binary '%': 'QString' does not define this operator

[Felix_F_.-]

Hi,

I'm trying to compile the QT source code for fun and I had an error at this code:

void toString(QString &appendTo, IPv4Address address)
{
    // reconstructing is easy
    // use the fast operator% that pre-calculates the size
    appendTo += number(address >> 24) % u'.'
                % number(address >> 16) % u'.'
                % number(address >> 8) % u'.'
                % number(address);
}

the error is:

error C2676: binary '%': 'QString' does not define this operator or a conversion to a type acceptable to the predefined operator

I must have some mistakes as to my local dev envirionment, which file / class should I debug into?

[Felix_F_.-]

I figured out that I need to define QT_USE_QSTRINGBUILDER during building of qt source code (https://stackoverflow.com/a/12187069/19171308)

and then operator% is defined:

template <typename A, typename B>
QStringBuilder<typename QConcatenable<A>::type, typename QConcatenable<B>::type>
operator%(const A &a, const B &b)
{
   return QStringBuilder<typename QConcatenable<A>::type, typename QConcatenable<B>::type>(a, b);
}

[JonB]

@Felix_F_
What version of Qt?
What compiler?
What compiler options and/or Qt configure options?
What file did you paste this from?

I am not as expert at C++ as some here, but https://doc.qt.io/qt-6/qstring.html shows no indication that QString overrides the % operator. Here it would need to be the overridden + operator to achieve what is desired.

[Felix_F_.-]

@JonB branch v6.5.1
the code is actually QT's source code, see https://github.com/qt/qtbase/blob/5ea0256b07495977a1f2740f6b2d99984cf927dc/src/corelib/io/qipaddress.cpp#L97

I also tried to find an operator% definition but I didn't find it.

from the comment, you can see it's '%' not '+'.

// use the fast operator% that pre-calculates the size

[Felix_F_.-]

I figured out that I need to define QT_USE_QSTRINGBUILDER during building of qt source code (https://stackoverflow.com/a/12187069/19171308)

and then operator% is defined:

template <typename A, typename B>
QStringBuilder<typename QConcatenable<A>::type, typename QConcatenable<B>::type>
operator%(const A &a, const B &b)
{
   return QStringBuilder<typename QConcatenable<A>::type, typename QConcatenable<B>::type>(a, b);
}

[JonB]

@Felix_F_
Interesting.

There's also mention of the QT_USE_QSTRINGBUILDER macro, which turns all + usage into %, provided that doesn't create problems elsewhere on your code.

But FWIW still no mention of this possibly-defined % operator in QString docs page.

[ChrisW67]

@JonB It is there "sort of": More Efficient String Construction (also in Qt 6).

The operator%() function is not part of the QString class, which may explain why it is not documented directly there.

[JonB]

@ChrisW67
Thanks. I don't totally understand how this works. Given that you write QString % QString, why does C++ go off and match that against some QStringBuilder template?