On Monday, 15 August 2016 at 19:40:37 UTC, Steven Schveighoffer
wrote:
On 8/15/16 3:31 PM, Engine Machine wrote:
Suppose I have a templated type like
struct S(T) { int x; static if (T is Y) int y; }
I would like to be able to create a reference to S(T) for any
T,
struct Q
{
S!* s; // Can hold any type of S.
}
and be able to access s.x, since it is common to all S.
Can D do anything like this? It is sort of like runtime
inheritance, but
at the compile time level.
I don't think so. You'd have to cast, as the compiler doesn't
have any understanding that all S instantiations will have an x
member.
This is a problem. How can I cast to something I don't know?
I do not want to have to cast to S!T every time just to access
x, e.g.,
struct Q
{
Object s;
}
which is too general as s can be things that are not of type
S!*.
This seems odd. You will lose the type information for s if you
were to succeed. Might as well just store an int.
Seems like what you want is a variant or some other kind of
tagged union.
No, what I want is a type that is the intersection of all the
types it can be.
The problem is I cannot cast because the type is complex and I
don't know the exact signature, but I do know that it has a basic
type embedded in it.
enum Types
{
Int, Word, Complex
}
struct MyType(string name, Types type, Args...)
{
string Name = name;
Types Type = type;
static if (type == Int) int val;
static if (type == word) ushort val;
static if (type == Complex) { ... };
}
So, how can I get val? I know if it is an int or a word, val
exists. But I can't specify the name or type at runtime to get at
the innards of the type.
Object o = MyType!("test", Types.Int);
Useless! I can never retrieve anything from o. I can't cast it
back. Yet I know that o has a name and a type and possibly a val
if I know the type... and I know all this at compile time. Yet D
doesn't seem to allow me to use this information.
I know one will say use either oop or constructors, but neither
of these are appropriate(although constructors are close). What I
need is a type constructor, which bridges the gap. This is how I
am using the data above, to construct the object, but D treats it
as part of the type, which I don't what.
I guess I will have to use string mixins to construct the types
properly ;/ What a pain in the ass ;/
