Implicit conversion from array of class to array of interface
Phil Deets
pjdeets2 at gmail.com
Sun Dec 13 01:08:19 PST 2009
On Sun, 13 Dec 2009 03:22:31 -0500, Phil Deets <pjdeets2 at gmail.com> wrote:
> (D 2.033) I have a need to do something like this code:
>
> interface I {}
> class C : I {}
> class D : I {}
>
> void f(I[]) {}
> void f(bool) {}
>
> void g(T)(T param) {
> f(param);
> }
>
> int main()
> {
> bool b;
> C[] c;
> D[] d;
> g(b);
> g(c);
> g(d);
> return 0;
> }
>
> This results in the output:
>
> temp.d(9): Error: function temp.f (I[] _param_0) is not callable using
> argument types (C[])
> temp.d(9): Error: cannot implicitly convert expression (param) of type
> C[] to bool
> temp.d(18): Error: template instance temp.g!(C[]) error instantiating
>
> As you can see, I can't cast C[] to I[] when I call f because T can be
> bool too. My workaround for now is:
>
> interface I {}
> class C : I {}
> class D : I {}
>
> void f(I[]) {}
> void f(C[] param) {
> f(cast(I[])param);
> }
> void f(D[] param) {
> f(cast(I[])param);
> }
> void f(bool) {}
>
> void g(T)(T param) {
> f(param);
> }
>
> int main()
> {
> bool b;
> C[] c;
> D[] d;
> g(b);
> g(c);
> g(d);
> return 0;
> }
>
> Is there a better way to deal with this? Is this behavior a design bug?
>
> Phil Deets
>
I found another workaround which doesn't require a bunch of extra
overloads. I'll probably update it to use that template someone wrote in
that thread about static duck-typing.
//interface I {void h();}
class C /*: I*/ {void h() {}}
class D /*: I*/ {void h() {}}
void f(T)(T param)
{
static if (__traits(compiles, param[0].h()))
{
// do I[] overload here
}
else
{
pragma(msg, "Unsupported type for f.")
static assert(0);
}
}
void f(T:bool)(T param) {}
void g(T)(T param) {
f(param);
}
int main()
{
bool b;
C[] c;
D[] d;
g(b);
g(c);
g(d);
return 0;
}
By the way, how do I move the bool specialization into the general
function?
void f(T)(T param)
{
static if (__traits(compiles, param[0].h()))
{
// do I[] overload here
}
else static if (T is bool) // how do I do this?
{
}
else
{
pragma(msg, "Unsupported type for f.")
static assert(0);
}
}
More information about the Digitalmars-d-learn
mailing list