cannot cast

[Jonathan M Davis]

On Thursday, May 03, 2012 09:33:01 Namespace wrote:
> 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?

If you want to restrict opCast, then use a template constraint, constraining 
it to what you want to work with it. Also, casting away const is generally a 
bad idea in D. Casting away const and mutating a variable is an _extremely_ 
bad idea. You _really_ shouldn't be doing it. So, the fact that you _have_ a 
function which is specifically trying to cast away const is almost certainly 
_not_ a good idea.

http://stackoverflow.com/questions/4219600/logical-const-in-d


- Jonathan M Davis