diff options
authorThiago Macieira <>2021-03-21 13:57:48 -0700
committerThiago Macieira <>2021-04-20 14:18:25 +0000
commitc7deb3400faa1d9389137fe63ffbffade8203850 (patch)
parent15b26935fca4ab14298abdcc70b3cb15b6cca195 (diff)
QUtf8StringView: ensure binary compatibility throughout 6.xv6.1.0-rc1
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. Pick-to: 6.1 dev Change-Id: I6bcbe88c072a438b8b4efffd166e77199ecb39e3 Reviewed-by: Allan Sandfeld Jensen <>
3 files changed, 5 insertions, 32 deletions
diff --git a/src/corelib/text/qstringalgorithms.h b/src/corelib/text/qstringalgorithms.h
index 5805e4c439..68d47b2893 100644
--- a/src/corelib/text/qstringalgorithms.h
+++ b/src/corelib/text/qstringalgorithms.h
@@ -48,7 +48,7 @@
-#ifdef __cpp_char8_t
# 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 {
diff --git a/src/corelib/text/qutf8stringview.h b/src/corelib/text/qutf8stringview.h
index 8e0656660d..e448667ae1 100644
--- a/src/corelib/text/qutf8stringview.h
+++ b/src/corelib/text/qutf8stringview.h
@@ -359,6 +359,7 @@ private:
template <bool UseChar8T>
+// ### Qt 7: remove the non-char8_t version of QUtf8StringView
using QUtf8StringView = QBasicUtf8StringView<false>;
diff --git a/src/corelib/text/qutf8stringview.qdoc b/src/corelib/text/qutf8stringview.qdoc
index 8ff208ff23..9c65799ca4 100644
--- a/src/corelib/text/qutf8stringview.qdoc
+++ b/src/corelib/text/qutf8stringview.qdoc
@@ -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