QHash ordered by value

[Lele]

Hi all,
I'd need to have a QHash where the elements are ordered by a certain filed inside the value.

struct MyData
	{
		MyData(): bitmap(0), weight(0)  {}
		MyData(qint64 b, qint32 w) : bitmap(b), weight(w) {}
 
		qint64 bitmap;
		qint32 weight;
	};
 
	QHash<qint32, MyData> mCurrentCache;

struct MyData
	{
		MyData(): bitmap(0), weight(0)  {}
		MyData(qint64 b, qint32 w) : bitmap(b), weight(w) {}

		qint64 bitmap;
		qint32 weight;
	};

	QHash<qint32, MyData> mCurrentCache;

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In my case I would like to order the elements depending on MyData::weight.
Is there a smart way to do that or do I have to resort to an additional QHash<qint32, qint32> as a look up table?

Thanks for any help
bye

[marcel]

1. QHash is not a data structure that can be sorted.
2. Why don't you use the weight as a key? Like this:

mCurrentCache[myDataObject.weight] = myData;

mCurrentCache[myDataObject.weight] = myData;

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[marcel]

Also, you need to declare MyData as a qt metatype, to be able to use it in a template container.

Q_DECLARE_METATYPE(MyStruct)

Q_DECLARE_METATYPE(MyStruct)

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Add this right after the struct definition.

[Lele]

Thanks for the quick reply.
I don't use the "weight" as a key because I need the key for other purposes.
The other thing I could do is to use a QMap<weight, key> knowing that the insert there preserve the order. But I though there was a smarter way to have only one QHash.
bye

[marcel]

You could create another class to be used as a key, just like in the QHash example:

class MyKey 
{
 public:
   MyKey();
   MyKey(const qint32& key, const qint32& weight);
 
  //add == operator
  //add uint qHash(const MyKey& k);
private:
  qint32 weight;
  qint32 key;
};

class MyKey 
{
 public:
   MyKey();
   MyKey(const qint32& key, const qint32& weight);
  
  //add == operator
  //add uint qHash(const MyKey& k);
private:
  qint32 weight;
  qint32 key;
};

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You could then use MyKey as a QHash key.

[Lele]

Interesting, would it be enought to redefine the operator > to order by the weight filed?
The only problem is that with this I won't have a fast look up on the key, probably I could redefine even the contains method and the [] operator.....

[marcel]

As I told you before, QHash is not sorted, so implementing the > or < operators would make no sense.

[jpn]

Why not a single map:

QMap<qint32, qint64> bitmaps;

QMap<qint32, qint64> bitmaps;

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where key is weight and value is bitmap identifier or whatever is it?