const member function

[Mike Parker via Digitalmars-d-learn]

On 2/21/2015 4:31 PM, rumbu wrote:

> My intention is not to have a read-only getter, I want to call SomeProp
> on a const object:
>
> class S
> {
>      private int cache = -1;
>      private int SomeExpensiveOp() { return 12345; }
>
>      public @property const(int) SomeProp()
>      {
>          if (cache = -1)
>              cache = SomeExpensiveOp();
>          return cache;
>      }
> }
>

const is transitive. If an instance of S is const, then all of its 
members are const, too. You cannot modify cache in that case. Assuming 
cache only needs to be initialized once, you can do this instead:

     this() { cache = SomeExpensiveOp(); }

     public @property const(int) SomeProp() const
     {
         return cache;
     }

Notice the const on the end of SomeProp. That makes the function 
callable on a const instance, but you still cannot modify cache inside 
of it.