Re: [boost] [convert] are you mixing default_value and error_value?

Subject: Re: [boost] [convert] are you mixing default_value and error_value?
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2009-07-09 09:23:31

On Thursday, July 9, 2009, Vladimir Batov <batov_at_[hidden]> wrote:
>
> From: "Vicente Botet Escriba" <vicente.botet_at_[hidden]>
> ...
>
> Do we need to give an initial value just for conversion to work at
> all, or is it only needed as a return in the error case?
>
> Tony
>
>
> It is needed for both usages: the initial value when the class is not
> default constructible, and the error value when we want this value to be
> returned when failure. Now we can not have an exception for the conversion
> to a class that is not default constructible because when we give a default
> value it is also interpreted as the error value. As I said we are mixing
> both concepts, which is not good, as both are orthogonal.
>
>
> The is only *one* value provided -- a conversion-failure value. It happens to be used internally for some initialization (for std::stream-based conversions). It's an internal implementation matter and not guaranteed to be that way. What do we achieve by stressing it out?
>

Maybe it should not be used for initialization at all.  Let's say
MyClass has a bad_state setting,  that, when set, stops the class from
doing anything, even processing << operators.
So if I pass in bad_state as a default return value, it won't work if
it is internally being used as an init value.

Tony
> V.
>
>
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