What's the technical reason that class ctors aren't virtual?

[Ali Çehreli]

On Wed, 24 Aug 2011 16:59:46 +0200, Andrej Mitrovic wrote:

> class Foo
> {
>     this(int x, int y) { }
> }
> 
> class Bar : Foo
> {
> }
> 
> Bar has to define its own ctor even if it only forwards the call to the
> super() ctor, e.g.:
> 
> class Bar : Foo
> {
>     this(int x, int y) { super(x, y); }
> }

That's because it is Bar that constructs its Foo part. The user 
constructs a Bar and must provide only what Bar needs, potentially 
nothing:

class Bar : Foo
{
    this() { super(42, 43); }

> But I'm curious why this works this way.

Another reason would be function hijacking: Subtle changes in types of 
contsructor parameters might bypass derived constructors just becaues 
they now match better to the super's.

> If I have a large inheritance
> tree of classes and I want to change the base class ctor (say I want to
> add another int as a parameter), I'll have to change all the ctors in
> the subclasses as well.
> 
> Isn't that counterproductive?

Sounds like it but is necessary: Which of the constructors of the derived 
constructors should the compiler call after calling super's matching one? 
If it didn't, the derived parts would be left unconstructed.

Ali