QMultiHash Class Reference
[QtCore module]

Detailed Description

The QMultiHash class is a convenience QHash subclass that provides multi-valued hashes.

QMultiHash<Key, T> is one of Qt's generic container classes. It inherits QHash and extends it with a few convenience functions that make it more suitable than QHash for storing multi-valued hashes. A multi-valued hash is a hash that allows multiple values with the same key; QHash normally doesn't allow that, unless you call QHash::insertMulti().

Because QMultiHash inherits QHash, all of QHash's functionality also applies to QMultiHash. For example, you can use isEmpty() to test whether the hash is empty, and you can traverse a QMultiHash using QHash's iterator classes (for example, QHashIterator). But in addition, it provides an insert() function that corresponds to QHash::insertMulti(), and a replace() function that corresponds to QHash::insert(). It also provides convenient operator+() and operator+=().

Example:

 QMultiHash<QString, int> hash1, hash2, hash3;

 hash1.insert("plenty", 100);
 hash1.insert("plenty", 2000);
 // hash1.size() == 2

 hash2.insert("plenty", 5000);
 // hash2.size() == 1

 hash3 = hash1 + hash2;
 // hash3.size() == 3

Unlike QHash, QMultiHash provides no operator[]. Use value() or replace() if you want to access the most recently inserted item with a certain key.

If you want to retrieve all the values for a single key, you can use values(const Key &key), which returns a QList<T>:

 QList<int> values = hash.values("plenty");
 for (int i = 0; i < values.size(); ++i)
     cout << values.at(i) << endl;

The items that share the same key are available from most recently to least recently inserted.

A more efficient approach is to call find() to get the STL-style iterator for the first item with a key and iterate from there:

 QMultiHash<QString, int>::iterator i = hash.find("plenty");
 while (i != hash.end() && i.key() == "plenty") {
     cout << i.value() << endl;
     ++i;
 }

QMultiHash's key and value data types must be assignable data types. You cannot, for example, store a QWidget as a value; instead, store a QWidget *. In addition, QMultiHash's key type must provide operator==(), and there must also be a global qHash() function that returns a hash value for an argument of the key's type. See the QHash documentation for details.

See also QHash, QHashIterator, QMutableHashIterator, and QMultiMap.

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