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digitalmars.D.learn - aliasing superclass constructors

reply Nick Treleaven <ntrel-public yahoo.co.uk> writes:
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.
Dec 26 2012
parent reply "Maxim Fomin" <maxim maxim-fomin.ru> writes:
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;
}
Dec 26 2012
parent reply Nick Treleaven <ntrel-public yahoo.co.uk> writes:
On 26/12/2012 15:13, Maxim Fomin wrote:

 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); }
 }


Thanks for the reply. But actually I'm not looking to define a 
parameter-less ctor for Exception subclasses, but to copy all of a 
parent class's constructors into a subclass. This is nice to do for 
Exception subclasses because sometimes I just want a new type that 
behaves exactly the same, but has different RTTI. But for other classes 
it can be useful if you want to override some methods, but don't need to 
add/change any constructors, apart from exposing them from the parent.

I've found a relevant discussion:
http://forum.dlang.org/post/op.u36pc3y4o7cclz korden-pc
Dec 26 2012
parent "Maxim Fomin" <maxim maxim-fomin.ru> writes:
On Wednesday, 26 December 2012 at 17:11:31 UTC, Nick Treleaven 
wrote:

 On 26/12/2012 15:13, Maxim Fomin wrote:

 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); }
 }


 Thanks for the reply. But actually I'm not looking to define a 
 parameter-less ctor for Exception subclasses, but to copy all 
 of a parent class's constructors into a subclass. This is nice 
 to do for Exception subclasses because sometimes I just want a 
 new type that behaves exactly the same, but has different RTTI. 
 But for other classes it can be useful if you want to override 
 some methods, but don't need to add/change any constructors, 
 apart from exposing them from the parent.

 I've found a relevant discussion:
 http://forum.dlang.org/post/op.u36pc3y4o7cclz korden-pc


Currently dmd always creates implicit constructor for derived 
classes if base class has a ctor. This can be prevented by 
annotating ctor as a  disable, but it is unacceptable because you 
have to write another constructor. So, there is no way for not 
creating implicit constructor in a convenient way.
Dec 26 2012