Cannot use QPoint as QHASH key

Question

I tried using this code:

QPoint pt;
QHash<QPoint, double> hexes;
hexes.contains(pt);

and I get the error:

error: no matching function for call to ‘qHash(const QPoint&)’

But if I replace 'QPoint' with 'int' it compiles just fine. Is it possible to use QPoint as a QHash key type?

You might want to submit a feature request to the bugtracker. — peppe, Jul 01 '14 at 06:37

score 5 · Answer 1 · answered Feb 15 '16 at 11:27

5

The qHash hashing function is not defined for QPoint, but you can write one using QPair<int,int>

inline uint qHash (const QPoint & key)
{
    return qHash (QPair<int,int>(key.x(), key.y()) );
}

answered Feb 15 '16 at 11:27

galinette

Which file would this function go in? – cid Sep 27 '19 at 18:31
@cid I put it in the same file I was declaring the QHash with the QPoint key in and it seems to work – Sv443 Dec 12 '20 at 23:37

Gluttton · Answer 2 · 2014-07-01T04:24:42.983

2

Is it possible to use QPoint as a QHash key type?

According to documentation it's possible. But you need define:

inline uint qHash (const QPoint & key)

Also QHash requires:

inline bool operator== (const QPoint & k1, const QPoint & k2)

but it's already defined.

Since qHash is already implemented for 64-bit integers I believe that this solution has sense:

inline uint qHash (const QPoint & key)
{
    return qHash (static_cast <qint64> (key.x () ) << 32 | key.y () );
}

Note: I don't know how to implement correct hash function which based on two 32-bit digits and output 32-bit hash.

edited Jul 01 '14 at 04:24

answered Jun 30 '14 at 21:22

Gluttton

Thx! I'll probably use inline uint qHash(const QPoint &key) { return (key.x() << 16) + key.y(); } – user2602914 Jun 30 '14 at 21:43

2 Answers2