QSet<myEnumClass> --> no matching function for call to 'qHash(myEnumClass&)'

SeDi

Hi and thanks for reading,
I have an enum class Operator::OperatorType that I want o use a QSet of. It's defined in the QObject-derived class Operator:

#include "expression.h"
class Operator: public Expression
{
    Q_OBJECT
public:

    enum class OperatorType {
        UNDEFINED,
        ADD, // add first to second
        SUB, // substract second from first
        MUL, // multiply first by second
        DIV, // divide first by second
        SQT, // secondth root of first
        SQU  // first to the power of second
    };
    Q_ENUM(OperatorType)
    //QHASH_FOR_CLASS_ENUM(OperatorType) // Ok
    Operator(OperatorType type, QSharedPointer<Expression> first, QSharedPointer<Expression> second);

At another point in my code I try to build a QSet of this type:

    QSet<Operator::OperatorType> testTypes;
    Operator::OperatorType t = Operator::OperatorType::ADD;
    testTypes.insert(t);     // <--- Error points to this line

This results in the following build error:

C:\Qt\5.11.2\mingw53_32\include\QtCore\qhashfunctions.h:112: error: no matching function for call to 'qHash(const Operator::OperatorType&)'
     Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(t))) // error points to (t)

As you can see (commented out in the above code), I have tried this idea:

#define QHASH_FRIEND_FOR_CLASS_ENUM(T) \
friend uint qHash(const T&, uint seed);

#define QHASH_FOR_CLASS_ENUM(T) \
inline uint qHash(const T &t, uint seed) { \
    return ::qHash(static_cast<typename std::underlying_type<T>::type>(t), seed); \
}

Unfortuntely the problem remained. Am I doing something stupid? I'd not have thought it to be problematic, having a set of enums….

What else would be recommendable? I'd rather not wrap the enum in a QVariant or attach it to a dummy object...

raven-worx

@SeDi
move the definition of qHash() outside the class.

As the error says, it is looking for qHash(const Operator::OperatorType&), not Operator::qHash(const OperatorType&)

sierdzio

To add to @raven-worx, here is the documentation on how to implement custom qHash.

SeDi

Aaaaah…. Yess… I didn't see that. Works perfectly now. Thank you so much, raven-worx and sierdzio!

VRonin

Are you sure you want to use QSet? QFlags seems more appropriate for enum values

SeDi

Thank you for the idea @VRonin - I have looked into QFlags and I'd actually prefer that. But, at some point, I have to iterate over the enums stored inside - I haven't yet found a way to iterate over QFlags.

What I actually try to achieve with the iteration (and it does work with QSet now) is to randomly select one of those enums.

I don't want to rely on the QSet as being un-sorted in a random manner, so I pick a number by random and iterate to it:

    int number = qrand() % types.count();
    QSetIterator<Operator::OperatorType> iter(types);
    int i = 0;
    while (iter.hasNext()) {
        if (i == number) {
            return(iter.peekNext());
        }
        i++;
        iter.next();
    }
    return Operator::OperatorType::UNDEFINED;

With QFlags I'd probably have to resort to some even uglier bitwise operators, I'd presume (without having given it too much thought).

VRonin

enum OperatorType {
        UNDEFINED = 0,
        ADD = 0x1, // add first to second
        SUB = 0x2, // substract second from first
        MUL= 0x4, // multiply first by second
        DIV= 0x8, // divide first by second
        SQT= 0x10, // secondth root of first
        SQU =0x20   // first to the power of second
    };
Q_FLAG(OperatorType)

But, at some point, I have to iterate over the enums stored inside

const auto flagMeta = QMetaEnum::fromType<Operator::OperatorType>();
    for(int i=0;i<flagMeta.keyCount();++i){
        qDebug() << flagMeta.key(i) << ": " << flagMeta.value(i);
    }

even uglier bitwise operators

const auto flagMeta = QMetaEnum::fromType<Operator::OperatorType>();
const auto randomOperator = static_cast<Operator::OperatorType>(1 << (qrand() % (flagMeta.keyCount()-1)));