>
> > i.e. instead of:
> >
> > throw network_error("connection failed (with layer = {}, protocol = {},
> > local_endpoint = {}, remote_endpoint = {}, error_code = {}, reason =
> > \"{}\", status = \"{}\"", layer, proto, lep, rep, ec, why, status);
> >
> > you should do:
> >
> > throw network_error("connection failed", layer, proto, lep, rep, ec,
> > why, status);


This is truly a better design, and very convincing. Thanks!


> A better design, that is unfortunately rarely followed, is to perform
> string formatting lazily in what() instead of when constructing an
> exception. With a fallback in the unlikely case when the formatting
> fails. This way the formatting won't be wasted if the formatted message
> is never requested by the caller. I think I saw this design
> recommendation in Boost somewhere, but I can't find where.
>

This might be due to std::exception requires what() to be **const**. Sad,
but however 'lazy formatting' itself is a good idea.


On Fri, Dec 12, 2025 at 8:00 PM <std-proposals-request_at_[hidden]>
wrote:

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> 1. Re: Formatting std::exception? (Andrey Semashev)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 12 Dec 2025 14:36:00 +0300
> From: Andrey Semashev <andrey.semashev_at_[hidden]>
> To: std-proposals_at_[hidden]
> Subject: Re: [std-proposals] Formatting std::exception?
> Message-ID: <af408c93-7675-4ac5-9a10-bf0625434fc5_at_[hidden]>
> Content-Type: text/plain; charset=UTF-8
>
> On 12 Dec 2025 14:03, Jonathan Wakely via Std-Proposals wrote:
> >
> > On Fri, 12 Dec 2025 at 10:59, Jonathan Wakely <cxx_at_[hidden]
> > <mailto:cxx_at_[hidden]>> wrote:
> >
> > For case B you are much better off capturing the data in the
> > exception object directly, not formatting it as a string.
> >
> > i.e. instead of:
> >
> > throw network_error("connection failed (with layer = {}, protocol = {},
> > local_endpoint = {}, remote_endpoint = {}, error_code = {}, reason =
> > \"{}\", status = \"{}\"", layer, proto, lep, rep, ec, why, status);
> >
> > you should do:
> >
> > throw network_error("connection failed", layer, proto, lep, rep, ec,
> > why, status);
> >
> > The exception type should store those values and provide getters to
> > access those values later. It can also use std::format internally to
> > create a string that is stored and returned from network_error::what()
> > (maybe by passing that string to a std::runtime_error base class, if it
> > derives from that).
>
> A better design, that is unfortunately rarely followed, is to perform
> string formatting lazily in what() instead of when constructing an
> exception. With a fallback in the unlikely case when the formatting
> fails. This way the formatting won't be wasted if the formatted message
> is never requested by the caller. I think I saw this design
> recommendation in Boost somewhere, but I can't find where.
>
>
>
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