QUtf8StringView: ensure binary compatibility throughout 6.x

We can't depend on whether the user compiles with -std=c++17 or
-std=c++20. So select what we can depend on and make that permanent.

Prior to this change:

 $ cat /tmp/test.cpp
 #include <QtCore/QUtf8StringView>
 void f(QUtf8StringView) {}
 $ qcd include
 $ g++ -S -o - -I. /tmp/test.cpp | grep globl | c++filt
         .globl  f(QBasicUtf8StringView<false>)
 $ g++ -fPIC -std=c++20 -S -o - -I. /tmp/test.cpp | grep globl | c++filt
         .globl  f(QBasicUtf8StringView<true>)

After this change, they're both "false". QUtf8StringView should have
been a concrete class that derived from QBsicUtf8StringView<whichever>
and inherited all its constructors. We'd cause ODR violations in C++20,
but nothing worse than what we usually do for BC reasons.

That solution is too late for Qt 6.x. Let's revisit in 7.0.

Change-Id: I6bcbe88c072a438b8b4efffd166e77199ecb39e3
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
(cherry picked from commit c7deb3400faa1d9389137fe63ffbffade8203850)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
This commit is contained in:
Thiago Macieira 2021-03-21 13:57:48 -07:00 committed by Qt Cherry-pick Bot
parent e5e6ca7836
commit 9c7a811327
3 changed files with 5 additions and 32 deletions

View File

@ -48,7 +48,7 @@
QT_BEGIN_NAMESPACE
#ifdef __cpp_char8_t
#if QT_VERSION >= QT_VERSION_CHECK(7, 0, 0)
# define QT_BEGIN_HAS_CHAR8_T_NAMESPACE inline namespace q_has_char8_t {
# define QT_BEGIN_NO_CHAR8_T_NAMESPACE namespace q_no_char8_t {
#else

View File

@ -359,6 +359,7 @@ private:
template <bool UseChar8T>
Q_DECLARE_TYPEINFO_BODY(QBasicUtf8StringView<UseChar8T>, Q_PRIMITIVE_TYPE);
// ### Qt 7: remove the non-char8_t version of QUtf8StringView
QT_BEGIN_NO_CHAR8_T_NAMESPACE
using QUtf8StringView = QBasicUtf8StringView<false>;
QT_END_NO_CHAR8_T_NAMESPACE

View File

@ -116,42 +116,14 @@
The second, in namespace \c{q_has_char8_t}, has a value_type of
\c{const char8_t} and is only available when compiling in C++20 mode.
In C++17 mode, \c{q_no_char8_t} is an inline namespace, in C++20 it's
\c{q_has_char8_t}. This means that the name "QUtf8StringView" (without
explicit namespace) will denote different types in C++17 and C++20 modes.
\c{q_no_char8_t} is an inline namespace regardless of C++ edition, to avoid
accidental binary incompatibilities. To use the \c{char8_t} version, you
need to name it explicitly with \c{q_has_char8_t::QUtf8StringView}.
Internally, both are instantiations of the same template class,
QBasicUtf8StringView. Please do not use the template class's name in your
source code.
All Qt APIs use \c{q_no_char8_t::QUtf8StringView} due to binary compatibility,
but these APIs accept \c{q_has_char8_t::QUtf8StringView} as well, since the
latter implicitly converts into the former, and vice versa.
In your own code, please use only \c QUtf8StringView and/or
\c{q_no_char8_t::QUtf8StringView}:
\list
\li If you only target C++20, then use "QUtf8StringView". It will be an alias
for \c{q_has_char8_t::QUtf8StringView} and you'll never look back.
\li If you only target C++17, then use "QUtf8StringView". It will be an alias
for \c{q_no_char8_t::QUtf8StringView} and for the time being, you're ok.
\li If you target both C++17 and C++20, then you have a choice to make:
\list
\li If you don't mind the source-incompatibility of return values of
QUtf8StringView::data() etc changing when compiling under C++17 or C++20,
use "QUtf8StringView". You will need to write your code in such a way that
it adapts to the differences in the QUtf8StringView API in different C++
versions.
\li If you don't want to deal with the above source-incompatibilities, or if
you need to maintain binary compatibility between C++20 and C++17 builds,
use "q_no_char8_t::QUtf8StringView" explicitly. Be aware that the
\c{q_no_char8_t} version will disappear in Qt 7.
\endlist
\endlist
Taken together: Just use QUtf8StringView unless you know what you're doing.
\sa QAnyStringView, QUtf8StringView, QString
*/