RE: [boost] Formal Review: Boost Cast Library

From: Jesse Jones (jesjones_at_[hidden])
Date: 2000-12-07 23:29:40

>Jesse Jones wrote:
>> >I hope that is sufficient to speak for the utility of these
>> functions. As to
>> >triviality, the same could be said for std::pair<>. Sometimes a trivial
>> >construct has great advantages in expressive power.
>>
>> Sure, but I think these two functions impair rather than enhance
>> readability. How is:
>>    derived = polymorphic_downcast <Derived*>(base);
>> better than
>>    derived = dynamic_cast<Derived*>(base);
>>    assert(derived != NULL);
>
>It's better because it wasn't supposed to be a 'dynamic_cast' in the first
>place. It should be
>
>derived = static_cast<Derived*>(base);
>
>because you use 'polymorphic_downcast' only then you KNOW that base actually
>points to Derived. But you don't want to deal with a mess you'll get in case
>if someone make changes that break your code, so you use
>'polymorphic_downcast' to trap that situation.

Right, and I'm saying that you can nearly always simply use dynamic_cast 
and do the assert yourself.

>> It's really only better because polymorphic_downcast doesn't do a
>> dynamic_cast in release. But I don't think this sort of
>> micro-optimization merits yet another cast operator.
>
>It's not about micro-optimization, it's about showing the intent of code in a
>less amount of lines and in the most expressive way. 'polymorphic_downcast'
>states that cast to Derived* should always succeed BY DEFINITION, point. 

Does it really show the intent? I don't know about you, but I have real 
problems remembering the difference between polymorphic_cast and 
polymorphic_downcast. I suspect most users won't have a clue what either 
one means. It's true that they can look up the definition, but this seems 
like an unneccesary burden to place on readers.

>/**/ }". Also, speaking about expressiveness, consider passing the result of
>the cast to a function:
>
>Derived* derived = dynamic_cast<Derived*>(base);
>assert(derived != NULL);
>foo(derived);
>
>vs.
>
>foo(polymorphic_downcast <Derived*>(base));
>
>I certainly prefer the last form.

I would put the assert in foo. :-)

  -- Jesse