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, 12 insertions, 0 deletions

diff --git a/tests/benchmarks/qstringbuilder/qstringbuilder.pro b/tests/benchmarks/qstringbuilder/qstringbuilder.pro
new file mode 100644
index 0000000..79171b4
--- /dev/null
+++ b/tests/benchmarks/qstringbuilder/qstringbuilder.pro
@@ -0,0 +1,12 @@
+load(qttest_p4)
+TEMPLATE = app
+TARGET = tst_qstringbuilder
+
+QMAKE_CXXFLAGS += -g
+QMAKE_CFLAGS += -g
+
+QT -= gui
+
+CONFIG += release
+
+SOURCES += main.cpp