Initializing const member post-construction?

Steven Schveighoffer schveiguy at
Tue Oct 28 09:40:12 PDT 2008

"Lars Kyllingstad" wrote
> 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.

Yes, I believe that is what final is for in D1.  But the OP is using D2, and 
I believe that context for final was removed (it is now only used on member 
functions to mean that the function is no longer virtual).


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