QSet：qHash 和自定义错误 class

Question

我想创建一个要在我的程序中使用的 QSet<Tag>。 Tag 是我的习惯 class.

当我构建代码时，我收到关于 qHash 重载的 gcc 错误：

Exercise.cpp.o
In file included from /opt/Qt/5.5/gcc_64/include/QtCore/qglobal.h:74:0,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/qchar.h:37,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/qstring.h:41,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/QString:1,
                 from /src/wmathtest/LaTeXText.h:4,
                 from /src/wmathtest/Exercise.h:4,
                 from /src/wmathtest/Exercise.cpp:1:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h: In instantiation of ‘uint qHash(const T&, uint) [with T = WMathTest::Tag; uint = unsigned int]’:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:952:32:   required from ‘QHash<Key, T>::Node** QHash<Key, T>::findNode(const Key&, uint*) const [with Key = WMathTest::Tag; T = QHashDummyValue; QHash<Key, T>::Node = QHashNode<WMathTest::Tag, QHashDummyValue>; uint = unsigned int]’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:808:36:   required from ‘QHash<Key, T>::iterator QHash<Key, T>::insert(const Key&, const T&) [with Key = WMathTest::Tag; T = QHashDummyValue]’
/opt/Qt/5.5/gcc_64/include/QtCore/qset.h:188:94:   required from ‘QSet<T>::iterator QSet<T>::insert(const T&) [with T = WMathTest::Tag]’
/src/wmathtest/Exercise.cpp:22:22:   required from here
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:103:5: error: no matching function for call to ‘qHash(const WMathTest::Tag&)’
     Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(t)))
     ^
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:103:5: note: candidates are:
In file included from /opt/Qt/5.5/gcc_64/include/QtCore/qset.h:37:0,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/QSet:1,
                 from /src/wmathtest/Tag.h:5,
                 from /src/wmathtest/Exercise.h:5,
                 from /src/wmathtest/Exercise.cpp:1:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:65:52: 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/5.5/gcc_64/include/QtCore/qhash.h:65:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘char’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:66:52: 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/5.5/gcc_64/include/QtCore/qhash.h:66:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘uchar {aka unsigned char}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:67:52: 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/5.5/gcc_64/include/QtCore/qhash.h:67:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘signed char’
In file included from /opt/Qt/5.5/gcc_64/include/QtCore/qset.h:37:0,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/QSet:1,
                 from /src/wmathtest/Tag.h:5,
                 from /src/wmathtest/Exercise.h:5,
                 from /src/wmathtest/Exercise.cpp:1:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:68:52: 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/5.5/gcc_64/include/QtCore/qhash.h:68:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘ushort {aka short unsigned int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:69:52: 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/5.5/gcc_64/include/QtCore/qhash.h:69:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘short int’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:70:52: 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/5.5/gcc_64/include/QtCore/qhash.h:70:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘uint {aka unsigned int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:71:52: 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/5.5/gcc_64/include/QtCore/qhash.h:71:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘int’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:72:52: 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/5.5/gcc_64/include/QtCore/qhash.h:72:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘ulong {aka long unsigned int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:78:52: 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/5.5/gcc_64/include/QtCore/qhash.h:78:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘long int’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:79:52: 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/5.5/gcc_64/include/QtCore/qhash.h:79:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘quint64 {aka long long unsigned int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:83:52: 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/5.5/gcc_64/include/QtCore/qhash.h:83:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘qint64 {aka long long int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:84:42: note: uint qHash(float, uint)
 Q_CORE_EXPORT Q_DECL_CONST_FUNCTION uint qHash(float key, uint seed = 0) Q_DECL_NOTHROW;
                                          ^
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:84:42: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘float’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:85:42: note: uint qHash(double, uint)
 Q_CORE_EXPORT Q_DECL_CONST_FUNCTION uint qHash(double key, uint seed = 0) Q_DECL_NOTHROW;
                                          ^
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:85:42: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘double’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:87:42: 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/5.5/gcc_64/include/QtCore/qhash.h:87:42: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘long double’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:89:52: 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/5.5/gcc_64/include/QtCore/qhash.h:89:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘QChar’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:90:41: 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/5.5/gcc_64/include/QtCore/qhash.h:90:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘const QByteArray&’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:91:41: 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/5.5/gcc_64/include/QtCore/qhash.h:91:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘const QString&’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:92:41: 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/5.5/gcc_64/include/QtCore/qhash.h:92:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘const QStringRef&’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:93:41: 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/5.5/gcc_64/include/QtCore/qhash.h:93:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘const QBitArray&’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:94:41: note: uint qHash(QLatin1String, uint)
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION uint qHash(QLatin1String key, uint seed = 0) Q_DECL_NOTHROW;
                                         ^
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:94:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘QLatin1String’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:98:32: 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/5.5/gcc_64/include/QtCore/qhash.h:98:32: note:   template argument deduction/substitution failed:
In file included from /opt/Qt/5.5/gcc_64/include/QtCore/qglobal.h:74:0,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/qchar.h:37,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/qstring.h:41,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/QString:1,
                 from /src/wmathtest/LaTeXText.h:4,
                 from /src/wmathtest/Exercise.h:4,
                 from /src/wmathtest/Exercise.cpp:1:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:103:5: note:   mismatched types ‘const T*’ and ‘WMathTest::Tag’
     Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(t)))
     ^
src/wmathtest/CMakeFiles/wmathtest.dir/build.make:146: recipe for target 'src/wmathtest/CMakeFiles/wmathtest.dir/Exercise.cpp.o' failed
make[2]: *** [src/wmathtest/CMakeFiles/wmathtest.dir/Exercise.cpp.o] Error 1
CMakeFiles/Makefile2:160: recipe for target 'src/wmathtest/CMakeFiles/wmathtest.dir/all' failed
make[1]: *** [src/wmathtest/CMakeFiles/wmathtest.dir/all] Error 2
Makefile:76: recipe for target 'all' failed
    Exercise.cpp.o
In file included from /opt/Qt/5.5/gcc_64/include/QtCore/qglobal.h:74:0,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/qchar.h:37,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/qstring.h:41,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/QString:1,
                 from /wmathtest/LaTeXText.h:4,
                 from /src/wmathtest/Exercise.h:4,
                 from /wmathtest/Exercise.cpp:1:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h: In instantiation of ‘uint qHash(const T&, uint) [with T = WMathTest::Tag; uint = unsigned int]’:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:952:32:   required from ‘QHash<Key, T>::Node** QHash<Key, T>::findNode(const Key&, uint*) const [with Key = WMathTest::Tag; T = QHashDummyValue; QHash<Key, T>::Node = QHashNode<WMathTest::Tag, QHashDummyValue>; uint = unsigned int]’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:808:36:   required from ‘QHash<Key, T>::iterator QHash<Key, T>::insert(const Key&, const T&) [with Key = WMathTest::Tag; T = QHashDummyValue]’
/opt/Qt/5.5/gcc_64/include/QtCore/qset.h:188:94:   required from ‘QSet<T>::iterator QSet<T>::insert(const T&) [with T = WMathTest::Tag]’
/wmathtest/Exercise.cpp:22:22:   required from here
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:103:5: error: no matching function for call to ‘qHash(const WMathTest::Tag&)’
     Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(t)))
     ^
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:103:5: note: candidates are:
In file included from /opt/Qt/5.5/gcc_64/include/QtCore/qset.h:37:0,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/QSet:1,
                 from /wmathtest/Tag.h:5,
                 from /wmathtest/Exercise.h:5,
                 from /wmathtest/Exercise.cpp:1:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:65:52: 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/5.5/gcc_64/include/QtCore/qhash.h:65:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘char’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:66:52: 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/5.5/gcc_64/include/QtCore/qhash.h:66:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘uchar {aka unsigned char}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:67:52: 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/5.5/gcc_64/include/QtCore/qhash.h:67:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘signed char’
In file included from /opt/Qt/5.5/gcc_64/include/QtCore/qset.h:37:0,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/QSet:1,
                 from /home/daniele/projects/wmathtestgenerator/src/wmathtest/Tag.h:5,
                 from /Exercise.h:5,
                 from /wmathtest/Exercise.cpp:1:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:68:52: 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/5.5/gcc_64/include/QtCore/qhash.h:68:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘ushort {aka short unsigned int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:69:52: 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/5.5/gcc_64/include/QtCore/qhash.h:69:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘short int’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:70:52: 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/5.5/gcc_64/include/QtCore/qhash.h:70:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘uint {aka unsigned int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:71:52: 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/5.5/gcc_64/include/QtCore/qhash.h:71:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘int’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:72:52: 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/5.5/gcc_64/include/QtCore/qhash.h:72:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘ulong {aka long unsigned int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:78:52: 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/5.5/gcc_64/include/QtCore/qhash.h:78:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘long int’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:79:52: 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/5.5/gcc_64/include/QtCore/qhash.h:79:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘quint64 {aka long long unsigned int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:83:52: 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/5.5/gcc_64/include/QtCore/qhash.h:83:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘qint64 {aka long long int}’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:84:42: note: uint qHash(float, uint)
 Q_CORE_EXPORT Q_DECL_CONST_FUNCTION uint qHash(float key, uint seed = 0) Q_DECL_NOTHROW;
                                          ^
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:84:42: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘float’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:85:42: note: uint qHash(double, uint)
 Q_CORE_EXPORT Q_DECL_CONST_FUNCTION uint qHash(double key, uint seed = 0) Q_DECL_NOTHROW;
                                          ^
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:85:42: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘double’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:87:42: 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/5.5/gcc_64/include/QtCore/qhash.h:87:42: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘long double’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:89:52: 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/5.5/gcc_64/include/QtCore/qhash.h:89:52: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘QChar’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:90:41: 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/5.5/gcc_64/include/QtCore/qhash.h:90:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘const QByteArray&’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:91:41: 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/5.5/gcc_64/include/QtCore/qhash.h:91:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘const QString&’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:92:41: 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/5.5/gcc_64/include/QtCore/qhash.h:92:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘const QStringRef&’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:93:41: 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/5.5/gcc_64/include/QtCore/qhash.h:93:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘const QBitArray&’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:94:41: note: uint qHash(QLatin1String, uint)
 Q_CORE_EXPORT Q_DECL_PURE_FUNCTION uint qHash(QLatin1String key, uint seed = 0) Q_DECL_NOTHROW;
                                         ^
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:94:41: note:   no known conversion for argument 1 from ‘const WMathTest::Tag’ to ‘QLatin1String’
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:98:32: 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/5.5/gcc_64/include/QtCore/qhash.h:98:32: note:   template argument deduction/substitution failed:
In file included from /opt/Qt/5.5/gcc_64/include/QtCore/qglobal.h:74:0,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/qchar.h:37,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/qstring.h:41,
                 from /opt/Qt/5.5/gcc_64/include/QtCore/QString:1,
                 from /home/daniele/projects/wmathtestgenerator/src/wmathtest/LaTeXText.h:4,
                 from /home/daniele/projects/wmathtestgenerator/src/wmathtest/Exercise.h:4,
                 from /thtest/Exercise.cpp:1:
/opt/Qt/5.5/gcc_64/include/QtCore/qhash.h:103:5: note:   mismatched types ‘const T*’ and ‘WMathTest::Tag’
     Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(t)))
     ^
src/wmathtest/CMakeFiles/wmathtest.dir/build.make:146: recipe for target 'src/wmathtest/CMakeFiles/wmathtest.dir/Exercise.cpp.o' failed
make[2]: *** [src/wmathtest/CMakeFiles/wmathtest.dir/Exercise.cpp.o] Error 1
CMakeFiles/Makefile2:160: recipe for target 'src/wmathtest/CMakeFiles/wmathtest.dir/all' failed
make[1]: *** [src/wmathtest/CMakeFiles/wmathtest.dir/all] Error 2
Makefile:76: recipe for target 'all' failed

我为 class 创建了 class、operator== 和 qHash。然后我为 class 创建了一个 typedef 但这不起作用。

我做错了什么？

Tag.h

#ifndef WMATHTEST_TAG_H_
#define WMATHTEST_TAG_H_

#include <QString>
#include <QSet>

namespace WMathTest {

class Tag {

public:

    Tag() = default;
    Tag(const QString &text);
    Tag(const Tag &tag) = default;
    virtual ~Tag() = default;
    void setText(const QString &text);
    const QString& getText() const;
    bool operator==(const Tag &other);

private:

QString m_text;
};

} // namespace WMathTest


inline uint qHash(const WMathTest::Tag &tag, uint seed) {
    return qHash(tag.getText(), seed);
}

typedef QSet<WMathTest::Tag> TagList;

#endif // !WMATHTEST_TAG_H_

Tag.cpp

#include "wmathtest/Tag.h"

namespace WMathTest {

///////////////////////////////////////////////////////////////////////////////
// PUBLIC SECTION                                                            //
///////////////////////////////////////////////////////////////////////////////

Tag::Tag(const QString &text) :
    m_text(text) {

}

void Tag::setText(const QString &text) {
    m_text = text;
}

const QString& Tag::getText() const {
    return m_text;
}

bool Tag::operator==(const Tag &other) {
    return m_text == other.m_text;
}

} // namespace WMathTest

Exercise.h

#ifndef WMATHTEST_EXERCISE_H_
#define WMATHTEST_EXERCISE_H_

#include "wmathtest/LaTeXText.h"
#include "wmathtest/Tag.h"
#include <QString>

namespace WMathTest {

class Exercise {

public:

    Exercise() = default;
    virtual ~Exercise() = default;
    void setID(const QString &id);
    void setProblem(const LaTeXText &problem);
    void setSolution(const LaTeXText &solution);
    void addArgumentTag(const Tag &tag);
    void addArgumentTag(const QString &tag);
    QString getID() const;
    LaTeXText getProblem() const;
    LaTeXText getSolution() const;
    const TagList& getArgumentTagList() const;

private:

    QString m_id;
    LaTeXText m_problem;
    LaTeXText m_solution;
    TagList m_tagList;
};

} // namespace WMathTest

#endif // !WMATHTEST_EXERCISE_H_

Exercise.cpp

#include "wmathtest/Exercise.h"

namespace WMathTest {

///////////////////////////////////////////////////////////////////////////////
// PUBLIC SECTION                                                            //
///////////////////////////////////////////////////////////////////////////////

void Exercise::setID(const QString &id) {
    m_id = id;
}

void Exercise::setProblem(const LaTeXText &problem) {
    m_problem = problem;
}

void Exercise::setSolution(const LaTeXText &solution) {
    m_solution = solution;
}

void Exercise::addArgumentTag(const Tag &tag) {
    m_tagList.insert(tag);
}

void Exercise::addArgumentTag(const QString &tag) {
    m_tagList.insert(Tag(tag));
}

QString Exercise::getID() const {
    return m_id;
}

LaTeXText Exercise::getProblem() const {
    return m_problem;
}

LaTeXText Exercise::getSolution() const {
    return m_solution;
}

const TagList& Exercise::getArgumentTagList() const {
    return m_tagList;
}

} // namespace WMathTest

Answer 1

散列是一个一元函数（只有一个参数），它是数学意义上的函数（return特定参数的相同值，并且 return 完整参数集的有效值).

哈希函数有一个必须始终满足才能正常工作的重要条件：如果两个对象被认为是相等的，那么该对象的哈希值也必须相等。因此，例如，如果您在不区分大小写的容器中使用字符串作为键，则必须提供不区分大小写的哈希函数。

请注意 qHash functions you are see in documentation 的附加参数：seed 始终具有默认值！所以这个函数可以用单个参数调用！这个参数只是一个选项，它让你有机会在为复杂类型构建自定义哈希函数时调整哈希函数。

像这样修正你的代码：

inline uint qHash(const WMathTest::Tag &tag) {
    return qHash(tag.getText(), 0xa03f); // arbitrary value
}

或者像这样：

inline uint qHash(const WMathTest::Tag &tag, uint seed = 0) {
    return qHash(tag.getText(), seed^0xa03f);
}

如果是更复杂的类型，散列函数可能如下所示（示例为什么种子有用）：

inline uint qHash(const DoubleStringValue &tag, uint seed = 0) {
    return qHash(tag.firstText(), seed^0xa03f)^qHash(tag.secondText(), seed^0x17a317a3);
}

请注意，如果有两个 DoubleStringValue 对象，其中 firstText 和 secondText 被翻转，则它们的哈希值将不同。

我建议阅读一些文档，了解带有哈希函数的容器如何工作，以及为什么它们比其他解决方案（例如红黑树）更快。

Answer 2

我自己找到了解决办法。显然这是我的错误，但与我的 qHash 实现无关。

我在 operator== 声明中犯了一个错误：

bool operator==(const Tag &other);

声明缺少 const 限定符。应该是

bool operator==(const Tag &other) const;

QSet：qHash 和自定义错误 class

QSet: error with qHash and custom class

c++

qt

qhash

qset