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, 17 insertions, 0 deletions

diff --git a/src/corelib/tools/qstring.cpp b/src/corelib/tools/qstring.cpp
index 29509c5..c64e1d6 100644
--- a/src/corelib/tools/qstring.cpp
+++ b/src/corelib/tools/qstring.cpp
@@ -944,6 +944,23 @@ QString::QString(int size, QChar ch)
     }
 }
 
+/*!
+    Constructs a string of the given \a size without initializing the
+    characters. This is only used in \c QStringBuilder::toString().
+
+    \internal
+*/
+
+QString::QString(int size, Uninitialized)
+{
+    d = (Data*) qMalloc(sizeof(Data)+size*sizeof(QChar));
+    d->ref = 1;
+    d->alloc = d->size = size;
+    d->clean = d->asciiCache = d->simpletext = d->righttoleft = d->capacity = 0;
+    d->data = d->array;
+    d->array[size] = '\0';
+}
+
 /*! \fn QString::QString(const QLatin1String &str)
 
     Constructs a copy of the Latin-1 string \a str.