[Development] QStringLiteral is broken(ish) on MSVC (compiler bug?)

Matthew Woehlke mwoehlke.floss at gmail.com
Thu Mar 14 21:54:03 CET 2019


On 14/03/2019 16.50, Matthew Woehlke wrote:
> While working on some modernization of my application — in particular,
> converting some UTF-8 literals to use QStringLiteral — I noticed a
> concerning compiler warning:
> 
>   warning C4566: character represented by universal-character-name
>   '\uXXXX' cannot be represented in the current code page (1252)
> 
> After doing some testing, it turns out that, given code like
> QStringLiteral("\u269E \U0001f387 \u269F"), MSVC is indeed butchering
> the string.
> 
> Further investigation shows that the problem seems to be with the
> implementation of QStringLiteral. In particular, it appears that the
> preprocessor initially sees just the raw string literal without the 'u'
> prefix, butchers it, then later "promotes" it to a UTF-16 literal, but
> by then the damage has been done.
> 
> While this absolutely feels like a compiler bug, it's an *awful* big
> gotcha that probably should be documented. Also, is there anything that
> Qt can do to work around it? (I know these sorts of macro expansions can
> be tricksy...)
> 
> Note: and the *local* work-around is apparently to include the 'u'
> prefix on my own literal; apparently doubling it (`uu"stuff"`) is okay.

...forgot to mention; previous mail attempted to have a complete test
case attached, except I accidentally applied the work-around and saved
it before sending the message. So, to see the problem, build the
attached source (with VC++), but remove the 'u' prefix from the
QStringLiteral.

-- 
Matthew


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