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>
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@ -48,7 +48,7 @@
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QT_BEGIN_NAMESPACE
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#ifdef __cpp_char8_t
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#if QT_VERSION >= QT_VERSION_CHECK(7, 0, 0)
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# define QT_BEGIN_HAS_CHAR8_T_NAMESPACE inline namespace q_has_char8_t {
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# define QT_BEGIN_NO_CHAR8_T_NAMESPACE namespace q_no_char8_t {
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#else
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@ -359,6 +359,7 @@ private:
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template <bool UseChar8T>
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Q_DECLARE_TYPEINFO_BODY(QBasicUtf8StringView<UseChar8T>, Q_PRIMITIVE_TYPE);
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// ### Qt 7: remove the non-char8_t version of QUtf8StringView
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QT_BEGIN_NO_CHAR8_T_NAMESPACE
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using QUtf8StringView = QBasicUtf8StringView<false>;
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QT_END_NO_CHAR8_T_NAMESPACE
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@ -116,42 +116,14 @@
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The second, in namespace \c{q_has_char8_t}, has a value_type of
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\c{const char8_t} and is only available when compiling in C++20 mode.
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In C++17 mode, \c{q_no_char8_t} is an inline namespace, in C++20 it's
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\c{q_has_char8_t}. This means that the name "QUtf8StringView" (without
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explicit namespace) will denote different types in C++17 and C++20 modes.
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\c{q_no_char8_t} is an inline namespace regardless of C++ edition, to avoid
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accidental binary incompatibilities. To use the \c{char8_t} version, you
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need to name it explicitly with \c{q_has_char8_t::QUtf8StringView}.
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Internally, both are instantiations of the same template class,
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QBasicUtf8StringView. Please do not use the template class's name in your
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source code.
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All Qt APIs use \c{q_no_char8_t::QUtf8StringView} due to binary compatibility,
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but these APIs accept \c{q_has_char8_t::QUtf8StringView} as well, since the
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latter implicitly converts into the former, and vice versa.
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In your own code, please use only \c QUtf8StringView and/or
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\c{q_no_char8_t::QUtf8StringView}:
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\list
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\li If you only target C++20, then use "QUtf8StringView". It will be an alias
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for \c{q_has_char8_t::QUtf8StringView} and you'll never look back.
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\li If you only target C++17, then use "QUtf8StringView". It will be an alias
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for \c{q_no_char8_t::QUtf8StringView} and for the time being, you're ok.
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\li If you target both C++17 and C++20, then you have a choice to make:
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\list
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\li If you don't mind the source-incompatibility of return values of
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QUtf8StringView::data() etc changing when compiling under C++17 or C++20,
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use "QUtf8StringView". You will need to write your code in such a way that
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it adapts to the differences in the QUtf8StringView API in different C++
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versions.
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\li If you don't want to deal with the above source-incompatibilities, or if
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you need to maintain binary compatibility between C++20 and C++17 builds,
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use "q_no_char8_t::QUtf8StringView" explicitly. Be aware that the
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\c{q_no_char8_t} version will disappear in Qt 7.
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\endlist
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\endlist
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Taken together: Just use QUtf8StringView unless you know what you're doing.
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\sa QAnyStringView, QUtf8StringView, QString
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*/
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