Could anybody explain what exactly the difference between a class allocator and constructor is?

erm..

Class[] className;
className.length=10;
className.function;

results in runtime access violation.
Why?

> 
> Class[] className;
> className.length=10;
> className.function;

may be this?

Class[] className;
className.length=10;
className[0]=new Class();
className[0].function;

> may be this?
>
> Class[] className;
> className.length=10;
> className[0]=new Class();
> className[0].function;
>

Yes it is :)

What then does Class[] className do exactly? Or, why do I need to use new?

"Saaa" <empty@needmail.com> wrote in message news:fqjjn5$2jb6$1@digitalmars.com...
>> may be this?
>>
>> Class[] className;
>> className.length=10;
>> className[0]=new Class();
>> className[0].function;
>>
>
> Yes it is :)
>
> What then does Class[] className do exactly? Or, why do I need to use new?
>

The same reason:

Class class;
class.function;

doesn't work.  Because all class variables are references which point to nothing until you use 'new'.

A Class[] is an array of references to class instances which is filled with nulls by default.  You have to actually initialize the array by creating instances of the class.

Class[] arr;

foreach(ref c; arr)
    c = new C;

Thanks,
Why isn't it initialized?
I mean, when are these null referenced class pointers useful?

>>> may be this?
>>>
>>> Class[] className;
>>> className.length=10;
>>> className[0]=new Class();
>>> className[0].function;
>>>
>>
>> Yes it is :)
>>
>> What then does Class[] className do exactly? Or, why do I need to use new?
>>
>
> The same reason:
>
> Class class;
> class.function;
>
> doesn't work.  Because all class variables are references which point to nothing until you use 'new'.
>
> A Class[] is an array of references to class instances which is filled with nulls by default.  You have to actually initialize the array by creating instances of the class.
>
> Class[] arr;
>
> foreach(ref c; arr)
>    c = new C;
> 

"Saaa" <empty@needmail.com> wrote in message news:fqjknh$2lrm$1@digitalmars.com...
> Thanks,
> Why isn't it initialized?
> I mean, when are these null referenced class pointers useful?

The same time any null pointer is useful - when you want to use it as a sentinel for some reason.

D is certainly not unique in this regard.  In all the languages I've ever used or seen, not one will automatically allocate an object when you declare a reference or pointer to one.

Maybe you're getting confused by C++ (I don't know your background) where:

Class c;
c.foo();

is legal, but something entirely different is happening here.  This is more like a D struct, where the class is allocated on the stack, not the heap. In D:

struct Struct
{
    void foo() {}
}

...
Struct s;
s.foo();

The D code:

Class c = new Class;
c.foo();

Translates to:

Class* c = new Class();
c->foo();

in C++.  Again, C++ will not automatically allocate a new class if you just write "Class* c".

Saaa wrote:
> Thanks,
> Why isn't it initialized?
> I mean, when are these null referenced class pointers useful?
> 

Two things.
First, a null reference is free to set up, whereas instantiating a class is not free.

Second:

---
class Bicycle {
   Human owner;
}

auto bike = new Bicycle();
---

The bike doesn't have an owner. Do you want to create a Human object that refers to nobody? Well, maybe, for some purposes, but not necessarily. That would probably alter your definition of Human.

I'm sorry, I've never used a null pointer (as far as I know) nor programmed
in any oo style before.
I just thought that it would work like structures and if I wanted to have a
nullpointer you could do something like:

Class[] className=void;

As I thought that most of the time you just want an instance and not just a
reference to nothing.
Thats why I asked why other people wanted this null pointer.



>
> The same time any null pointer is useful - when you want to use it as a sentinel for some reason.
>
> D is certainly not unique in this regard.  In all the languages I've ever used or seen, not one will automatically allocate an object when you declare a reference or pointer to one.
>
> Maybe you're getting confused by C++ (I don't know your background) where:
>
> Class c;
> c.foo();
>
> is legal, but something entirely different is happening here.  This is more like a D struct, where the class is allocated on the stack, not the heap. In D:
>
> struct Struct
> {
>    void foo() {}
> }
>
> ...
> Struct s;
> s.foo();
>
> The D code:
>
> Class c = new Class;
> c.foo();
>
> Translates to:
>
> Class* c = new Class();
> c->foo();
>
> in C++.  Again, C++ will not automatically allocate a new class if you just write "Class* c".
> 

What is Class doesn't have a default constructor, or has many?

Saaa wrote:
> I'm sorry, I've never used a null pointer (as far as I know) nor programmed in any oo style before.
> I just thought that it would work like structures and if I wanted to have a nullpointer you could do something like:
> 
> Class[] className=void;
> 
> As I thought that most of the time you just want an instance and not just a reference to nothing.
> Thats why I asked why other people wanted this null pointer.
> 
> 
> 
>> The same time any null pointer is useful - when you want to use it as a sentinel for some reason.
>>
>> D is certainly not unique in this regard.  In all the languages I've ever used or seen, not one will automatically allocate an object when you declare a reference or pointer to one.
>>
>> Maybe you're getting confused by C++ (I don't know your background) where:
>>
>> Class c;
>> c.foo();
>>
>> is legal, but something entirely different is happening here.  This is more like a D struct, where the class is allocated on the stack, not the heap. In D:
>>
>> struct Struct
>> {
>>    void foo() {}
>> }
>>
>> ...
>> Struct s;
>> s.foo();
>>
>> The D code:
>>
>> Class c = new Class;
>> c.foo();
>>
>> Translates to:
>>
>> Class* c = new Class();
>> c->foo();
>>
>> in C++.  Again, C++ will not automatically allocate a new class if you just write "Class* c".
>>
> 
>