String-like containers: add implicit conversions towards std:: string views

In C++20 std::basic_string_view has gained a range constructor (like
QStringView always had), but that range constructor has been made
explicit. This means we can't just pass a QString(View) to a function
taking a u16string_view. The consensus seems to be that that types that
should implictly convert towards stdlib's string views should do that
via implicit conversion operators. This patch adds them for

* QByteArrayView => std::string_view
* QString(View) => std::u16string_view
* QUtf8StringView => std::string_view or std::u8string_view, depending
  on the storage_type

QLatin1StringView doesn't have a matching std:: view so I'm not enabling
its conversion.

QByteArray poses a challenge, in that it already defines a conversion
towards const char *. (One can disable that conversion with a macro.)
That conversion makes it impossible to support:

  QByteArray ba;
  std::string_view sv1(ba);  // 1
  std::string_view sv2 = ba; // 2

because:

* if only operator const char *() is defined, then (2) doesn't work
  (situation right now);

* if both conversions to const char * and string_view are defined, then
  (1) is ambiguous on certain compilers (MSVC, QCC). Interestingly
  enough, not on GCC/Clang, but only in C++17 and later modes.

I can't kill the conversion towards const char * (API break, and we use
it *everywhere* in Qt), hence, QByteArray does not get the implicit
conversion, at least not in this patch.

[ChangeLog][QtCore][QByteArrayView] Added an implicit conversion
operator towards std::string_view.

[ChangeLog][QtCore][QString] Added an implicit conversion operator
towards std::u16string_view.

[ChangeLog][QtCore][QStringView] Added an implicit conversion operator
towards std::u16string_view.

[ChangeLog][QtCore][QUtf8StringView] Added an implicit conversion
operator towards std::string_view (QUtf8StringView is using char
as its storage type in Qt 6). Note that QUtf8StringView is planned to
use char8_t in Qt 7, therefore it is expected that the conversion will
change towards std::u8string_view in Qt 7.

Change-Id: I6d3b64d211a386241ae157765cd1b03f531f909a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/corelib/text/qstring.h b/src/corelib/text/qstring.h
index e050ec356e..602bbb3e97 100644
--- a/src/corelib/text/qstring.h
+++ b/src/corelib/text/qstring.h
@@ -883,6 +883,8 @@ public:
     static inline QString fromStdU32String(const std::u32string &s);
     inline std::u32string toStdU32String() const;
 
+    Q_IMPLICIT inline operator std::u16string_view() const noexcept;
+
 #if defined(Q_OS_DARWIN) || defined(Q_QDOC)
     static QString fromCFString(CFStringRef string);
     CFStringRef toCFString() const Q_DECL_CF_RETURNS_RETAINED;
@@ -1351,6 +1353,11 @@ inline std::u32string QString::toStdU32String() const
     return u32str;
 }
 
+QString::operator std::u16string_view() const noexcept
+{
+    return std::u16string_view(d.data(), size_t(d.size));
+}
+
 #if !defined(QT_NO_DATASTREAM) || defined(QT_BOOTSTRAPPED)
 Q_CORE_EXPORT QDataStream &operator<<(QDataStream &, const QString &);
 Q_CORE_EXPORT QDataStream &operator>>(QDataStream &, QString &);