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digitalmars.D.learn - override '.' member access

reply spir <denis.spir gmail.com> writes:
Hello,

Cannot find corresponding opSomething method, if any. (opDispatch seems to 
specialise for method call.)
Else, how to catch obj.member?

Denis
-- 
_________________
vita es estrany
spir.wikidot.com
Jan 25 2011
parent reply "Simen kjaeraas" <simen.kjaras gmail.com> writes:
spir <denis.spir gmail.com> wrote:


 Hello,

 Cannot find corresponding opSomething method, if any. (opDispatch seems  
 to specialise for method call.)
 Else, how to catch obj.member?


opDispatch is likely what you want. with the  property annotation, it
will readily support obj.member; and obj.member = foo; syntax.

-- 
Simen
Jan 25 2011
next sibling parent reply spir <denis.spir gmail.com> writes:
On 01/25/2011 10:29 PM, Simen kjaeraas wrote:

 spir <denis.spir gmail.com> wrote:


 Hello,

 Cannot find corresponding opSomething method, if any. (opDispatch seems to
 specialise for method call.)
 Else, how to catch obj.member?


 opDispatch is likely what you want. with the  property annotation, it
 will readily support obj.member; and obj.member = foo; syntax.


Thank you, Simen, i'll try using opDispatch with  property. But I'm not sure 
how to write that concretely. My use case is of a type holding a 
string[AnyThing] AA called symbols. Then, I wish to map
	obj.id
to
	obj.symbols["id"]
(hope I'm clear)

Denis
-- 
_________________
vita es estrany
spir.wikidot.com
Jan 25 2011
parent reply "Simen kjaeraas" <simen.kjaras gmail.com> writes:
spir <denis.spir gmail.com> wrote:


 On 01/25/2011 10:29 PM, Simen kjaeraas wrote:

 spir <denis.spir gmail.com> wrote:


 Hello,

 Cannot find corresponding opSomething method, if any. (opDispatch  
 seems to
 specialise for method call.)
 Else, how to catch obj.member?


 opDispatch is likely what you want. with the  property annotation, it
 will readily support obj.member; and obj.member = foo; syntax.


 Thank you, Simen, i'll try using opDispatch with  property. But I'm not  
 sure how to write that concretely. My use case is of a type holding a  
 string[AnyThing] AA called symbols. Then, I wish to map
 	obj.id
 to
 	obj.symbols["id"]
 (hope I'm clear)


I just found that it is, in fact, unpossible. That is, you can support
either a = foo.id; or foo.id = a; - not both. This is caused by bug 620:
http://d.puremagic.com/issues/show_bug.cgi?id=620

-- 
Simen
Jan 25 2011
parent spir <denis.spir gmail.com> writes:
On 01/26/2011 01:06 AM, Simen kjaeraas wrote:

 spir <denis.spir gmail.com> wrote:


 On 01/25/2011 10:29 PM, Simen kjaeraas wrote:

 spir <denis.spir gmail.com> wrote:


 Hello,

 Cannot find corresponding opSomething method, if any. (opDispatch seems to
 specialise for method call.)
 Else, how to catch obj.member?


 opDispatch is likely what you want. with the  property annotation, it
 will readily support obj.member; and obj.member = foo; syntax.


 Thank you, Simen, i'll try using opDispatch with  property. But I'm not sure
 how to write that concretely. My use case is of a type holding a
 string[AnyThing] AA called symbols. Then, I wish to map
 obj.id
 to
 obj.symbols["id"]
 (hope I'm clear)


 I just found that it is, in fact, unpossible. That is, you can support
 either a = foo.id; or foo.id = a; - not both. This is caused by bug 620:
 http://d.puremagic.com/issues/show_bug.cgi?id=620


I do not really understand the subtleties of this bug, but indeed, I found no 
way to use opDispatch to set data members.
In other words, I could implement python's __getattr__ or Lua's __index,
not  python's __setattr__ or Lua's __newindex.

EDIT: Could do this as a workaround.

alias int[string] Symbols;

class T {
     int i;
     Symbols symbols;
     this (int i, Symbols symbols) {
         this.i = i;
         this.symbols = symbols;
     }
     auto opDispatch (string name) () {
         writeln("get");
         auto p = (name in this.symbols);
         if (p)
             return *p;
         throw new Exception("No symbol called "~name~".");
     }
     auto opDispatch (string name, Value) (Value value) {
         writeln("set ");
         auto p = (name in this.symbols);
         if (p) {
             *p = value;
             return;
         }
         throw new Exception("No symbol called '"~name~"'.");
     }
}
unittest {
     auto t = new T(1, ["j":2, "k":3]);
     writefln("<%s %s %s>", t.i, t.j,t.k);
     t.i = 11;
     t.j(22);
     t.k(33);
     writefln("<%s %s %s>", t.i, t.j,t.k);
//~     writeln(t.l);   // throws as expected
//~     t.l(44);   // throws as expected
     t.j = 222;  // Error: function
                 // __trials__.T.opDispatch!("j").opDispatch ()
                 // is not callable using argument types (int)
}

The last example shows how normal member-set syntax fails. I would like to know 
into what
	obj.name = val
is rewritten.

Denis
-- 
_________________
vita es estrany
spir.wikidot.com
Jan 26 2011
prev sibling parent spir <denis.spir gmail.com> writes:
On 01/26/2011 12:05 AM, spir wrote:

 On 01/25/2011 10:29 PM, Simen kjaeraas wrote:

 spir <denis.spir gmail.com> wrote:


 Hello,

 Cannot find corresponding opSomething method, if any. (opDispatch seems to
 specialise for method call.)
 Else, how to catch obj.member?


 opDispatch is likely what you want. with the  property annotation, it
 will readily support obj.member; and obj.member = foo; syntax.


 Thank you, Simen, i'll try using opDispatch with  property. But I'm not sure
 how to write that concretely. My use case is of a type holding a
 string[AnyThing] AA called symbols. Then, I wish to map
 obj.id
 to
 obj.symbols["id"]
 (hope I'm clear)

 Denis


Right, just understood that you meant, I guess: since D supports properties 
with "x.a" syntax mapping to a func call x.a() under the hood, then one can use 
opDispatch intended for func calls to dispatch plain data membar access.
(woof!).
FWIW: the following works fine:

alias int[string] Symbols;

class T {
     int i;
     Symbols symbols;
     this (int i, Symbols symbols) {
         this.i = i;
         this.symbols = symbols;
     }
     auto opDispatch (string name) () {
         auto p = (name in this.symbols);
         if (p)
             return *p;
         throw new Exception("No symbol called "~name~".");
     }
}
unittest {
     auto t = new T(1, ["j":2, "k":3]);
     writeln(t.i);
     writeln(t.j);
     writeln(t.k);
     writeln(t.l);   // throws as expected
}


Denis
-- 
_________________
vita es estrany
spir.wikidot.com
Jan 26 2011