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Subject: Re: Handling literals throughout the translation phases
From: Peter Brett (pbrett_at_[hidden])
Date: 2021-01-04 08:54:41


Please could someone remind me of the *downsides* of allowing escape sequences to be synthesized into string literals through pre-processor concatenation?

Many thanks,

              Peter

> -----Original Message-----
> From: SG16 <sg16-bounces_at_[hidden]> On Behalf Of Jens Maurer via SG16
> Sent: 19 December 2020 22:45
> To: Corentin Jabot <corentinjabot_at_[hidden]>; SG16 <sg16_at_[hidden]>
> Cc: Jens Maurer <Jens.Maurer_at_[hidden]>
> Subject: Re: [SG16] Handling literals throughout the translation phases
>
> EXTERNAL MAIL
>
>
> On 18/12/2020 10.33, Corentin Jabot wrote:
> > On Thu, Dec 17, 2020, 22:33 Jens Maurer via SG16 <sg16_at_[hidden]
> <mailto:sg16_at_[hidden]>> wrote:
> >
> >
> > I'm working on a paper that switches C++ to a modified "model B"
> approach for
> > universal-character-names as described in the C99 Rationale v5.10,
> section 5.2.1.
> >
> >
> > I thought sg16 agreed to not replace ucn until phase 5 a few meetings ago,
> did I completely missunderstood what sg16 agreed ?
>
> The difference is that we do not produce UCNs is phase 1.
> Instead, phase 1 simply produces Unicode scalar values.
> Any UCNs that appeared in the original source are replaced later.
>
> > My current idea is to focus on the creation of the string literal
> > object; that's when transcoding to execution (literal) encoding
> > happens. All other uses of string-literals don't produce objects,
> > so aren't transcoded.
> >
> > In order to be able to interpret escape-sequences in phase 5/6,
> > we need a "tunnel" for numeric-escape-sequences.  One idea would
> > be to add "code unit characters" to the translation character set,
> > where each such character represents a code unit coming from a
> > numeric-escape-sequence.  The sole purpose is to keep the
> > code units safe until we produce the initializer for the
> > string literal object.
> >
> > The alternative would be to delay all interpretation of escape-
> > sequences to when we produce the initializer for the string
> > literal object, but that also means we need to delay string
> > literal concatenation until that time (see first item above).
> >
> >
> > Would that cause any issue? This would otherwise be my preferred solution!
>
> We currently support operator "" "" "" in [over.literal], for example.
> We'd need to make string-literal concatenation first-class citizens
> in phase 7 (e.g. making it a constant expression or so), which is a fairly
> large hammer.
>
> Jens
>
>
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