Initializing const member post-construction?

Lars Kyllingstad public at kyllingen.NOSPAMnet
Tue Oct 28 09:25:14 PDT 2008

Steven Schveighoffer wrote:
> "Jerry Quinn" wrote
>> Hi.  I'm trying to port a C++ program to D as an exercise in exploring D. 
>> As I'm doing this, I've run into a bit of confusion with the const system.
>> I have something like
>> class A {}
>> class B {
>>  const A a;
>>  void init(A aa) { a = aa; }
>> }
>> This doesn't work, because dmd (2.020) complains that you can't initialize 
>> a const member after the constructor.  The catch is that the value of aa 
>> is not available at construction time, but only later on.  However, I'd 
>> still like to declare that once set, the object referred to by a is const.
>> The C++ code used a pointer, but it seemed to me like D's references were 
>> more capable than C++'s, so I'm trying to use them.
>> To me it seems like this should still be allowed.  Even though the object 
>> referred to by a is const, the reference itself shouldn't need to be. 
>> This seems morally equivalent to:
>> const(A)* a;
>> which is allowed by dmd.  In both cases I'm trying to tell the compiler 
>> that the object referred to by a is const.
>> Is there a way to do what I'm trying to do?
> You can hide a behind a property:
> class B {
>   private A aPriv;
>   void init(A aa)
>   {
>      aPriv = aa;
>   }
>   const(A) a() const { return aPriv;}
> }
> Now, do not use aPriv anywhere else in your code, and you should be all set. 
> Use -inline when compiling and you should see no performance penalty.
>> What's the reason for not allowing this?
> I was unaware you could even set a in the constructor.  I don't think 
> there's any general 'set once' type modifier.
> -Steve 

Isn't that what 'final' is for? I can't seem to find any details on it 
in the D1 specs, but at least that's what it does in Java. I'm not sure 
it's implemented in D1 (yet?), though.

BTW, if you don't initialise a const member variable upon declaration, 
then it *must* be initialised in every constructor, I think.


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