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, 16 insertions, 0 deletions

diff --git a/src/corelib/text/qstringview.h b/src/corelib/text/qstringview.h
index 2dd933284b..7e70238cb2 100644
--- a/src/corelib/text/qstringview.h
+++ b/src/corelib/text/qstringview.h
@@ -351,6 +351,22 @@ template <typename QStringLike, typename std::enable_if<
 inline QStringView qToStringViewIgnoringNull(const QStringLike &s) noexcept
 { return QStringView(s.data(), s.size()); }
 
+// QChar inline functions:
+
+Q_REQUIRED_RESULT constexpr auto QChar::fromUcs4(char32_t c) noexcept
+{
+    struct R {
+        char16_t chars[2];
+        Q_REQUIRED_RESULT constexpr operator QStringView() const noexcept { return {begin(), end()}; }
+        Q_REQUIRED_RESULT constexpr qsizetype size() const noexcept { return chars[1] ? 2 : 1; }
+        Q_REQUIRED_RESULT constexpr const char16_t *begin() const noexcept { return chars; }
+        Q_REQUIRED_RESULT constexpr const char16_t *end() const noexcept { return begin() + size(); }
+    };
+    return requiresSurrogates(c) ? R{{QChar::highSurrogate(c),
+                                      QChar::lowSurrogate(c)}} :
+                                   R{{char16_t(c), u'\0'}} ;
+}
+
 QT_END_NAMESPACE
 
 #endif /* QSTRINGVIEW_H */