efficiently compare QString and std::string for equality

c++ qt std stdstring qstring
Ber picture Ber · Aug 14, 2013 · Viewed 7k times · Source

I want to efficiently compare a QString and a std::string for (in)equality. Which is the best way to do it, possibly without creating intermediate objects?

Answer

Shf picture Shf · Aug 14, 2013

QString::fromStdString() and QString::toStdString() comes to mind, but they create temporary copy of the string, so afaik, if you don't want to have temporary objects, you will have to write this function yourself (though what is more efficient is a question).

Example:

    QString string="string";
    std::string stdstring="string";
    qDebug()<< (string.toStdString()==stdstring); // true


    QString string="string";
    std::string stdstring="std string";
    qDebug()<< (str==QString::fromStdString(stdstring)); // false

By the way in qt5, QString::toStdString() now uses QString::toUtf8() to perform the conversion, so the Unicode properties of the string will not be lost (qt-project.org/doc/qt-5.0/qtcore/qstring.html#toStdString

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