Re: [boost] [log] Comments

Subject: Re: [boost] [log] Comments
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2010-03-16 11:26:57

On 03/16/2010 01:49 AM, Steven Watanabe wrote:
> AMDG
>
> Andrey Semashev wrote:
>>>> Yes. attr is a formatter and a filter, for example.
>>>
>>> Oh, indeed! But.. why? Cannot one object serve both roles?
>>
>> One is filter and the other is formatter. They have different
>> interfaces (with regard to the library and the user)
>
> Are the interfaces incompatible?

Yes. Filters and formatters do different things, how can they share the 
interface?

>> and serve different purposes (one - to compose filters, the other -
>> formatters).
>
> Why does this matter? spirit uses the same placeholders in both
> qi and karma, for instance.

I'm not a specialist in Qi and Karma internals and cannot judge on how 
that design decision is applicable to Boost.Log. But from my viewpoint, 
mixing filters and formatters is absolutely not a good idea.

>> Also, thread safety requirements are different.
>
> Why? In any sane implementation that I can think of,
> both attr's should store the same immutable data.

The formatter attr can contain a boost::format member, which makes it 
thread-unsafe. Thread unsafe design is common for most formatters.