How to convert QString to std::string?

Question

I am trying to do something like this:

QString string;
// do things...
std::cout << string << std::endl;

but the code doesn't compile. How to output the content of qstring into the console (e.g. for debugging purposes or other reasons)? How to convert QString to std::string?

Solution 1

One of the things you should remember when converting QString to std::string is the fact that QString is UTF-16 encoded while std::string... May have any encodings.

So the best would be either:

QString qs;

// Either this if you use UTF-8 anywhere
std::string utf8_text = qs.toUtf8().constData();

// or this if you're on Windows :-)
std::string current_locale_text = qs.toLocal8Bit().constData();

The suggested (accepted) method may work if you specify codec.

See: http://doc.qt.io/qt-5/qstring.html#toLatin1

Solution 2

You can use:

QString qs;
// do things
std::cout << qs.toStdString() << std::endl;

It internally uses QString::toUtf8() function to create std::string, so it's Unicode safe as well. Here's reference documentation for QString.

Solution 3

If your ultimate aim is to get debugging messages to the console, you can use qDebug().

You can use like,

qDebug()<<string; which will print the contents to the console.

This way is better than converting it into std::string just for the sake of debugging messages.

Solution 4

QString qstr;
std::string str = qstr.toStdString();

However, if you're using Qt:

QTextStream out(stdout);
out << qstr;

Solution 5

Best thing to do would be to overload operator<< yourself, so that QString can be passed as a type to any library expecting an output-able type.

std::ostream& operator<<(std::ostream& str, const QString& string) {
    return str << string.toStdString();
}

View more solutions

Comments

[augustin]

I had tried out << qstr first, before asking, but it didn't compile. It works with qstr.toStdString(), though.

[augustin]

Why the down votes, folks? It's an overkill in my case, but who knows, it might be useful (to me or someone else).

[chris]

I don't think so. You tried std::cout << qstr, not QTextString(stdout) << qstr;

[Kamil Klimek]

qDebug() would be much better, because it supports more Qt types.

[Vitali]

This isn't safe & is slightly slower than the proper way. You're accessing the data of a QByteArray created on the stack. The destructor for the QByteArray may be called before the constructor of the STL string. The safest way to create a helper function. static inline std::string toUtf8(const QString& s) { QByteArray sUtf8 = s.toUtf8(); return std::string(sUtf8.constData(), sUtf8.size()); }

[Artyom]

@Vitali not correct. "The destructor for the QByteArray may be called before the constructor of the STL string" is not correct statement: Quoting the standard: 12.2.3 Temporary objects are destroyed as the last step in evaluating the full-expression (1.9) that (lexically) contains the point where they were created. And the full expression there is std::string utf8_text = qs.toUtf8().constData(); So your statement is not correct

[Vitali]

That's true - I was thinking about const char *x = qs.ToUtf8().constData(). Still, isn't it easier to just call qs.toStdString()?

[Notinlist]

@Vitali No. That loses non-latin1 characters. Try this: QString s = QString::fromUtf8("árvíztűrő tükörfúrógép ÁRVÍZTŰRŐ TÜKÖRFÚRÓGÉP"); std::cout << s.toStdString() << std::endl; std::cout << s.toUtf8().constData() << std::endl;. The first is incorrect, the second is perfect. You need an utf8 terminal to test this.

[Daniel Saner]

For what it's worth, .toStdString() for me always results in an access violation in the pipe operator, irrespective of the QString's contents (non-latin1 or not). This is on Qt 4.8.3/MSVC++ 10/Win 7.

[Emile Cormier]

As of Qt 5.0, 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).

[Senči]

this works even with a Qt-Build with -no-stl-Option set. some more info

[thuga]

And if you want to check the source code for QString::toStdString, here it is.

[Tarod]

@Artyom The answer should be edited because QByteArray QString::toAscii() const is obsolete

[Yousuf Azad]

How can i test to see if it indeed loses the Unicode property? Will using Unicode characters(e.g. say from a language other than English) will do?

[Anantha Raju C]

please add some details as to what the mistake was and why your answer works

[eyllanesc]

The question is very clear: convert QString to std::string, not to print it.

[Den-Jason]

I like this because Qt have a habit of changing the way their strings work - and this puts the conversion in one place.

[Laszlo]

@notinlist Well, both methods result in the same giberish: "�rv�zt?r? t�k�rf�r�g�p �RV�ZT?R? T�K�RF�R�G�P". They are the same at least.

[M.M]

@eyllanesc the question text says "How to output the content of qstring into the console?" , it seems OP assumes converting to std::string is the only way. It's really two questions being asked at once .

[eyllanesc]

@M.M The question seems unclear since the question in the title says How to convert QString to std::string?, Maybe it's an XY problem.

[IceBerg0]

stackoverflow.com/a/4214397/8533873 should be the accepted answer.