[Issue 4000] New: Function pointer/delegate covariance
d-bugmail at puremagic.com
d-bugmail at puremagic.com
Mon Mar 22 07:11:15 PDT 2010
http://d.puremagic.com/issues/show_bug.cgi?id=4000
Summary: Function pointer/delegate covariance
Product: D
Version: future
Platform: All
OS/Version: All
Status: NEW
Keywords: rejects-valid
Severity: enhancement
Priority: P2
Component: DMD
AssignedTo: nobody at puremagic.com
ReportedBy: bearophile_hugs at eml.cc
--- Comment #0 from bearophile_hugs at eml.cc 2010-03-22 07:11:14 PDT ---
This page about C#:
http://msdn.microsoft.com/en-us/library/ms173174%28VS.80%29.aspx
contains this working C# code:
class Mammals {}
class Dogs : Mammals {}
class Program {
public delegate Mammals HandlerMethod();
public static Mammals FirstHandler() { return null; }
public static Dogs SecondHandler() { return null; }
static void Main() {
HandlerMethod handler1 = FirstHandler;
HandlerMethod handler2 = SecondHandler;
}
}
This is a possible translation to D2:
class Mammals {}
class Dogs : Mammals {}
alias Mammals function() HandlerMethod;
Mammals function() handler;
Mammals FirstHandler() { return null; }
Dogs SecondHandler() { return null; }
void main() {
HandlerMethod handler1 = &FirstHandler;
HandlerMethod handler2 = &SecondHandler; // line 9, err
handler = &SecondHandler; // line 10, err
}
But the D2 code gives two compile errors:
test.d(9): Error: cannot implicitly convert expression (& SecondHandler) of
type Dogs function() to Mammals function()
test.d(10): Error: cannot implicitly convert expression (& SecondHandler) of
type Dogs function() to Mammals function()
It can be positive for the D type system to accept this code too.
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