QChar: add fromUcs{2,4}()

The fromUcs2() named ctor is designed to replace all the non-char
integral-type constructors of QChar which make it very hard to control
the implicit QChar conversions, which have caused a few bugs in Qt
itself. As a classical named contructor, it simply returns QChar.

The fromUcs4() named "ctor", however, needs to expand surrogate pairs,
and thus can't just return QChar. Instead, it returns a small struct
that contains one or two char16_t's, can be iterated over and be
implicitly converted to QStringView.  To avoid bikeshedding the name
(FromUcs4Result, of course :), it's defined inline and thus can't be
named outside the function. This function replaces most uses of
QChar::requiresSurrogates() in QtBase.

[ChangeLog][QtCore][QChar] Added fromUcs2(), fromUcs4().

Change-Id: I803708c14001040f75cb599e33c24a3fb8d2579c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/corelib/text/qchar.h b/src/corelib/text/qchar.h
index ea1ce1e4bb..9fea104ec1 100644
--- a/src/corelib/text/qchar.h
+++ b/src/corelib/text/qchar.h
@@ -120,6 +120,10 @@ public:
     QT_ASCII_CAST_WARN Q_DECL_CONSTEXPR QChar(uchar c) noexcept : ucs(c) { }
 #endif
 #endif
+
+    static constexpr QChar fromUcs2(char16_t c) noexcept { return QChar{c}; }
+    static constexpr inline auto fromUcs4(char32_t c) noexcept;
+
     // Unicode information
 
     enum Category
@@ -680,3 +684,5 @@ struct hash<QT_PREPEND_NAMESPACE(QChar)>
 } // namespace std
 
 #endif // QCHAR_H
+
+#include <QtCore/qstringview.h> // for QChar::fromUcs4() definition