cannot cast

[Namespace]

On Wednesday, 2 May 2012 at 22:38:36 UTC, Namespace wrote:
> Other, shorter example:
>
> [code]
> import std.stdio, std.traits;
>
> class A {
> 	int val;
>
> 	alias val this;
> 	
> 	T opCast(T : Object)() {
> 		writeln("FOO");
> 		
> 		return to!(T)(this);
> 	}
> }
>
> class B : A {
>
> }
>
> T to(T : Object, U : Object)(const U obj) {
> 	return *(cast(T*) &obj);
> }
>
> T const_cast(T)(const T obj) {
> 	return cast(T) obj;
> }
>
> void main () {
> 	A a = new B();
> 	a.val = 42;
> 	
> 	writefln("a.val: %d", a.val);
>
> 	B* b = cast(B*) &a;
> 	writefln("*b.val: %d", b.val);
>
> 	B b1 = to!(B)(a);
> 	writefln("b1.val: %d", b1.val);
> 	
> 	B b2 = cast(B) a;
> 	writefln("b2.val: %d", b2.val);
> 	
> 	const B b3 = cast(B) a;
> 	
> 	B b4 = const_cast(b3);
> }
> [/code]
>
> print:
>
> alias_this_impl.d(24): Error: function 
> alias_this_impl.A.opCast!(B).opCast () is
>  not callable using argument types ()
> alias_this_impl.d(44): Error: template instance 
> alias_this_impl.const_cast!(B) e
> rror instantiating
>
> I'm not very skillful in such "template" stories. Maybe someone 
> can help me?

Solved with

T const_cast(T)(const T obj) {
	return to!(T)(obj);
}

But i think that there must exist a more nicer way to cast away 
const, isn't there?

To cast away "const" with a simple cast to "T" fails (see my post 
above), because i have no idea, how i can restrict my opCast. So 
i have to convert it again with "to". Do some of you have any 
ideas how i can restrict my opCast, so my const_cast doesn't 
match it, e.g. with some template magic?