- Qt 6.7
- Qt Core
- C++ Classes
- const_iterator
- Qt 6.7.0 Reference Documentation

# const_iterator Class

class QSet::const_iterator

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

- List of all members, including inherited members

## Public Types

| iterator_category

## Public Functions

| const_iterator()
| const_iterator(const iterator &other)
| const_iterator(const const_iterator &other)
bool | operator!=(const const_iterator &other) const
const T & | operator*() const
const_iterator & | operator++()
const_iterator | operator++(int)
const T * | operator->() const
const_iterator & | operator=(const const_iterator &other)
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<Key, T>::const_iterator allows you to iterate over a QSet. If you want to modify the QSet as you iterate over it, you must use QSet::iterator instead. 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::const_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 = {"January", "February", ... "December"};

     // i is QSet<QString>::const_iterator
     for (auto i = set.cbegin(), end = set.cend(); i != end; ++i)
         qDebug() << *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>::const_iterator it = std::find_if(set.cbegin(), set.cend(), predicate);
     if (it != set.constEnd())
         cout << "Found Jeanette" << endl;

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::iterator and QSetIterator.

## Member Type Documentation

### const_iterator::iterator_category

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

## Member Function Documentation

### const_iterator::const_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().

### const_iterator::const_iterator(const const_iterator &other)

Constructs a copy of other.

### const_iterator &const_iterator::operator=(const const_iterator &other)

Assigns other to this iterator.

### const T &const_iterator::operator*() const

Returns a reference to the current item.

See also operator->().

### const T *const_iterator::operator->() const

Returns a pointer to the current item.

See also operator*().

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

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

See also operator!=().

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

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

See also operator==().

### const_iterator &const_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.

### const_iterator const_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.

### const_iterator::const_iterator(const iterator &other)

This is an overloaded function.

Constructs a copy of other.