q{,Utf8}Printable: avoid creating a copy of a QString

We have this QString() constructor call to permit things that convert to
QString but aren't QString to be used in qPrintable, like a
QStringBuilder-powered fast operator+ expression, like:

    qPrintable(string1 + ": " + string2)

Unfortunately, it meant that we unnecessarily created a QString copy if
the input was already QString.

Change-Id: Iecab8770aa5840aba8edfffd1516bc94cec791a9
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/src/corelib/tools/qstring.h b/src/corelib/tools/qstring.h
index 0138ae4098..b8f4d49831 100644
--- a/src/corelib/tools/qstring.h
+++ b/src/corelib/tools/qstring.h
@@ -1857,6 +1857,12 @@ QT_DEPRECATED inline QString escape(const QString &plain) {
 #endif
 }
 
+namespace QtPrivate {
+// used by qPrintable() and qUtf8Printable() macros
+inline const QString &asString(const QString &s)    { return s; }
+inline QString &&asString(QString &&s)              { return std::move(s); }
+}
+
 QT_END_NAMESPACE
 
 #if defined(QT_USE_FAST_OPERATOR_PLUS) || defined(QT_USE_QSTRINGBUILDER)