QMap Class

The QMap class is a template class that provides a red-black-tree-based dictionary. More...

Header: #include <QMap>
qmake: QT += core
Inherited By:

QMultiMap

Note: All functions in this class are reentrant.

Public Types

class const_iterator
class iterator
class key_iterator
typedef ConstIterator
typedef Iterator
typedef difference_type
typedef key_type
typedef mapped_type
typedef size_type

Public Functions

QMap()
QMap(std::initializer_list<std::pair<Key, T> > list)
QMap(const QMap<Key, T> &other)
QMap(QMap<Key, T> &&other)
QMap(const std::map<Key, T> &other)
~QMap()
iterator begin()
const_iterator begin() const
const_iterator cbegin() const
const_iterator cend() const
void clear()
const_iterator constBegin() const
const_iterator constEnd() const
const_iterator constFind(const Key &key) const
bool contains(const Key &key) const
int count(const Key &key) const
int count() const
bool empty() const
iterator end()
const_iterator end() const
QPair<iterator, iterator> equal_range(const Key &key)
QPair<const_iterator, const_iterator> equal_range(const Key &key) const
iterator erase(iterator pos)
iterator find(const Key &key)
const_iterator find(const Key &key) const
T &first()
const T &first() const
const Key &firstKey() const
iterator insert(const Key &key, const T &value)
iterator insert(const_iterator pos, const Key &key, const T &value)
iterator insertMulti(const Key &key, const T &value)
iterator insertMulti(const_iterator pos, const Key &key, const T &value)
bool isEmpty() const
const Key key(const T &value, const Key &defaultKey = Key()) const
key_iterator keyBegin() const
key_iterator keyEnd() const
QList<Key> keys() const
QList<Key> keys(const T &value) const
T &last()
const T &last() const
const Key &lastKey() const
iterator lowerBound(const Key &key)
const_iterator lowerBound(const Key &key) const
int remove(const Key &key)
int size() const
void swap(QMap<Key, T> &other)
T take(const Key &key)
std::map<Key, T> toStdMap() const
QList<Key> uniqueKeys() const
QMap<Key, T> &unite(const QMap<Key, T> &other)
iterator upperBound(const Key &key)
const_iterator upperBound(const Key &key) const
const T value(const Key &key, const T &defaultValue = T()) const
QList<T> values() const
QList<T> values(const Key &key) const
bool operator!=(const QMap<Key, T> &other) const
QMap<Key, T> &operator=(const QMap<Key, T> &other)
QMap<Key, T> &operator=(QMap<Key, T> &&other)
bool operator==(const QMap<Key, T> &other) const
T &operator[](const Key &key)
const T operator[](const Key &key) const
QDataStream &operator<<(QDataStream &out, const QMap<Key, T> &map)
QDataStream &operator>>(QDataStream &in, QMap<Key, T> &map)

Detailed Description

The QMap class is a template class that provides a red-black-tree-based dictionary.

QMap<Key, T> is one of Qt's generic container classes. It stores (key, value) pairs and provides fast lookup of the value associated with a key.

QMap and QHash provide very similar functionality. The differences are:

  • QHash provides average faster lookups than QMap. (See Algorithmic Complexity for details.)
  • When iterating over a QHash, the items are arbitrarily ordered. With QMap, the items are always sorted by key.
  • The key type of a QHash must provide operator==() and a global qHash(Key) function. The key type of a QMap must provide operator<() specifying a total order.

Here's an example QMap with QString keys and int values:

QMap<QString, int> map;

To insert a (key, value) pair into the map, you can use operator[]():

map["one"] = 1;
map["three"] = 3;
map["seven"] = 7;

This inserts the following three (key, value) pairs into the QMap: ("one", 1), ("three", 3), and ("seven", 7). Another way to insert items into the map is to use insert():

map.insert("twelve", 12);

To look up a value, use operator[]() or value():

int num1 = map["thirteen"];
int num2 = map.value("thirteen");

If there is no item with the specified key in the map, these functions return a default-constructed value.

If you want to check whether the map contains a certain key, use contains():

int timeout = 30;
if (map.contains("TIMEOUT"))
    timeout = map.value("TIMEOUT");

There is also a value() overload that uses its second argument as a default value if there is no item with the specified key:

int timeout = map.value("TIMEOUT", 30);

In general, we recommend that you use contains() and value() rather than operator[]() for looking up a key in a map. The reason is that operator[]() silently inserts an item into the map if no item exists with the same key (unless the map is const). For example, the following code snippet will create 1000 items in memory:

// WRONG
QMap<int, QWidget *> map;
...
for (int i = 0; i < 1000; ++i) {
    if (map[i] == okButton)
        cout << "Found button at index " << i << endl;
}

To avoid this problem, replace map[i] with map.value(i) in the code above.

If you want to navigate through all the (key, value) pairs stored in a QMap, you can use an iterator. QMap provides both Java-style iterators (QMapIterator and QMutableMapIterator) and STL-style iterators (QMap::const_iterator and QMap::iterator). Here's how to iterate over a QMap<QString, int> using a Java-style iterator:

QMapIterator<QString, int> i(map);
while (i.hasNext()) {
    i.next();
    cout << i.key() << ": " << i.value() << endl;
}

Here's the same code, but using an STL-style iterator this time:

QMap<QString, int>::const_iterator i = map.constBegin();
while (i != map.constEnd()) {
    cout << i.key() << ": " << i.value() << endl;
    ++i;
}

The items are traversed in ascending key order.

Normally, a QMap allows only one value per key. If you call insert() with a key that already exists in the QMap, the previous value will be erased. For example:

map.insert("plenty", 100);
map.insert("plenty", 2000);
// map.value("plenty") == 2000

However, you can store multiple values per key by using insertMulti() instead of insert() (or using the convenience subclass QMultiMap). 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 = map.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. Another approach is to call find() to get the STL-style iterator for the first item with a key and iterate from there:

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

If you only need to extract the values from a map (not the keys), you can also use foreach:

QMap<QString, int> map;
...
foreach (int value, map)
    cout << value << endl;

Items can be removed from the map in several ways. One way is to call remove(); this will remove any item with the given key. Another way is to use QMutableMapIterator::remove(). In addition, you can clear the entire map using clear().

QMap's key and value data types must be assignable data types. This covers most data types you are likely to encounter, but the compiler won't let you, for example, store a QWidget as a value; instead, store a QWidget *. In addition, QMap's key type must provide operator<(). QMap uses it to keep its items sorted, and assumes that two keys x and y are equal if neither x < y nor y < x is true.

Example:

#ifndef EMPLOYEE_H
#define EMPLOYEE_H

class Employee
{
public:
    Employee() {}
    Employee(const QString &name, const QDate &dateOfBirth);
    ...

private:
    QString myName;
    QDate myDateOfBirth;
};

inline bool operator<(const Employee &e1, const Employee &e2)
{
    if (e1.name() != e2.name())
        return e1.name() < e2.name();
    return e1.dateOfBirth() < e2.dateOfBirth();
}

#endif // EMPLOYEE_H

In the example, we start by comparing the employees' names. If they're equal, we compare their dates of birth to break the tie.

See also QMapIterator, QMutableMapIterator, QHash, and QSet.

Member Type Documentation

typedef QMap::ConstIterator

Qt-style synonym for QMap::const_iterator.

typedef QMap::Iterator

Qt-style synonym for QMap::iterator.

typedef QMap::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

typedef QMap::key_type

Typedef for Key. Provided for STL compatibility.

typedef QMap::mapped_type

Typedef for T. Provided for STL compatibility.

typedef QMap::size_type

Typedef for int. Provided for STL compatibility.

Member Function Documentation

QMap::QMap()

Constructs an empty map.

See also clear().

QMap::QMap(std::initializer_list<std::pair<Key, T> > list)

Constructs a map with a copy of each of the elements in the initializer list list.

This function is only available if the program is being compiled in C++11 mode.

This function was introduced in Qt 5.1.

QMap::QMap(const QMap<Key, T> &other)

Constructs a copy of other.

This operation occurs in constant time, because QMap is implicitly shared. This makes returning a QMap from a function very fast. If a shared instance is modified, it will be copied (copy-on-write), and this takes linear time.

See also operator=().

QMap::QMap(QMap<Key, T> &&other)

Move-constructs a QMap instance, making it point at the same object that other was pointing to.

This function was introduced in Qt 5.2.

QMap::QMap(const std::map<Key, T> &other)

Constructs a copy of other.

This function is only available if Qt is configured with STL compatibility enabled.

See also toStdMap().

QMap::~QMap()

Destroys the map. References to the values in the map, and all iterators over this map, become invalid.

iterator QMap::begin()

Returns an STL-style iterator pointing to the first item in the map.

See also constBegin() and end().

const_iterator QMap::begin() const

This is an overloaded function.

const_iterator QMap::cbegin() const

Returns a const STL-style iterator pointing to the first item in the map.

This function was introduced in Qt 5.0.

See also begin() and cend().

const_iterator QMap::cend() const

Returns a const STL-style iterator pointing to the imaginary item after the last item in the map.

This function was introduced in Qt 5.0.

See also cbegin() and end().

void QMap::clear()

Removes all items from the map.

See also remove().

const_iterator QMap::constBegin() const

Returns a const STL-style iterator pointing to the first item in the map.

See also begin() and constEnd().

const_iterator QMap::constEnd() const

Returns a const STL-style iterator pointing to the imaginary item after the last item in the map.

See also constBegin() and end().

const_iterator QMap::constFind(const Key &key) const

Returns an const iterator pointing to the item with key key in the map.

If the map contains no item with key key, the function returns constEnd().

This function was introduced in Qt 4.1.

See also find() and QMultiMap::constFind().

bool QMap::contains(const Key &key) const

Returns true if the map contains an item with key key; otherwise returns false.

See also count() and QMultiMap::contains().

int QMap::count(const Key &key) const

Returns the number of items associated with key key.

See also contains(), insertMulti(), and QMultiMap::count().

int QMap::count() const

This is an overloaded function.

Same as size().

bool QMap::empty() const

This function is provided for STL compatibility. It is equivalent to isEmpty(), returning true if the map is empty; otherwise returning false.

iterator QMap::end()

Returns an STL-style iterator pointing to the imaginary item after the last item in the map.

See also begin() and constEnd().

const_iterator QMap::end() const

This is an overloaded function.

QPair<iterator, iterator> QMap::equal_range(const Key &key)

Returns a pair of iterators delimiting the range of values [first, second), that are stored under key.

QPair<const_iterator, const_iterator> QMap::equal_range(const Key &key) const

This is an overloaded function.

This function was introduced in Qt 5.6.

iterator QMap::erase(iterator pos)

Removes the (key, value) pair pointed to by the iterator pos from the map, and returns an iterator to the next item in the map.

See also remove().

iterator QMap::find(const Key &key)

Returns an iterator pointing to the item with key key in the map.

If the map contains no item with key key, the function returns end().

If the map contains multiple items with key key, this function returns an iterator that points to the most recently inserted value. The other values are accessible by incrementing the iterator. For example, here's some code that iterates over all the items with the same key:

QMap<QString, int> map;
...
QMap<QString, int>::const_iterator i = map.find("HDR");
while (i != map.end() && i.key() == "HDR") {
    cout << i.value() << endl;
    ++i;
}

See also constFind(), value(), values(), lowerBound(), upperBound(), and QMultiMap::find().

const_iterator QMap::find(const Key &key) const

This is an overloaded function.

T &QMap::first()

Returns a reference to the first value in the map, that is the value mapped to the smallest key. This function assumes that the map is not empty.

When unshared (or const version is called), this executes in constant time.

This function was introduced in Qt 5.2.

See also last(), firstKey(), and isEmpty().

const T &QMap::first() const

This is an overloaded function.

This function was introduced in Qt 5.2.

const Key &QMap::firstKey() const

Returns a reference to the smallest key in the map. This function assumes that the map is not empty.

This executes in constant time.

This function was introduced in Qt 5.2.

See also lastKey(), first(), keyBegin(), and isEmpty().

iterator QMap::insert(const Key &key, const T &value)

Inserts a new item with the key key and a value of value.

If there is already an item with the key key, that item's value is replaced with value.

If there are multiple items with the key key, the most recently inserted item's value is replaced with value.

See also insertMulti().

iterator QMap::insert(const_iterator pos, const Key &key, const T &value)

This is an overloaded function.

Inserts a new item with the key key and value value and with hint pos suggesting where to do the insert.

If constBegin() is used as hint it indicates that the key is less than any key in the map while constEnd() suggests that the key is (strictly) larger than any key in the map. Otherwise the hint should meet the condition (pos - 1).key() < key <= pos.key(). If the hint pos is wrong it is ignored and a regular insert is done.

If there is already an item with the key key, that item's value is replaced with value.

If there are multiple items with the key key, then exactly one of them is replaced with value.

If the hint is correct and the map is unshared, the insert executes in amortized constant time.

When creating a map from sorted data inserting the largest key first with constBegin() is faster than inserting in sorted order with constEnd(), since constEnd() - 1 (which is needed to check if the hint is valid) needs logarithmic time.

Note: Be careful with the hint. Providing an iterator from an older shared instance might crash but there is also a risk that it will silently corrupt both the map and the pos map.

This function was introduced in Qt 5.1.

See also insertMulti().

iterator QMap::insertMulti(const Key &key, const T &value)

Inserts a new item with the key key and a value of value.

If there is already an item with the same key in the map, this function will simply create a new one. (This behavior is different from insert(), which overwrites the value of an existing item.)

See also insert() and values().

iterator QMap::insertMulti(const_iterator pos, const Key &key, const T &value)

This is an overloaded function.

Inserts a new item with the key key and value value and with hint pos suggesting where to do the insert.

If constBegin() is used as hint it indicates that the key is less than any key in the map while constEnd() suggests that the key is larger than any key in the map. Otherwise the hint should meet the condition (pos - 1).key() < key <= pos.key(). If the hint pos is wrong it is ignored and a regular insertMulti is done.

If there is already an item with the same key in the map, this function will simply create a new one.

Note: Be careful with the hint. Providing an iterator from an older shared instance might crash but there is also a risk that it will silently corrupt both the map and the pos map.

This function was introduced in Qt 5.1.

See also insert().

bool QMap::isEmpty() const

Returns true if the map contains no items; otherwise returns false.

See also size().

const Key QMap::key(const T &value, const Key &defaultKey = Key()) const

This is an overloaded function.

Returns the first key with value value, or defaultKey if the map contains no item with value value. If no defaultKey is provided the function returns a default-constructed key.

This function can be slow (linear time), because QMap's internal data structure is optimized for fast lookup by key, not by value.

This function was introduced in Qt 4.3.

See also value() and keys().

key_iterator QMap::keyBegin() const

Returns a const STL-style iterator pointing to the first key in the map.

This function was introduced in Qt 5.6.

See also keyEnd() and firstKey().

key_iterator QMap::keyEnd() const

Returns a const STL-style iterator pointing to the imaginary item after the last key in the map.

This function was introduced in Qt 5.6.

See also keyBegin() and lastKey().

QList<Key> QMap::keys() const

Returns a list containing all the keys in the map in ascending order. Keys that occur multiple times in the map (because items were inserted with insertMulti(), or unite() was used) also occur multiple times in the list.

To obtain a list of unique keys, where each key from the map only occurs once, use uniqueKeys().

The order is guaranteed to be the same as that used by values().

See also uniqueKeys(), values(), and key().

QList<Key> QMap::keys(const T &value) const

This is an overloaded function.

Returns a list containing all the keys associated with value value in ascending order.

This function can be slow (linear time), because QMap's internal data structure is optimized for fast lookup by key, not by value.

T &QMap::last()

Returns a reference to the last value in the map, that is the value mapped to the largest key. This function assumes that the map is not empty.

When unshared (or const version is called), this executes in logarithmic time.

This function was introduced in Qt 5.2.

See also first(), lastKey(), and isEmpty().

const T &QMap::last() const

This is an overloaded function.

This function was introduced in Qt 5.2.

const Key &QMap::lastKey() const

Returns a reference to the largest key in the map. This function assumes that the map is not empty.

This executes in logarithmic time.

This function was introduced in Qt 5.2.

See also firstKey(), last(), keyEnd(), and isEmpty().

iterator QMap::lowerBound(const Key &key)

Returns an iterator pointing to the first item with key key in the map. If the map contains no item with key key, the function returns an iterator to the nearest item with a greater key.

Example:

QMap<int, QString> map;
map.insert(1, "one");
map.insert(5, "five");
map.insert(10, "ten");

map.lowerBound(0);      // returns iterator to (1, "one")
map.lowerBound(1);      // returns iterator to (1, "one")
map.lowerBound(2);      // returns iterator to (5, "five")
map.lowerBound(10);     // returns iterator to (10, "ten")
map.lowerBound(999);    // returns end()

If the map contains multiple items with key key, this function returns an iterator that points to the most recently inserted value. The other values are accessible by incrementing the iterator. For example, here's some code that iterates over all the items with the same key:

QMap<QString, int> map;
...
QMap<QString, int>::const_iterator i = map.lowerBound("HDR");
QMap<QString, int>::const_iterator upperBound = map.upperBound("HDR");
while (i != upperBound) {
    cout << i.value() << endl;
    ++i;
}

See also upperBound() and find().

const_iterator QMap::lowerBound(const Key &key) const

This is an overloaded function.

int QMap::remove(const Key &key)

Removes all the items that have the key key from the map. Returns the number of items removed which is usually 1 but will be 0 if the key isn't in the map, or > 1 if insertMulti() has been used with the key.

See also clear(), take(), and QMultiMap::remove().

int QMap::size() const

Returns the number of (key, value) pairs in the map.

See also isEmpty() and count().

void QMap::swap(QMap<Key, T> &other)

Swaps map other with this map. This operation is very fast and never fails.

This function was introduced in Qt 4.8.

T QMap::take(const Key &key)

Removes the item with the key key from the map and returns the value associated with it.

If the item does not exist in the map, the function simply returns a default-constructed value. If there are multiple items for key in the map, only the most recently inserted one is removed and returned.

If you don't use the return value, remove() is more efficient.

See also remove().

std::map<Key, T> QMap::toStdMap() const

Returns an STL map equivalent to this QMap.

This function is only available if Qt is configured with STL compatibility enabled.

QList<Key> QMap::uniqueKeys() const

Returns a list containing all the keys in the map in ascending order. Keys that occur multiple times in the map (because items were inserted with insertMulti(), or unite() was used) occur only once in the returned list.

This function was introduced in Qt 4.2.

See also keys() and values().

QMap<Key, T> &QMap::unite(const QMap<Key, T> &other)

Inserts all the items in the other map into this map. If a key is common to both maps, the resulting map will contain the key multiple times.

See also insertMulti().

iterator QMap::upperBound(const Key &key)

Returns an iterator pointing to the item that immediately follows the last item with key key in the map. If the map contains no item with key key, the function returns an iterator to the nearest item with a greater key.

Example:

QMap<int, QString> map;
map.insert(1, "one");
map.insert(5, "five");
map.insert(10, "ten");

map.upperBound(0);      // returns iterator to (1, "one")
map.upperBound(1);      // returns iterator to (5, "five")
map.upperBound(2);      // returns iterator to (5, "five")
map.upperBound(10);     // returns end()
map.upperBound(999);    // returns end()

See also lowerBound() and find().

const_iterator QMap::upperBound(const Key &key) const

This is an overloaded function.

const T QMap::value(const Key &key, const T &defaultValue = T()) const

Returns the value associated with the key key.

If the map contains no item with key key, the function returns defaultValue. If no defaultValue is specified, the function returns a default-constructed value. If there are multiple items for key in the map, the value of the most recently inserted one is returned.

See also key(), values(), contains(), and operator[]().

QList<T> QMap::values() const

Returns a list containing all the values in the map, in ascending order of their keys. If a key is associated with multiple values, all of its values will be in the list, and not just the most recently inserted one.

See also keys() and value().

QList<T> QMap::values(const Key &key) const

This is an overloaded function.

Returns a list containing all the values associated with key key, from the most recently inserted to the least recently inserted one.

See also count() and insertMulti().

bool QMap::operator!=(const QMap<Key, T> &other) const

Returns true if other is not equal to this map; otherwise returns false.

Two maps are considered equal if they contain the same (key, value) pairs.

This function requires the value type to implement operator==().

See also operator==().

QMap<Key, T> &QMap::operator=(const QMap<Key, T> &other)

Assigns other to this map and returns a reference to this map.

QMap<Key, T> &QMap::operator=(QMap<Key, T> &&other)

Move-assigns other to this QMap instance.

This function was introduced in Qt 5.2.

bool QMap::operator==(const QMap<Key, T> &other) const

Returns true if other is equal to this map; otherwise returns false.

Two maps are considered equal if they contain the same (key, value) pairs.

This function requires the value type to implement operator==().

See also operator!=().

T &QMap::operator[](const Key &key)

Returns the value associated with the key key as a modifiable reference.

If the map contains no item with key key, the function inserts a default-constructed value into the map with key key, and returns a reference to it. If the map contains multiple items with key key, this function returns a reference to the most recently inserted value.

See also insert() and value().

const T QMap::operator[](const Key &key) const

This is an overloaded function.

Same as value().

Related Non-Members

QDataStream &operator<<(QDataStream &out, const QMap<Key, T> &map)

Writes the map map to stream out.

This function requires the key and value types to implement operator<<().

See also Format of the QDataStream operators.

QDataStream &operator>>(QDataStream &in, QMap<Key, T> &map)

Reads a map from stream in into map.

This function requires the key and value types to implement operator>>().

See also Format of the QDataStream operators.

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