how to use opdot

[Ary Borenszweig]

Steven Schveighoffer wrote:
> "Ary Borenszweig" wrote
>> Steven Schveighoffer wrote:
>>> "Morusaka" wrote
>>>> Hi,
>>>>
>>>> I've read about opdot in D language spec operator overload section, but 
>>>> the little snippet of code provided isn't enough, for me, to figure out 
>>>> what it is supposed to do and how to use it or what it could be usefull 
>>>> for.
>>>>
>>>> Could you please help me to get the right way?
>>> opDot is useful if you want to make a 'wrapper' type.  That is, you want 
>>> to mimic another type, but you want to slightly alter the behavior. 
>>> opDot allows you to 'inherit' all the member functions and fields from 
>>> the wrapped type.  For example, if I wanted to create a wrapper type that 
>>> added a 'blahblah' integer to the type, I could do this:
>>>
>>> struct AddBlahBlah(T)
>>> {
>>>    T _t;
>>>    int blahblah;
>>>
>>>    T *opDot() { return &_t;}
>>> }
>>>
>>> Now, if I declare an AddBlahBlah!(C) and class C has a member foo():
>>>
>>> C c;
>>> AddBlahBlah!(C) abb = AddBlahBlah!(C)(c);
>>>
>>> abb.foo(); // translates to abb.opDot().foo()
>>> abb.blahblah = 5; // sets abb.blahblah to 5, doesn't affect _t
>> Wow. That's incredibly useful for doing decorators!
>>
>> http://en.wikipedia.org/wiki/Decorator_pattern
> 
> Not exactly ;)  The wrapped type is not equivalent to inheritance.

Ah, right. I forgot the inheritance part. :(

> 
> For example, if you have a function that takes a class C, you can't pass an 
> AddBlahBlah!(C) type into it.
> 
> However, a template function which expects a type C or a wrapped C, could 
> possibly be used as you say.
> 
> -Steve 
> 
>