Initialize a QHash

SeDi

Hi,
this might be a noob question and I am not sure if it's rather about how QHash's initializer lists work / don't work or perhaps just about the basic c++ means of list initialization.

    Note test0 = Note("f","#",4);

    QHash<QVector<int>, QVector<int>> test1{ {QVector<int>{1,2,3}, QVector<int>{1,2,3} } };

    QHash<Note, QVector<int>> test3;

    QHash<Note, QVector<int>> test4{ {Note("f","#",4), QVector<int>{1,2,3} } };

The first three variables are initialized without problems. But "test4" gives a mess of build errors, the most sensible being:

C:/Qt/5.15.0/mingw81_64/include/QtCore/qhashfunctions.h:118:28: error: no matching function for call to 'qHash(const Note&)'
     noexcept(noexcept(qHash(t)))

I don't understand this error message.
The non-default constructor of the class Note is

Note::Note(QString name, QString accidental, int midiOctave, Note::Duration duration, bool dotted)
{
    m_name = name;
    m_accidental = accidental;
    m_midiOctave = midiOctave;
    m_duration = duration;
    m_dotted = dotted;
}

Is what I try to achieve possible? Can I change the Constructor or write an additional one for this purpose?

Christian Ehrlicher

@SeDi said in Initialize a QHash:

no matching function for call to 'qHash(const Note&)'

Isn't this obvious enough? It's also in the docs: The key type of a QHash must provide operator==() and a global hash function called qHash().

SeDi

Ouch. Thank you. I have missed that. Sorry. Thank you very much!

SeDi

Unfortunately I still can't get it to work. I have defined an:

inline bool operator==(const Note &n2) const { return n2.midiNumber() == this->midiNumber(); }

This compiles well, at least as long as I comment out:

//QHash<Note, QVector<int>> test4{ {Note("f","#",4), QVector<int>{1,2,3} } };

I have tried dozens of versions and places for my:

uint qHash(const Note& key, uint seed = 0) {
    Q_UNUSED(seed)
    return uint(key.midiNumber());
}

It is now located in Note.h right after the end of the class definition. Shortened declaration:

#ifndef NOTE_H
#define NOTE_H
#include <QHash>
class Note
{
public:
    explicit Note();
    Note(QString name, QString accidental, int midiOctave, Note::Duration duration = Note::Duration::WHOLE, bool dotted = false);
    int midiNumber() const;
    inline bool operator==(const Note &n2) const { return n2.midiNumber() == this->midiNumber(); }
private:
    QString m_name = "c";
    QString m_accidental = "";
    int m_midiOctave = 5;
};
/// after class definition has ended:
uint qHash(const Note& key, uint seed = 0) {
    Q_UNUSED(seed)
    return uint(key.midiNumber());
}
#endif // NOTE_H

I get this build error log (test4 still commented out):

debug/main.o: In function `qHash(Note const&, unsigned int)':
C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/../BrassTutor/note.h:71: multiple definition of `qHash(Note const&, unsigned int)'
debug/controller.o:C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/../BrassTutor/note.h:71: first defined here
debug/note.o: In function `std::operator&(std::memory_order, std::__memory_order_modifier)':
C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/../BrassTutor/note.h:71: multiple definition of `qHash(Note const&, unsigned int)'
debug/controller.o:C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/../BrassTutor/note.h:71: first defined here
debug/task.o: In function `operator new(unsigned long long, void*)':
C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/../BrassTutor/note.h:71: multiple definition of `qHash(Note const&, unsigned int)'
debug/controller.o:C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/../BrassTutor/note.h:71: first defined here
debug/moc_controller.o: In function `QHash<int, QHashDummyValue>::concrete(QHashData::Node*)':
C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/debug/../../BrassTutor/note.h:71: multiple definition of `qHash(Note const&, unsigned int)'
debug/controller.o:C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/../BrassTutor/note.h:71: first defined here
debug/moc_task.o: In function `QMetaObject const* QMetaObject::staticMetaObject<QObject::staticMetaObject>()':
C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/debug/../../BrassTutor/note.h:71: multiple definition of `qHash(Note const&, unsigned int)'
debug/controller.o:C:\Users\SeDi\cpp\build-BrassTutor-Desktop_Qt_5_15_0_MinGW_64_bit-Debug/../BrassTutor/note.h:71: first defined here
collect2.exe: error: ld returned 1 exit status
mingw32-make[1]: *** [Makefile.Debug:90: debug/BrassTutor.exe] Error 1
mingw32-make: *** [Makefile:45: debug] Error 2

I don't understand how this can be possible. I am 100% sure that I don't have a second definition of qHash() anywhere in my code and the header file is guarded with #ifndef against multiple includes.
There is probably a very basic thing that I totally don't see or understand.

SeDi

After days of searching I've found it myself right after posting this. Sorry.
Following this article I re-applied "inline" and now it works.
Thanks!
For posterity:

#ifndef NOTE_H
#define NOTE_H
#include <QHash>
class Note
{
    ///...
    inline bool operator==(const Note &n2) const { return n2.midiNumber() == this->midiNumber(); }
    ///...
};
/// after class definition has ended:
inline uint qHash(const Note& key, uint seed = 0) {
    Q_UNUSED(seed)
    return uint(key.midiNumber());
}
#endif // NOTE_H