aliasing superclass constructors

[Maxim Fomin]

On Wednesday, 26 December 2012 at 14:38:45 UTC, Nick Treleaven 
wrote:
> Hi,
> Is there an easy way to forward/expose all the constructors of 
> a parent class? Just curious. Here are my attempts:
>
> class EFoo : Exception
> {
> 	//alias typeof(super).this this;	// line 12
> 	//alias typeof(super).__ctor __ctor;	// line 13
> }
>
> uncommenting line 12:
> superthis.d(12): Error: identifier expected following '.' 
> instead of 'this'
> superthis.d(12): Error: no identifier for declarator 
> typeof(super)
> superthis.d(12): Error: semicolon expected to close alias 
> declaration
> superthis.d(12): Error: found 'this' when expecting '('
> superthis.d(12): Error: basic type expected, not ;
> superthis.d(12): Error: found ';' when expecting ')'
> superthis.d(17): Error: semicolon expected following function 
> declaration
> Compilation failed.
>
> uncommenting line 13:
> superthis.d(13): Error: this is not a type
>
> I've got a mixin to work though:
>
> class EFoo : Exception
> {
> 	mixin SuperCtors;
> }
>
> mixin template SuperCtors()
> {
> 	this(T...)(T args)
> 	if (__traits(compiles, {new typeof(super)(args);}))
> 	{super(args);}
> }
>
> But it would be nicer if an alias statement could work to avoid 
> defining the mixin in every project that needs it.
>
> Partly I'm interested in this as I'm wondering if 'alias this' 
> conflicts with 'normal' alias syntax when aliasing constructors.

The problem is that Exception class ctor requires at least one 
explicit argument 
(https://github.com/D-Programming-Language/druntime/blob/master/src/object.di#L339).

To solve the problem in general case you can either supply 
default arguments for all ctor parameters or define 
parameter-less ctor.

In your particular case because you cannot change Exception you 
can inherit not from Exception class but from intermediate one, 
which does one of the mentioned things:

import std.stdio;

class DException : Exception
{
	this(string value = "") { super(value); }
}

class A : DException
{

}

void main()
{
	A a = new A;
}