iterator Class

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

This class was introduced in Qt 4.2.

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Publié le 27 décembre 2024 - Mis à jour le 27 décembre 2024

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iterator Class▲

Header: iterator
Since: Qt 4.2
qmake: QT += core

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:

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QSet&lt;QString&gt; set;
set &lt;&lt; "January" &lt;&lt; "February" &lt;&lt; ... &lt;&lt; "December";

QSet&lt;QString&gt;::iterator i;
for (i = set.begin(); i != set.end(); ++i)
    qDebug() &lt;&lt; *i;

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

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QSet&lt;QString&gt; set;
set &lt;&lt; "January" &lt;&lt; "February" &lt;&lt; ... &lt;&lt; "December";

QSet&lt;QString&gt;::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:

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

Multiple iterators can be used on the same set.

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.

Member Type Documentation▲

iterator::iterator_category▲

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

Member Function Documentation▲

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.

iterator::iterator(const iterator &other)▲

Constructs a copy of other.

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

Assigns other to this iterator.

const T &iterator::operator*() const▲

Returns a reference to the current item.

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

Returns a pointer to the current item.

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

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

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 a different item than this iterator; otherwise returns false.

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.

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.

Obsolete Members for iterator▲

The following members of class iterator are deprecated. We strongly advise against using them in new code.