QHash add comparison function, how?

[holst]

Hi,

In std::map (stl) you can define the key and value types, but also the comparison function for the keys.
I want to do the same for a QHash, but I reading QtAssistant and googling and the only things I can define are key and value type.

Can I set the comparison function for a QHash? If yes, how?

Thanks.

[spud]

You not only can, but you have to define the function:

quint32 qHash(const Key &key);

quint32 qHash(const Key &key);

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unless you're working with some standard type, for which the function already has been provided.

[holst]

I'm using a QPair as a key, and I want to use another comparison function and not the given one.
I tried to derived QPair an use it as a key:

template <class T1, class T2>
struct stEvPairKey : QPair<T1, T2>
{
    stEvPairKey() : QPair<T1, T2>() {}
    stEvPairKey(const T1 &t1, const T2 &t2) : QPair<T1, T2>(t1, t2) {}
};
template <class T1, class T2>
bool operator==(const stEvPairKey<T1, T2> &p1, const stEvPairKey<T1, T2> &p2)
{
    return (p1.first == p2.first) && (p1.second & p2.second);
}

template <class T1, class T2>
struct stEvPairKey : QPair<T1, T2>
{
    stEvPairKey() : QPair<T1, T2>() {}
    stEvPairKey(const T1 &t1, const T2 &t2) : QPair<T1, T2>(t1, t2) {}
};
template <class T1, class T2>
bool operator==(const stEvPairKey<T1, T2> &p1, const stEvPairKey<T1, T2> &p2)
{
    return (p1.first == p2.first) && (p1.second & p2.second);
}

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and use this struct as a key:

QHash<stEvPairKey<int, unsigned long>, CMyClass*> m_hash;

QHash<stEvPairKey<int, unsigned long>, CMyClass*> m_hash;

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But never goes into my "operator==".
The function you give:

quint32 qHash(const Key &key);

quint32 qHash(const Key &key);

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is to compute the hash value for the key, but not to compare 2 keys.

What is wrong?

Thank you.

[wysota]

Doesn't QPair have the operator already defined the way you defined it?

[spud]

As you will see in the following program, both the equality operator and the hash function get called.

#include <QHash>
#include <QDebug>
 
template <class T1, class T2>
struct stEvPairKey : QPair<T1, T2>
{
	stEvPairKey() : QPair<T1, T2>() {}
	stEvPairKey(const T1 &t1, const T2 &t2) : QPair<T1, T2>(t1, t2) {}
};
template <class T1, class T2>
bool operator==(const stEvPairKey<T1, T2> &p1, const stEvPairKey<T1, T2> &p2)
{
	qDebug()<<"comparison";
	return (p1.first == p2.first) && (p1.second & p2.second);
}
template <class T1, class T2>
quint32 qHash(const stEvPairKey<T1,T2>&key)
{
	qDebug()<<"hash";
	return qHash((QPair<T1,T2>)key);
}
void main()
{
 
	QHash<stEvPairKey<int, unsigned long>, bool> m_hash;
	m_hash.insert(stEvPairKey<int, unsigned long>(1, 2), true);
	if(m_hash.contains(stEvPairKey<int, unsigned long>(1, 2)))
		qDebug()<<"true";
}

#include <QHash>
#include <QDebug>

template <class T1, class T2>
struct stEvPairKey : QPair<T1, T2>
{
	stEvPairKey() : QPair<T1, T2>() {}
	stEvPairKey(const T1 &t1, const T2 &t2) : QPair<T1, T2>(t1, t2) {}
};
template <class T1, class T2>
bool operator==(const stEvPairKey<T1, T2> &p1, const stEvPairKey<T1, T2> &p2)
{
	qDebug()<<"comparison";
	return (p1.first == p2.first) && (p1.second & p2.second);
}
template <class T1, class T2>
quint32 qHash(const stEvPairKey<T1,T2>&key)
{
	qDebug()<<"hash";
	return qHash((QPair<T1,T2>)key);
}
void main()
{

	QHash<stEvPairKey<int, unsigned long>, bool> m_hash;
	m_hash.insert(stEvPairKey<int, unsigned long>(1, 2), true);
	if(m_hash.contains(stEvPairKey<int, unsigned long>(1, 2)))
		qDebug()<<"true";
}

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Although, as Wysota pointed out, both functions are unnecessary because QPair provides those functions.

[wysota]

By the way... qHash() is for comparing two keys.

[holst]

ok, at last I understand what is happening.
The second part of the key T2 are some flags. For example this code

#include <QtCore/QCoreApplication>
 
#include <QMultiHash>
#include <QDebug>
#include <QString>
 
template <class T1, class T2>
struct stEvPairKey : QPair<T1, T2>
{
    stEvPairKey() : QPair<T1, T2>() {}
    stEvPairKey(const T1 &t1, const T2 &t2) : QPair<T1, T2>(t1, t2) {}
};
template <class T1, class T2>
bool operator==(const stEvPairKey<T1, T2> &p1, const stEvPairKey<T1, T2> &p2)
{
    qDebug()<<"comparison";
    return (p1.first == p2.first) && (p1.second & p2.second);
}
template <class T1, class T2>
quint32 qHash(const stEvPairKey<T1,T2>&key)
{
    qDebug()<<"hash: " << qHash((QPair<T1,T2>)key);
    return qHash((QPair<T1,T2>)key);
}
 
enum EFlags
{
    NONE_FLAGS      = 0,
 
    FLAG1        = 1,
    FLAG2        = 2,
    FLAG3        = 4,
};
 
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
 
    QHash<stEvPairKey<int, unsigned long>, bool> m_hash;
    m_hash.insert(stEvPairKey<int, unsigned long>(1, FLAG1 | FLAG3), true);
    if(m_hash.contains(stEvPairKey<int, unsigned long>(1, FLAG1)))
        qDebug()<<"FLAG1: true";
    if(m_hash.contains(stEvPairKey<int, unsigned long>(1, FLAG1 | FLAG3)))
        qDebug()<<"FLAG1 | FLAG3: true";
 
    return a.exec();
}

#include <QtCore/QCoreApplication>

#include <QMultiHash>
#include <QDebug>
#include <QString>

template <class T1, class T2>
struct stEvPairKey : QPair<T1, T2>
{
    stEvPairKey() : QPair<T1, T2>() {}
    stEvPairKey(const T1 &t1, const T2 &t2) : QPair<T1, T2>(t1, t2) {}
};
template <class T1, class T2>
bool operator==(const stEvPairKey<T1, T2> &p1, const stEvPairKey<T1, T2> &p2)
{
    qDebug()<<"comparison";
    return (p1.first == p2.first) && (p1.second & p2.second);
}
template <class T1, class T2>
quint32 qHash(const stEvPairKey<T1,T2>&key)
{
    qDebug()<<"hash: " << qHash((QPair<T1,T2>)key);
    return qHash((QPair<T1,T2>)key);
}

enum EFlags
{
    NONE_FLAGS      = 0,

    FLAG1        = 1,
    FLAG2        = 2,
    FLAG3        = 4,
};

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

    QHash<stEvPairKey<int, unsigned long>, bool> m_hash;
    m_hash.insert(stEvPairKey<int, unsigned long>(1, FLAG1 | FLAG3), true);
    if(m_hash.contains(stEvPairKey<int, unsigned long>(1, FLAG1)))
        qDebug()<<"FLAG1: true";
    if(m_hash.contains(stEvPairKey<int, unsigned long>(1, FLAG1 | FLAG3)))
        qDebug()<<"FLAG1 | FLAG3: true";

    return a.exec();
}

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The output is:

hash: 65541
hash: 65541
hash: 65537
hash: 65541
comparison
FLAG1 | FLAG3: true

My idea was to implement the operator== for stEvPairKey so the 1st "if()" be also true. But as is shown in the output the hash value is different for "FLAG1" and for "FLAG1 | FLAG3".
I was confused, the flags can't be part of the hash key.

Thank you very much

[wysota]

Maybe they should be a part of value and not the key then?

[holst]

Yes, that's right.
I made the change and works. I was confused, I was only thinking in the comparison function, when "contains()" is called, and no the "qHash()".

Thank you again.

[wysota]

QHash::contains() will call qHash() for the key.