Making containers that "go both ways"

[Bill Baxter]

Jarrett Billingsley wrote:
> "janderson" <askme at me.com> wrote in message 
> news:fnem61$2054$1 at digitalmars.com...
> 
>> Personally I'd make it a class.  Yes its going to go on the heap however 
>> the resizeable array part or whatever the container contains, will 
>> probably go on the heap anyway.
> 
> Good point.  Also keep in mind that even if it's a class, it can be 
> allocated on the stack in functions if you use 'scope x = new X()', so 
> there's at least one heap allocation avoided. 

And if it's a struct it can be allocated on the heap using new if you like.

I would be much more inclined to use classes for containers if they 
either didn't require new, or could be new'ed at point of declaration 
(like I think they can in Java?).

This scattering of information all over the place is one of the things 
that annoys me most about C++.

// Annoying
class D {
     MyContainer c; // must remember to 'new' this later in constructor
     int[] d; // no worries, it's good.
     MyStructContainer e; // also no worries

     ...
     this() {
         c = new MyContainer;
     }
}

// This would be less annoying
class D {
     MyContainer c = new MyContainer;

     ...
     this() {}
}

// Or this
class D {
     scope MyContainer c;

     ...
     this() {}
}

And the second reason I think containers should be structs is because if 
you want to "upgrade" some existing code from using a built-in array or 
AA to using a custom container, it's much less headache if it's a 
struct-based container.  Also if you want to write generic code that can 
handle built-in and custom containers, then it's going to be easier if 
your custom containers are structs.

I think main important difference is the way copying is handled, but 
unfortunately "places where a container of type X is copied and 
subsequently it is assumed that X.member is a distinct pointer" is 
pretty difficult to grep for.

Initialization is also different but you initialize things far less 
often than you pass them around or assign them to other variables. 
Initialization patterns are also easier to grep for.

That's my 2c.

--bb