Re: cannot cast

[sclytrack]

On 05/03/2012 09:33 AM, 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?

Unqual können Sie finden in std.traits.


template Unqual(T)
{
    version (none) // Error: recursive alias declaration @@@BUG1308@@@
    {
             static if (is(T U == const U)) alias Unqual!U Unqual;
        else static if (is(T U == immutable U)) alias Unqual!U Unqual;
        else static if (is(T U == inout U)) alias Unqual!U Unqual;
        else static if (is(T U == shared U)) alias Unqual!U Unqual;
        else alias T Unqual;
    }
    else // workaround
    {
             static if (is(T U == shared(const U))) alias U Unqual;
        else static if (is(T U == const U )) alias U Unqual;
        else static if (is(T U == immutable U )) alias U Unqual;
        else static if (is(T U == inout U )) alias U Unqual;
        else static if (is(T U == shared U )) alias U Unqual;
        else alias T Unqual;
    }
}

unittest
{
    static assert(is(Unqual!(int) == int));
    static assert(is(Unqual!(const int) == int));
    static assert(is(Unqual!(immutable int) == int));
    static assert(is(Unqual!(inout int) == int));
    static assert(is(Unqual!(shared int) == int));
    static assert(is(Unqual!(shared(const int)) == int));
    alias immutable(int[]) ImmIntArr;
    static assert(is(Unqual!(ImmIntArr) == immutable(int)[]));
}