#Qstring startswith ida code

Same as for QString, but Qt lets you implicitly convert QByteArray to QString, this is why there is an overload that converts from ASCII to unicode. Static inline void appendTo(const QByteArray &ba, char *&out) QAbstractConcatenable::convertFromAscii(ba.constData(), Static inline void appendTo(const QByteArray &ba, QChar *&out) QStringBuilder(const A &a_, const B &b_) : a(a_), b(b_) Now we can have a look at this QStringBuilder class The result of something like string1 + string2 + string3 would be of type QStringBuilder, QString> The operator+(a,b) simply returns QStringBuilder(a, b). Since, for example, QConcatenable::type does not exist, that operator+ is not enabled if used with QVariant. QConcatenable::type is a typedef to the type T only for the specialized types. Indeed, QConcatenable is an internal template class that is specialized only for: QString, QLatin1String, QChar, QStringRef, QCharRef, and also QByteArray and char*. This operator uses SFINAE to enable itself only for types that support concatenation to strings. QStringBuilder::type, typename QConcatenable::type> Let's start by looking at the implementation of the operator+: It is highly technical, and absolutely not required to be understood to use it.Įverything is in qtringbuilder.h, but the snippets pasted in this article may be slightly simplified to ease the understanding. Some of the commits that fixes the sources compatibility problems are: 5d3eb7a1 for the previous version that did not support QByteArray yet, and 7101a3fa which was required to add support for QByteArray in Qt 4.8īecause I think the implementation shows many nice template features and I thought it would be fun to explain a bit of the implementation details of this class in this article. QT_USE_QSTRINGBUILDER is already enabled when compiling Qt itself and creator. QString s = QString(someString + someOtherString).toUpper() QVariant v = QString(someString + someOtherString) The solution is to explicitly cast to QString: QString s = (someString + someOtherString).toUpper() QVariant v = someString + someOtherString

#Qstring startswith ida code

It is not source compatible because you might have code that assumes that the result of the operator+ is of the type QString.

String1 + string2 would be of the type QStringBuilder that is implicitly casted to QString.

We changed some of the operator+ that takes strings to return a special template class that will lazily compute the results.įor instance, with QT_USE_QSTRINGBUILDER defined, In order to make it work, we use a technique called Expression template. You need to use that new macro to benefit from the QByteArray changes. The way to enable it with Qt 4.7 is described in the 4.7 QString documentationīut this method is now deprecated, and in Qt 4.8, the macro has been replaced with the new QT_USE_QSTRINGBUILDER macro. In Qt 4.6 we introduced a kind of hidden class in Qt: QStringBuilderĪnd it was improved in Qt 4.8 by adding support for QByteArray as well.īecause it is source incompatible (see below), you need to explicitly enable it. What if the first expression could be as fast as the above? The good news is that it is possible. But it unfortunately does not look as good. Only one allocation and one copy, this is the optimum. It would be much faster to do something like that.ĭataFile.reserve(directory.size() + 1 + name.size() + 4) We have something very convenient, but what about the performance of that kind of expression?Įach operator+ will create a temporary string that is then discarded, that means, many allocations and copies. The QLatin1Char and QLatin1String are just there for correctness, you could omit those while writing your application. QString dataFile = directory + QLatin1Char('/') + name + QLatin1String('.dat')

QString and QByteArray comes with very handy operator+ which allows you to write stuff like this: