Subj : Re: spidermonkey api
To   : netscape.public.mozilla.jseng
From : Jens Thiele
Date : Fri Nov 05 2004 11:52 am

Brendan Eich schrieb:
> Jens Thiele wrote:
> 
>> I am currently mapping c++ class-based inheritance to prototype based
>> inheritance.
>>
>> In JavaScript:
>>
>> function Base() {
>> };
>> Base.prototype.baseMethod = ...
>>
>> function Sub() {
>> //  Base.call(this);
>> };
>>
>> Sub.prototype.__proto__   = Base.prototype;
>> Sub.prototype.specialized = ...
> 
> 
> Why aren't you setting Sub.prototype = new Base; ?
> 
> /be

IMHO
Sub.prototype.__proto__   = Base.prototype;
is the cleaner way to modell class-based inheritance via prototyping
(each class is modelled by a prototype object) because:
- new Base is in fact called to set __proto__ and we don't want to call
the constructor on the sub prototype object
(in the simple example above not a problem since the constructor
function is a nop)
- we must set the constructor property manually and afterwards it is
enumerable (Sub.prototype.constructor = Sub;)

see also:
http://egachine.berlios.de/inheritance/
(compare figure 3 and figure 4)

the assertions to be true in the example on
http://egachine.berlios.de/inheritance/
(missing on that page)

function assert(f){
  if (!f()) throw Error("assertion failed: "+f.toSource());
};

base=new Base(0);
assert(function(){return !base.specialized;});
assert(function(){return (base.baseMethod()+base.polymorph()==0);});
assert(function(){
         return base instanceof Base && (!(base instanceof Sub));
       });

sub=new Sub(90,0);
assert(function(){return sub.specialized;});
assert(function(){
         return (sub.baseMethod()
                 +sub.polymorph()
                 +sub.specialized()==100);
       });
assert(function(){return sub instanceof Base && sub instanceof Sub;});

Jens

.