Re: [boost] [challenge: Dynamic Overloads]

From: Joel de Guzman (joel_at_[hidden])
Date: 2007-10-09 02:53:59

Marco Costalba wrote:
> On 10/8/07, Joel de Guzman <joel_at_[hidden]> wrote:
>> Marco Costalba wrote:
>>
>>>> It has the right semantics and is very idiomatic. This is how
>>>> Boost.Function does it. Only in this case, we can have many mappings
>>>> instead of just one.
>>>>
>>> I would prefer 'add' or 'assign' here because 'overload' is a kind of
>>> a container while fooX are single functions.
>>>
>>> My intuitive idea of operator=() is that at the end what is at the
>>> left is == of what is at the right, in other words:
>>>
>>> std::string a;
>>> a = "apple";  // now I would think 'a' is an apple
>>> a = "blueberry"; // now 'a' is *no more* an apple, is a blueberry
>>>
>>> assert(a=="apple"); // ??? strange last assignement was a bluebarry here!
>> Assignment can be lossy, you know. Here's a sample:
>>
>>     int x;
>>     x = 0.0001;
>>     x == 0.0001; // ??? huh
>>
>> That "overloads" is a container is an implementation detail. Or
>> how about:
>>
>>      std::string a;
>>      a = 'x';
>>      a == 'x'; // oops can't compare char and string
>>
>> at any rate, It *is* possible to follow the semantics of Boost.Function:
>> http://boost.org/doc/html/function/tutorial.html#id1187293
>>
> 
> I've tried to understand why it seems strange to me the proposed
> operator=() for assigning functions to overload:
> 
> void foo1();
> int foo2(int);
> 
> overload f;
> f = &foo1;
> f = &foo2;
> 
> I came to the conclusion that all boils down to transitive property of
> equality. In other words until childhood we are teached that
> 
> If a == b and b == c, then a == c
> 
> Because the following line
> 
> if (a = b)  // here = instead of == is intended
>     assert(a==b);
> 
> should never fail for any properly defined operator=() and
> operator==() it derives that operator=() as proposed for our overload
> does not satisfy the above very intuitive concepts (because &foo1 !=
> &foo2) so I would say it cannot be called 'idiomatic' for this case.

Yep. Agreed. See my other post why I am convinced your intuition
is correct :-) No need to convince me more ;)

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net