qHash implementation for scoped enum type

apbarrero · wrote on 24 Sept 2015, 07:11

I want to have a QSet of scoped enum values within a namespace. I tried implementing a specialization of qHash function for my scoped enum type following instructions in the docs. I also checked the forum for an answer on a similar problem and the only difference is that my enum type is defined within a namespace. I'm getting an obscure compilation error I haven't been able to fully understand.

If I define the enum in the global namespace I don't get the compilation errors, but I'd like to keep my enum defined under its namespace.

Here's my testing program:

#include <QCoreApplication>
#include <QSet>

namespace foo {
namespace hash {
    enum class MyEnum
    {
        ZERO = 0
        , ONE
    };
}
}

inline uint qHash(foo::hash::MyEnum key, uint seed)
{
    return qHash(static_cast<uint>(key), seed);
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QSet<foo::hash::MyEnum> set;
    set.insert(foo::hash::MyEnum::ZERO);
    set.insert(foo::hash::MyEnum::ONE);

    return a.exec();
}

And the compilation errors:

g++ -c -pipe -std=c++11 -g -Wall -W -D_REENTRANT -fPIC -DQT_CORE_LIB -I../hash -I. -I/opt/Qt/5.4/gcc_64/include -I/opt/Qt/5.4/gcc_64/include/QtCore -I. -I/opt/Qt/5.4/gcc_64/mkspecs/linux-g++ -o main.o ../hash/main.cpp
In file included from /opt/Qt/5.4/gcc_64/include/QtCore/qglobal.h:70:0,
                 from /opt/Qt/5.4/gcc_64/include/QtCore/qcoreapplication.h:37,
                 from /opt/Qt/5.4/gcc_64/include/QtCore/QCoreApplication:1,
                 from ../hash/main.cpp:1:
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h: In instantiation of 'uint qHash(const T&, uint) [with T = foo::hash::MyEnum; uint = unsigned int]':
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:912:32:   required from 'QHash<Key, T>::Node** QHash<Key, T>::findNode(const Key&, uint*) const [with Key = foo::hash::MyEnum; T = QHashDummyValue; QHash<Key, T>::Node = QHashNode<foo::hash::MyEnum, QHashDummyValue>; uint = unsigned int]'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:768:36:   required from 'QHash<Key, T>::iterator QHash<Key, T>::insert(const Key&, const T&) [with Key = foo::hash::MyEnum; T = QHashDummyValue]'
/opt/Qt/5.4/gcc_64/include/QtCore/qset.h:188:94:   required from 'QSet<T>::iterator QSet<T>::insert(const T&) [with T = foo::hash::MyEnum]'
../hash/main.cpp:24:39:   required from here
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:101:42: error: no matching function for call to 'qHash(const foo::hash::MyEnum&)'
     Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(t)))
                                          ^
/opt/Qt/5.4/gcc_64/include/QtCore/qcompilerdetection.h:977:43: note: in definition of macro 'Q_DECL_NOEXCEPT_EXPR'
 # define Q_DECL_NOEXCEPT_EXPR(x) noexcept(x)
                                           ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:101:42: note: candidates are:
     Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(t)))
                                          ^
/opt/Qt/5.4/gcc_64/include/QtCore/qcompilerdetection.h:977:43: note: in definition of macro 'Q_DECL_NOEXCEPT_EXPR'
 # define Q_DECL_NOEXCEPT_EXPR(x) noexcept(x)
                                           ^
In file included from /opt/Qt/5.4/gcc_64/include/QtCore/qset.h:37:0,
                 from /opt/Qt/5.4/gcc_64/include/QtCore/QSet:1,
                 from ../hash/main.cpp:2:
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:63:52: note: constexpr uint qHash(char, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(char key, uint seed = 0) Q_DECL_NOTHROW { return uint(key) ^ seed; }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:63:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'char'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:64:52: note: constexpr uint qHash(uchar, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(uchar key, uint seed = 0) Q_DECL_NOTHROW { return uint(key) ^ seed; }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:64:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'uchar {aka unsigned char}'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:65:52: note: constexpr uint qHash(signed char, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(signed char key, uint seed = 0) Q_DECL_NOTHROW { return uint(key) ^ seed; }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:65:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'signed char'
In file included from /opt/Qt/5.4/gcc_64/include/QtCore/qset.h:37:0,
                 from /opt/Qt/5.4/gcc_64/include/QtCore/QSet:1,
                 from ../hash/main.cpp:2:
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:66:52: note: constexpr uint qHash(ushort, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(ushort key, uint seed = 0) Q_DECL_NOTHROW { return uint(key) ^ seed; }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:66:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'ushort {aka short unsigned int}'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:67:52: note: constexpr uint qHash(short int, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(short key, uint seed = 0) Q_DECL_NOTHROW { return uint(key) ^ seed; }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:67:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'short int'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:68:52: note: constexpr uint qHash(uint, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(uint key, uint seed = 0) Q_DECL_NOTHROW { return key ^ seed; }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:68:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'uint {aka unsigned int}'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:69:52: note: constexpr uint qHash(int, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(int key, uint seed = 0) Q_DECL_NOTHROW { return uint(key) ^ seed; }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:69:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'int'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:70:52: note: constexpr uint qHash(ulong, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(ulong key, uint seed = 0) Q_DECL_NOTHROW
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:70:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'ulong {aka long unsigned int}'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:76:52: note: constexpr uint qHash(long int, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(long key, uint seed = 0) Q_DECL_NOTHROW { return qHash(ulong(key), seed); }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:76:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'long int'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:77:52: note: constexpr uint qHash(quint64, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(quint64 key, uint seed = 0) Q_DECL_NOTHROW
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:77:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'quint64 {aka long long unsigned int}'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:81:52: note: constexpr uint qHash(qint64, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(qint64 key, uint seed = 0) Q_DECL_NOTHROW { return qHash(quint64(key), seed); }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:81:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'qint64 {aka long long int}'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:82:42: note: uint qHash(float, uint)
 Q_CORE_EXPORT Q_DECL_CONST_FUNCTION uint qHash(float key, uint seed = 0) Q_DECL_NOTHROW;
                                          ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:82:42: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'float'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:83:42: note: uint qHash(double, uint)
 Q_CORE_EXPORT Q_DECL_CONST_FUNCTION uint qHash(double key, uint seed = 0) Q_DECL_NOTHROW;
                                          ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:83:42: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'double'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:85:42: note: uint qHash(long double, uint)
 Q_CORE_EXPORT Q_DECL_CONST_FUNCTION uint qHash(long double key, uint seed = 0) Q_DECL_NOTHROW;
                                          ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:85:42: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'long double'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:87:52: note: constexpr uint qHash(QChar, uint)
 Q_DECL_CONST_FUNCTION Q_DECL_CONSTEXPR inline uint qHash(const QChar key, uint seed = 0) Q_DECL_NOTHROW { return qHash(key.unicode(), seed); }
                                                    ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:87:52: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'QChar'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:88:41: note: uint qHash(const QByteArray&, uint)
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION uint qHash(const QByteArray &key, uint seed = 0) Q_DECL_NOTHROW;
                                         ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:88:41: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'const QByteArray&'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:89:41: note: uint qHash(const QString&, uint)
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION uint qHash(const QString &key, uint seed = 0) Q_DECL_NOTHROW;
                                         ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:89:41: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'const QString&'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:90:41: note: uint qHash(const QStringRef&, uint)
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION uint qHash(const QStringRef &key, uint seed = 0) Q_DECL_NOTHROW;
                                         ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:90:41: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'const QStringRef&'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:91:41: note: uint qHash(const QBitArray&, uint)
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION uint qHash(const QBitArray &key, uint seed = 0) Q_DECL_NOTHROW;
                                         ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:91:41: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'const QBitArray&'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:92:41: note: uint qHash(QLatin1String, uint)
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION uint qHash(QLatin1String key, uint seed = 0) Q_DECL_NOTHROW;
                                         ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:92:41: note:   no known conversion for argument 1 from 'const foo::hash::MyEnum' to 'QLatin1String'
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:96:32: note: template<class T> uint qHash(const T*, uint)
 template <class T> inline uint qHash(const T *key, uint seed = 0) Q_DECL_NOTHROW
                                ^
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:96:32: note:   template argument deduction/substitution failed:
In file included from /opt/Qt/5.4/gcc_64/include/QtCore/qglobal.h:70:0,
                 from /opt/Qt/5.4/gcc_64/include/QtCore/qcoreapplication.h:37,
                 from /opt/Qt/5.4/gcc_64/include/QtCore/QCoreApplication:1,
                 from ../hash/main.cpp:1:
/opt/Qt/5.4/gcc_64/include/QtCore/qhash.h:101:42: note:   mismatched types 'const T*' and 'foo::hash::MyEnum'
     Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(t)))
                                          ^
/opt/Qt/5.4/gcc_64/include/QtCore/qcompilerdetection.h:977:43: note: in definition of macro 'Q_DECL_NOEXCEPT_EXPR'
 # define Q_DECL_NOEXCEPT_EXPR(x) noexcept(x)
                                           ^

apbarrero · wrote on 24 Sept 2015, 07:25

I found a way to get the code compiling. Defining the qHash function within the same scope as the enum type makes it work. But I fail to see why the definition in the global namespace with the fully qualified parameter type fails to compile.

Here's the working code:

#include <QCoreApplication>
#include <QSet>

namespace foo {
namespace hash {
    enum class MyEnum
    {
        ZERO = 0
        , ONE
    };

    inline uint qHash(MyEnum key, uint seed)
    {
        return ::qHash(static_cast<uint>(key), seed);
    }
}
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QSet<foo::hash::MyEnum> set;
    set.insert(foo::hash::MyEnum::ZERO);
    set.insert(foo::hash::MyEnum::ONE);

    return a.exec();
}

SGaist · wrote on 24 Sept 2015, 21:35

Hi and welcome to devnet,

Good question, however the documentation about the qHash function states that the function must be in the namespace