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digitalmars.D.learn - opAdd and opAdd_r both match

reply Li Jie <cpunion gmail.com> writes:
Test CODE:
---------------------------
class Foo{
    public:
    Foo opAdd(Foo foo){
        return null;
    }

    Foo opAdd_r(Foo foo){
        return null;
    }
}


void main(){
    Foo foo1 = new Foo;
    Foo foo2 = new Foo;
    Foo foo3 = foo1 + foo2;
}

---------------------------
$ dmd zzzz.d
zzzz.d(16): Error: overloads Foo(Foo foo) and Foo(Foo foo) both match argument
list for opAdd


Why dose not select opAdd automatically? like python:

class Foo:
    def __add__(self, v):
        print "call add"
    def __radd__(self, v):
        print "call radd"

foo1 = Foo()
foo2 = Foo()
foo1 + foo2         # call __add__
Dec 10 2006
parent reply Karen Lanrap <karen digitaldaemon.com> writes:
Li Jie wrote:

 Why dose not select opAdd automatically? like python:

Because more than one match is a hint, that the coder might not have noticed, that there are indeed at least two matches, especially when there is a hirarchy of deriving. In your case one of the overloads is essentially dead code with its possibly harmful impact on maintenance.
Dec 10 2006
parent reply Li Jie <cpunion gmail.com> writes:
== Quote from Karen Lanrap (karen digitaldaemon.com)'s article
 Because more than one match is a hint, that the coder might not have
 noticed, that there are indeed at least two matches, especially when
 there is a hirarchy of deriving.
 In your case one of the overloads is essentially dead code with its
 possibly harmful impact on maintenance.

Thanks. Sometimes I need it. A new program: -------------------- class Bar{} class Foo{ Bar opAdd(T)(T v){ return null; } Bar opAdd_r(T)(T v){ return null; } } auto foo1 = new Foo; auto foo2 = new Foo; auto bar1 = foo1 + 1; // OK auto bar2 = 1 + foo1; // OK auto bar3 = "a" + foo1; // OK auto bar4 = [1,2,3] + foo1; // OK auto bar5 = foo1 + foo2 // Not OK -------------------- How to write opAdd_r? I try to write it: -------------------- template opAdd_r(T){ static if (!is(T == Foo)){ Bar opAdd_r(T v){ return null; } } } -------------------- Compile failed too: zzz.d(61): template zzz.Foo.opAdd_r(T) is not a function template zzz.d(10): template zzz.Foo.opAdd_r(T) cannot deduce template function from argument types (Foo) May be I need a NO-MATCH Template Specialization: ------------------ class Foo{ Bar opAdd_r(T: !Foo)(T v){ // *(T: !Foo)* return null; } } ------------------
Dec 10 2006
parent Karen Lanrap <karen digitaldaemon.com> writes:
Li Jie wrote:

 May be I need a NO-MATCH Template Specialization:

Because in a real world example you need specializations for every other type, you simply do not provide a specialization for the conflicting type.
Dec 11 2006