[Development] Is qsizetype *documented* to be ptrdiff_t?

Giuseppe D'Angelo giuseppe.dangelo at kdab.com
Thu Sep 3 04:25:39 CEST 2020

On 02/09/2020 16:49, Thiago Macieira wrote:
> Lars has asked for qsizetype to be defined as the same type (not just the same
> size) as ptrdiff_t. Effectively, this means defining qint64 as the same type
> as int64_t. Sounds easy, right?
> Please remember that qlonglong is the same type in Qt as qint64. That means on
> these platforms, qlonglong would then be "long", not "long long". This is also
> reflected in our API: QString::toLongLong and QVariant::toLongLong -- will
> they return long? Those types are probably also used in a lot of overload sets
> and could break existing code. Moreover, it could make maintaining Qt5 and Qt6
> codebases working together cumbersome.
> I'm not completely opposed to changing this. But I am asking whether it's
> worth the trouble.

I'm lacking some context here, why does the definition of qsizetype 
affect toLongLong? I would imagine that toLongLong functions deal with 
"long long" no matter what. Why does this prevent making qsizetype == 

> Tor Arne wrote:
>> As a user of this API I was also stumped by not being able to just call
>> printf with %z and a qsizetype, under the assumption that qsizetype’s
>> purpose in life was to mask_away_  the differences of what a size was
>> represented as.
> That is indeed why C99 added the "z" and "t" modifiers. But unfortunately we
> still support one pre-C99 runtime, so you can't use them in cross-platform
> code.

We're proposing every now and then to drop compilers and platforms for 
much less, but we do bother with supporting a toolchain that still 
doesn't implement 20+ years old standards? What's the percentage of 
users of that one?

> For restricted scenarios (MSVC-only or Unix-only code), you can use those
> modifiers. And in MSVC-only scenarios, there will be no type mismatch either,
> in both 32- and 64-bit.
> On 64-bit Unix, the result will be correct too, since the types are the right
> size. The only thing is you may get a compiler warning that the types
> mismatched. We could disable -Wformat.

That sounds evil. It catches real bugs (just caught 
https://codereview.qt-project.org/c/qt/qtbase/+/299726 ).

My 2 c,
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software Engineer
KDAB (France) S.A.S., a KDAB Group company
Tel. France +33 (0)4 90 84 08 53, http://www.kdab.com
KDAB - The Qt, C++ and OpenGL Experts

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 4329 bytes
Desc: S/MIME Cryptographic Signature
URL: <http://lists.qt-project.org/pipermail/development/attachments/20200903/e5197c6f/attachment.bin>

More information about the Development mailing list