Re: [boost] Review Request: Boost Exception

From: Matthew Herrmann (matthew.herrmann_at_[hidden])
Date: 2006-11-15 17:54:20

>
> Message: 9
> Date: Tue, 14 Nov 2006 18:06:03 -0800
> From: "Emil Dotchevski" <emildotchevski_at_[hidden]>
> Subject: Re: [boost] Review Request: Boost Exception
> To: <boost_at_[hidden]>
> Message-ID: <BAY125-DAV966A5057842FC29C1782BD4EA0_at_phx.gbl>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> 	reply-type=original
>   
>> >
>> > No, that's not quite what I meant.
>> >
>> > void tag_exception(boost::exception& x)
>> > {
>> >    // do something to x
>> > }
>> >
>> > void read_file()
>> > {
>> >   boost::adorn_exception e( boost::bind( tag_exception, _1 ) );
>> >   ...
>> > }
>>     
>
> Could you provide more detail on the "do something to x" part? It seems to 
> me that it would require something similar to the boost::exception::pimpl 
> class, in that it must be able to store arbitrary values, but unlike 
> boost::exception::pimpl it would have to keep some kind of proxy for each 
> stored object, to be evaluated later if something throws.
>
> Wouldn't that amount to executing a lot of code and using a lot of storage, 
> even if nothing throws? 
Here's how it would work:

> void tag_exception(boost::exception& x, std::string* filename)
> {
>    // append filename tag to exception here
> }
>
> void read_file(const std::string& filename)
> {
>   boost::adorn_exception e( boost::bind( tag_exception, _1, &filename) );
>   ...
> }

There is a (small) fixed cost of creating the functor for boost::bind 
and copying the pointer. By using pointers (or references, though the 
syntax is heavier), you avoid hidden copy-construction costs. Since 
boost::adorn_exception has to be declared after the variables it refers 
to, the pointers will still be valid when it is destroyed.

Regards,
Matthew Herrmann