Long live Q{Any,Utf8}StringView!

We need to add these two classes at the same time, because
QAnyStringView makes all QUtf8StringView relational operators moot. We
might want to add some later, esp. for UTF-8/UTf-8 comparisons, to
avoid the pessimization that we can't early-out on size() mismatch in
QAnyStringView equality operators, but that's an optimization, not a
correctness issue, and can be fixed in a source-compatible way even
after Qt 6 is released.

To deal with the char8_t problem in C++20, make QUtf8StringView a
class template out of which two UTF-8 views can be instantiated: the
Qt 7 version, which depends on C++20 char8_t as value_type, and the Qt
6 version where value_type is a char. Use inline namespaces to map the
QUtf8StringView identifier to one or the other, depending on the C++
version used to compile the user code. The inline namespace names must
needs be a bit ugly, as their inline'ness depends on __cpp_char8_t. If
we simply used q_v1/q_v2 we'd be blocking these names for Qt inline
namespaces forever, because it's likely that inline'ness of other
users of inline namespaces in Qt depends on things other than
__cpp_char8_t. While inline'ness of namespaces is, theoretically
speaking, a compile-time-only property, at least Clang warns about
mixed use of inline on a given namespace, so we need to bite the
bullet here. This is also the reason for the QT_BEGIN_..._NAMESPACE
macros: GCC is ok with the first declaration making a namespace
inline, while Clang warns upon re-opening an inline namespace as a
non-inline one.

[ChangeLog][QtCore][QUtf8StringView] New class.

[ChangeLog][QtCore][QAnyStringView] New class.

Change-Id: Ia7179760fca0e0b67d52f5accb0a62e389b17913
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 28 insertions, 0 deletions

diff --git a/src/corelib/serialization/qxmlstream_p.h b/src/corelib/serialization/qxmlstream_p.h
index 0759e92ce7..21229a949e 100644
--- a/src/corelib/serialization/qxmlstream_p.h
+++ b/src/corelib/serialization/qxmlstream_p.h
@@ -94,6 +94,34 @@ public:
     bool isEmpty() const { return m_size == 0; }
     bool isNull() const { return !m_string; }
     QString toString() const { return view().toString(); }
+
+#define MAKE_OP(op) \
+    friend auto operator op(const XmlStringRef &lhs, const XmlStringRef &rhs) noexcept { return lhs.view() op rhs.view(); } \
+    /*end*/
+    MAKE_OP(==)
+    MAKE_OP(!=)
+    MAKE_OP(<=)
+    MAKE_OP(>=)
+    MAKE_OP(<)
+    MAKE_OP(>)
+#ifdef __cpp_impl_three_way_comparison
+    MAKE_OP(<=>)
+#endif
+#undef MAKE_OP
+#define MAKE_OP(op) \
+    friend auto operator op(const XmlStringRef &lhs, QStringView rhs) noexcept { return lhs.view() op rhs; } \
+    friend auto operator op(QStringView lhs, const XmlStringRef &rhs) noexcept { return lhs op rhs.view(); } \
+    /*end*/
+    MAKE_OP(==)
+    MAKE_OP(!=)
+    MAKE_OP(<=)
+    MAKE_OP(>=)
+    MAKE_OP(<)
+    MAKE_OP(>)
+#ifdef __cpp_impl_three_way_comparison
+    MAKE_OP(<=>)
+#endif
+#undef MAKE_OP
 };
 
 }