Re: [Boost-users] boost any questions and praise

Subject: Re: [Boost-users] boost any questions and praise
From: Alan Tennant (alan2here_at_[hidden])
Date: 2010-02-03 10:03:28

>
> Incidentally, you may be interested in the Constrained Value library (
> http://student.agh.edu.pl/~kawulak/constrained_value/index.html
>

> )
>

I like that idea.

I quite like the idea of things like an 8bit number that stores values
between 0 and 1.

I like the idea of a number type, that can store numbers in all sorts of
forms, an example could be a "8bit integer / 8bit integer" or "4bit float"
or whatever, you could instruct it with methods to do things such as. Have a
scale defining it's limits and possibly also set limits, store 2 numbers in
3 bits. Store a value in a number of forms so that it is always quick to
access for different uses or have it store it in the most specially
efficient form for the value, have a number that expanded until a certain
number of decimal places was reached, or forever, or all sorts of other
related things.

I have some idea about a possible way to represent the expandable number in
binary as well.

At least in C++ I would expect this to have such a performance overhead as
to make it not worth it even trying to implement though, I guess I don't
much like lexical casting automatically or using the cast syntax, prefer
.toString() type methods, but especially between numerical types where it
seems like it's always about fundamentally the same thing, a number.

> I do appreciate there are different views here, however if one has
> > chosen boost::any already then they already know what they want,
> > especially given the stuff in std. Suggesting that because you can
> > cause an error an error could unexpectedly occur when you pass
> > unknown objects into an unknown method in a non sensible example
> > that wouldn't happen isn't helpful.
>
> You have to appreciate where we are coming from. Your original post had
> code where you tried to pass the result of a runtime method as a template
> parameter.
> This suggests you are not hugely familiar with C++ and perhaps Boost.Any,
> so people naturally ponder whether Boost.Any really is the right choice
> (often it isn't) and try to help by making sure the OP has heard about the
> alternatives.
>

Your right, I am fairly new to C++.

> The documentation says that for boost::any a program doesn't crash
> > if you try and cast an any into a type it was not set as, it gives a
> > null instead.
>
> Actually, If try to retrieve a pointer to the type, it would return a null,
> if you tried to retrieve a value, it would throw bad_any_cast.

Thanks.

> For crashing on stream insertion when it's contents can't be
> > inserted vs giving an error that will stop the program more safely
> > and tell the coder what went wrong, I think the latter is more
> > desirable, unless there is a significant performance difference in
> > which case either one is valid.
>
> Right, if you went down a has_operator_stream route, you could throw an
> exception instead of blowing the stack.
>
> Feel free to offer a patch along those lines if you want, but it opens up a
> can worms. If operator<<, why not operator+, or member foo() etc... I am
> sceptical that the maintainer would accept it.
>

Does adobe::polly offer a way to call methods on the anys content without
unwrapping it? Could this be done generically for any method (I know there
would be type/method safety issues)?

I'm using Peters version. My stuff is ambitious but higher level, I can't
write things like this.

It would possibly make a good case to have two versions of boost::any if the
generic methods could be implemented, would also close the can of worms,
nothing would ever need to be added to it again.

Maybe I have overlooked that a stream and other such things are probably not
proper methods at any level and so there are a few worms, but a finite
number.