It's worse than I thought

[Sönke Ludwig]

Walter Bright schrieb:
> Sönke Ludwig wrote:
>> I've noticed some things that seem to be inconsistent with this 
>> behavior for local variables and where I've not found a workaround for 
>> (with the exception of casts):
>>
>>
>> 1. Arrays or associative arrays of references to const instances:
>>   class C {}
>>   const(C)[] a;
>>   const(C)[string] b;
>>
>>   It seems to be impossible here to make the array elements 
>> assignable, while at the same time keeping the class instances const. 
>> I currently need this in a function, where I'm getting a const array 
>> of instances and need to build a map to those.
>>
>>   void fn( in C[] objs ){
>>     const(C)[string] map;
>>     map["test"] = objs[0]; // error, not mutable
>>   }
>>
>>
>> 2. Class references as members:
>>   class D {
>>     const(C) c;
>>     const C d;
>>     this( C c ){
>>       this.c = c; // error
>>       this.d = d; // works, but, of course, only inside of the 
>> constructor
>>     }
>>   }
> 
> There isn't much to be done about this problem except make the 
> implementation of the class itself const.

As far as I understand it, it's currently only possible to have mutable 
references to const/invariant data when using local/global variables, 
but not inside classes/structs/arrays.

Now, after playing some more, I have made some classes "invariant class 
C {}" (a really nice feature). I think it is absolutely _vital_ to be 
able to have at least arrays/maps of such classes. However, in this case 
it is not even possible to cast away the invariant, as it now belongs to 
the type C.
The workarounds I've found are to make a proxy "struct CS { C c; }" and 
store that, or to cast the class reference to void*. But both solutions 
are ugly.

Having said that, especially after using the system now for more than 
some const methods, I actually like the current system - of course with 
the above exception :) - and although it is of course quite different to 
C++'s solution, it's actually quite easy to grasp.

PS: on a side note, I found that invariantness is lost in a few 
situations, when a const modifier is applied. Unfortunately I don't have 
an example at hand, will post one if I'm hitting it again.