key_iterator Class

Detailed Description

QMultiHash::key_iterator is essentially the same as QMultiHash::const_iterator with the difference that operator*() and operator->() return a key instead of a value.

For most uses QMultiHash::iterator and QMultiHash::const_iterator should be used, you can easily access the key by calling QMultiHash::iterator::key():

 for (auto it = hash.cbegin(), end = hash.cend(); it != end; ++it) {
     cout << "The key: " << it.key() << endl;
     cout << "The value: " << qPrintable(it.value()) << endl;
     cout << "Also the value: " << qPrintable(*it) << endl;
 }

However, to have interoperability between QMultiHash's keys and STL-style algorithms we need an iterator that dereferences to a key instead of a value. With QMultiHash::key_iterator we can apply an algorithm to a range of keys without having to call QMultiHash::keys(), which is inefficient as it costs one QMultiHash iteration and memory allocation to create a temporary QList.

 // Inefficient, keys() is expensive
 QList<int> keys = hash.keys();
 int numPrimes = std::count_if(keys.cbegin(), keys.cend(), isPrimeNumber);
 qDeleteAll(hash2.keys());

 // Efficient, no memory allocation needed
 int numPrimes = std::count_if(hash.keyBegin(), hash.keyEnd(), isPrimeNumber);
 qDeleteAll(hash2.keyBegin(), hash2.keyEnd());

QMultiHash::key_iterator is const, it's not possible to modify the key.

The default QMultiHash::key_iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiHash function like QMultiHash::keyBegin() or QMultiHash::keyEnd().