Qt Documentation

iterator Class

class QSet::iterator

The QSet::iterator class provides an STL-style non-const iterator for QSet. More...

List of all members, including inherited members

Public Types

	difference_type
	iterator_category
	pointer
	reference
	value_type

Public Functions

	iterator(const iterator &other)
	iterator()
iterator &	operator=(const iterator &other)
bool	operator!=(const iterator &other) const
bool	operator!=(const const_iterator &other) const
const T &	operator*() const
iterator &	operator++()
iterator	operator++(int)
const T *	operator->() const
bool	operator==(const iterator &other) const
bool	operator==(const const_iterator &other) const

Detailed Description

QSet features both STL-style iterators and Java-style iterators. The STL-style iterators are more low-level and more cumbersome to use; on the other hand, they are slightly faster and, for developers who already know STL, have the advantage of familiarity.

QSet<T>::iterator allows you to iterate over a QSet and to remove items (using QSet::erase()) while you iterate. (QSet doesn't let you modify a value through an iterator, because that would potentially require moving the value in the internal hash table used by QSet.) If you want to iterate over a const QSet, you should use QSet::const_iterator. It is generally good practice to use QSet::const_iterator on a non-const QSet as well, unless you need to change the QSet through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QSet::iterator constructor creates an uninitialized iterator. You must initialize it using a function like QSet::begin(), QSet::end(), or QSet::insert() before you can start iterating. Here's a typical loop that prints all the items stored in a set:

QSet<QString> set;
set << "January" << "February" << ... << "December";

QSet<QString>::iterator i;
for (i = set.begin(); i != set.end(); ++i)
    qDebug() << *i;

Here's a loop that removes certain items (all those that start with 'J') from a set while iterating:

QSet<QString> set;
set << "January" << "February" << ... << "December";

QSet<QString>::iterator i = set.begin();
while (i != set.end()) {
    if ((*i).startsWith('J')) {
        i = set.erase(i);
    } else {
        ++i;
    }
}

STL-style iterators can be used as arguments to generic algorithms. For example, here's how to find an item in the set using the qFind() algorithm:

QSet<QString> set;
...
const auto predicate = [](const QString &s) { return s.compare("Jeanette", Qt::CaseInsensitive) == 0; };
QSet<QString>::iterator it = std::find_if(set.begin(), set.end(), predicate);
if (it != set.end())
    cout << "Found Jeanette" << endl;

Multiple iterators can be used on the same set.

Warning: Iterators on implicitly shared containers do not work exactly like STL-iterators. You should avoid copying a container while iterators are active on that container. For more information, read Implicit sharing iterator problem.

See also QSet::const_iterator and QMutableSetIterator.

Member Type Documentation

iterator::iterator_category

Synonyms for std::bidirectional_iterator_tag indicating these iterators are bidirectional iterators.

Member Function Documentation

iterator iterator::operator++(int)

This is an overloaded function.

The postfix ++ operator (it++) advances the iterator to the next item in the set and returns an iterator to the previously current item.

iterator &iterator::operator++()

The prefix ++ operator (++it) advances the iterator to the next item in the set and returns an iterator to the new current item.

Calling this function on QSet<T>::constEnd() leads to undefined results.

bool iterator::operator!=(const iterator &other) const

Returns true if other points to a different item than this iterator; otherwise returns false.

See also operator==().

bool iterator::operator!=(const const_iterator &other) const

bool iterator::operator==(const const_iterator &other) const

This is an overloaded function.

bool iterator::operator==(const iterator &other) const

Returns true if other points to the same item as this iterator; otherwise returns false.

See also operator!=().

const T *iterator::operator->() const

Returns a pointer to the current item.

See also operator*().

const T &iterator::operator*() const

Returns a reference to the current item.

See also operator->().

iterator &iterator::operator=(const iterator &other)

Assigns other to this iterator.

iterator::iterator(const iterator &other)

Constructs a copy of other.

iterator::iterator()

Constructs an uninitialized iterator.

Functions like operator*() and operator++() should not be called on an uninitialized iterator. Use operator=() to assign a value to it before using it.

See also QSet::begin() and QSet::end().

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