Introduce a new class QStringBuilder to speed up the creation of

QString objects from smaller chunks.

The QStringBuilder class:

QStringBuilder uses expression templates (using the '%' operator)
to postpone any actual concatenation until it is assigned to an
actual QString. At that time it knows the exact sizes of all chunks,
can compute the required space, allocates once a QString of
appriopriate size and then copies over the chunk data one-by-one.

In addition, QLatin1Literal is a drop-in replacement for QLatin1String
(which we can't change for compatibility reasons) that knows its
size, therefore saving a few cycles when computing the size of the
resulting string.

Some further saved cycles stem from inlining and reduced reference
counting logic (the QString created from a QStringBuilder has typically
ref count equal to 1, while QString::append() needs an extra test)

Minor changes to the existing QString class:

- Introduce QString constructor to create an uninitialized QString of a given size.
This particular constructor is used by QStringBuilder class.

- Introduce a QT_USE_FAST_CONCATENATION macro to disable the existing
overloads of operator+() and helps finding the places where they are used in code.

- Introduce QT_USE_FAST_OPERATOR_PLUS.  This also disables the existing
overloads of operator+() and creates a new templated operator+() with
identical implementation of operator%().  This allows code that is compilable
QT_CAST_{TO,FROM}_ASCII to use QStringBuilder almost transparently. The only
case that is not covered is creating objects like QUrl that are implicitly
constructible from a QString from a QStringBuilder result.  This needs to be
converted explicitly to a QString first, e.g. by using QUrl
url(QString(QLatin1String("http://") + hostName));

Reviewed-by: MariusSO

1 files changed, 2 insertions, 0 deletions

diff --git a/src/corelib/tools/tools.pri b/src/corelib/tools/tools.pri
index 90287cb..aea0c6c 100644
--- a/src/corelib/tools/tools.pri
+++ b/src/corelib/tools/tools.pri
@@ -32,6 +32,7 @@ HEADERS +=  \
         tools/qsize.h \
 	tools/qstack.h \
 	tools/qstring.h \
+	tools/qstringbuilder.h \
 	tools/qstringlist.h \
 	tools/qstringmatcher.h \
 	tools/qtextboundaryfinder.h \
@@ -62,6 +63,7 @@ SOURCES += \
 	tools/qsharedpointer.cpp \
         tools/qsize.cpp \
 	tools/qstring.cpp \
+	tools/qstringbuilder.cpp \
 	tools/qstringlist.cpp \
 	tools/qtextboundaryfinder.cpp \
 	tools/qtimeline.cpp \