Why is QString printed with quotation marks?

问题:

So when you use qDebug() to print a QString, quotation marks appears suddenly in the output.

int main()
{
    QString str = "hello world"; //Classic
    qDebug() << str; //Output: "hello world"
    //Expected Ouput: hello world
}

I know we can solve this with qPrintable(const QString), but I was just wondering why does QString work like that?, and Is there a method inside QString to change the way it's printed?

回答1:

Why?

It's because of the implementation of qDebug().

From the source code:

inline QDebug &operator<<(QChar t) { stream->ts << '\'' << t << '\''; return maybeSpace(); }
inline QDebug &operator<<(const char* t) { stream->ts << QString::fromAscii(t); return maybeSpace(); }
inline QDebug &operator<<(const QString & t) { stream->ts << '\"' << t  << '\"'; return maybeSpace(); }

Therefore,

QChar a = 'H';
char b = 'H';
QString c = "Hello";

qDebug()<<a;
qDebug()<<b;
qDebug()<<c;

outputs

'H' 
 H 
"Hello"

---

Comment

So why Qt do this? Since qDebug is for the purpose of debugging, the inputs of various kinds of type will become text stream output through qDebug.

For example, qDebug print boolean value into text expression true / false:

inline QDebug &operator<<(bool t) { stream->ts << (t ? "true" : "false"); return maybeSpace(); }

It outputs true or false to your terminal. Therefore, if you had a QString which store true, you need a quote mark " to specify the type.

回答2:

Qt 5.4 has a new feature that lets you disable this. To quote the documentation:

QDebug & QDebug::​noquote()

Disables automatic insertion of quotation characters around QChar, QString and QByteArray contents and returns a reference to the stream.

This function was introduced in Qt 5.4.

See also quote() and maybeQuote().

(Emphasis mine.)

Here's an example of how you'd use this feature:

QDebug debug = qDebug();
debug << QString("This string is quoted") << endl;
debug.noquote();
debug << QString("This string is not") << endl;

Another option is to use QTextStream with stdout. There's an example of this in the documentation:

QTextStream out(stdout);
out << "Qt rocks!" << endl;

回答3:

Qt 4: If the string contains just ASCII, the following workaround helps:

qDebug() << QString("TEST").toLatin1().data();

回答4:

Simply cast to const char *

qDebug() << (const char *)yourQString.toStdString().c_str();