Re: How to check class type ?

[DrPi <314@drpi.fr>]

Le 21/04/2021 à 19:40, Dmitry A. Kazakov a écrit :
> On 2021-04-21 19:08, DrPi wrote:
>> Hi,
>>
>> I have the following declarations (just for the example) :
>>
>> type Root is tagged null record;
>> type Root_Ptr is access all Root'Class;
>>
>> type Leaf1 is new Root with null record;
>> type Leaf1_Ptr is access all Leaf1'Class;
>>
>> type Leaf2 is new Root with null record;
>> type Leaf2_Ptr is access all Leaf2'Class;
>>
>>
>> To check if a variable is of type Leaf1_Ptr or Leaf2_Ptr, I do :
> 
> That does not make sense, Leaf1_Ptr is not tagged.
> 
>> if var'Tag = Leaf1'Tag then
>>    ...
>> elsif var'Tag = Leaf2'Tag then
>>    ...
>> end if;
> 
> 
>    if Var in Leaf1'Class then
>       ... -- Class rooted in Leaf1
>    elsif Var in Leaf2'Class then
>       ... -- Class rooted in Leaf2
> 
>> It works. But is this correct ? Should the test be done differently to 
>> suit Ada coding style ?
> 
> It depends on the intent. Usually you should never check for the class 
> except two cases:
> 
> 1. Defensive programming of a downcast:
> 
>    if X in T'Class then
>       declare
>          Y : T'Class renames T'Class (X);
>       begin
>          ...
>       end;
> 
> Downcasts are by themselves problematic, to be avoided when possible.
> 
> 2. Emulation of multiple dispatch.
> 
> In most cases you leave all tag checks to dispatch.
> 
> This may lead to so-called God-classes when primitive operations migrate 
> to the root. E.g. if you had a primitive Foo on Leaf1, but not in Leaf2 
> and the purpose of check would be if you could call to Foo, you might 
> move Foo as abstract to Root and override it as raising exception in 
> Leaf2. Then instead of checking for the tag, you would simply dispatch:
> 
>    Var.Foo; -- Will raise if not Leaf1
> 
> This would be a God-class case.
> 
Thanks for the explanations. As always, very instructive.