Comparing QStringView with std::string and char*

[AlQuemist]

QStringView is introduced as a light-weight reference to QString, similar to the relation of std::string_view to std::string in C++.
Yet it apparently lacks some important functionality: Comparison to char* or std::string with the == operator.
One has always to convert either the lhs or rhs of the comparison to QString which entails a full copy and hence, violates the raison d’être of QStringView.
See also https://stackoverflow.com/q/18228720.

Is there a better way for such comparisons (in Qt5 or Qt6)?

• Minimal example

#include <string>
#include <iostream>

#include <QString>
#include <QStringView>

int main()
{
    constexpr const char* const str0 {"my string"};
    QStringView qstrv0(QString::fromStdString(str0));
    if(qstrv0 == QString::fromStdString(str0))
        std::cout << "1) equal" << std::endl;
    if(qstrv0.toString() == str0)
        std::cout << "2) equal" << std::endl;

    std::string str1 = "my string";
    QStringView qstrv1(QString::fromStdString(str1));
    if(qstrv1 == QString::fromStdString(str1))
        std::cout << "3) equal" << std::endl;

    if(qstrv1.toString() == str1)  // does not compile
        std::cout << "4) equal" << std::endl;

    if(qstrv0 == str0)  // does not compile
        std::cout << "5) equal" << std::endl;

    if(qstrv0 == str1)  // does not compile
        std::cout << "6) equal" << std::endl;

    return 0;
}

• Compile & link:

$ qmake -project
$ qmake
$ make

[Christian Ehrlicher]

@AlQuemist said in Comparomg QStringView with std::string and char*:

Comparison to char* or std::string with the == operator.

How should this work - char* and std::string are just a bunch of bytes without any encoding. QString on the other side is well encoded utf-16.

[Roy Qu]

@Christian-Ehrlicher Sometimes you need to compare a QStringView with string literals. In the following example, when parsing a xml file, I need to differentiate tags by their name. Since QXmlStreamReader.name() returns a QStringView, compares it to a const char * literal is very natural. Wrap all comparable literals using QLatin1String is very inconvinient.

    QXmlStreamReader xml;
    xml.setDevice(&file);
    while(!xml.atEnd()) {
        xml.readNext();
        switch (xml.tokenType()) {
        case QXmlStreamReader::TokenType::StartElement:
            if (xml.name() == "item") {
                ...
            } 
            ...
        }
    }

[Christian Ehrlicher]

And what the big difference to

if (xml.name() == "item"_L1) {

Are we really discussing here about three characters?