaticMatrix {
>         DynamicMatrix opMul(DynamicMatrix m) {...}
>         ...
>    }
> 
> I think this will work fine with DMD as long as you have both
> DynamicMatrix and StaticMatrix in the same file.
> 
> If you *do* want to put them in separate modules, then, yes, you would
> probably run into trouble.
>

Yes.

In addition, in the overloaded methods will probably need to implement a 
different mechanism for creating all types of matrices (this need for 
returning values), eg, static matrix template will have to pass an array 
pattern in a string like this (very cumbersome):

==========================================
char[] matBySize(int size) {
	char[] init;
	for (int i = 0; i < size; i++) {
		init ~= "[";
		for (int j = 0; j < size - 1; j++) {
			init ~="0, ";
		}
		init ~= "0]";
		if (i == size - 1) init ~= "]"; else init ~= ",\n";
	}
	return "mat([" ~ init ~ ")";
}


struct mat {
	int[][] value;

	static mat opCall(int[][] init) {
		mat m;
		m.value = init;
		return m;
	}

	mat opPostInc() {
		foreach (row; value) {
			foreach (ref item; row) {
				item++;
			}
		}
		return this;
	}
}

void myFunc() {
	static sm = mixin(matBySize(2));
	auto dm = mixin(matBySize(3));
}

// (thanks naryl for this code sample)
==========================================


In addition, if I want to add other types of matrices (I have already 
talked about the matrix, which is part of another matrix) it will be 
difficult to do so, because the structures do not support the interfaces 
and will have to manually monitor so that all methods have been 
implemented correctly.

Figuratively speaking, in the spirit of the matrix is class, not the 
structure.


> It is kind of annoying that issues like this end up dictating how
> source code is split among files in D.  I really think it should be
> *my* choice to split up a file if I think it's getting too big.
> 
> --bb