As a practical habit, once I stumble upon a very tricky error, I usually
share the valuable knowledge of "when you do this ... and get that ...
it's probably because ... "
Damn, sometimes they can even become cool quizzes...
So to warn those oblivious to the dangers of opDispatch, here is my the
yesterday nightmare, the striped down code below.
import std.algorithm;
import std.array;
class Widget {
string _name;
Widget[] _children;
this(in string name){
_name = name.idup;
}
Widget opDispatch(string nm)(){
auto r = find!((Widget c){ return c._name == nm; })(_children);
return r.front();
}
}
void main(){
Widget g = new Widget("G");
Widget warr[] = [new Widget("W"),g];
find(warr,g);
}
produces:
Error 1 Error: template std.algorithm.startsWith(alias pred = "a
== b",Range,Ranges...) if (isInputRange!(Range) && Ranges.length > 0 &&
is(typeof(binaryFun!(pred)(doesThisStart.front,withOneOfThese[0].front))))
startsWith(alias pred = "a == b",Range,Ranges...) if
(isInputRange!(Range) && Ranges.length > 0 &&
is(typeof(binaryFun!(pred)(doesThisStart.front,withOneOfThese[0].front))))
matches more than one template declaration,
C:\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(1892):startsWith(alias
pred = "a == b",Range,Ranges...) if (isInputRange!(Range) &&
Ranges.length > 0 &&
is(typeof(binaryFun!(pred)(doesThisStart.front,withOneOfThese[0].front))))
and
C:\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(1980):startsWith(alias
pred = "a == b",Range,Elements...) if (isInputRange!(Range) &&
Elements.length > 0 &&
is(typeof(binaryFun!(pred)(doesThisStart.front,withOneOfThese[0]))))
C:\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d 1488
The tricky part is that *any* class with unconstrained (or loosely
constrained) opDispatch is also a Range, and at least a bidirectional
one, since it "provides" all the primitives: front, popFront etc.
In fact such classes could penetrate almost any attempts at C++
trait-like stuff and should be avoided.
The moral: unconstrainted opDispatch == TROUBLE.
Hope that helps!
P.S. Strangely enough, that problem haven't showed up until update to
2.047 release, so it's probably detonated by some changes to Phobos. I
guess better sooner than later.
--
Dmitry Olshansky
