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+** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (
+** This file is part of the documentation of the Qt Toolkit.
+** GNU Free Documentation License
+** Alternatively, this file may be used under the terms of the GNU Free
+** Documentation License version 1.3 as published by the Free Software
+** Foundation and appearing in the file included in the packaging of
+** this file.
+** Other Usage
+** Alternatively, this file may be used in accordance with the terms
+** and conditions contained in a signed written agreement between you
+** and Nokia.
+ \group tools
+ \title Non-GUI Classes
+ \ingroup groups
+ \brief Collection classes such as list, queue, stack and string, along
+ with other classes that can be used without needing QApplication.
+ The non-GUI classes are general-purpose collection and string classes
+ that may be used independently of the GUI classes.
+ In particular, these classes do not depend on QApplication at all,
+ and so can be used in non-GUI programs.
+ \page containers.html
+ \title Container Classes
+ \ingroup technology-apis
+ \ingroup groups
+ \ingroup qt-basic-concepts
+ \keyword container class
+ \keyword container classes
+ \brief Qt's template-based container classes.
+ \tableofcontents
+ \section1 Introduction
+ The Qt library provides a set of general purpose template-based
+ container classes. These classes can be used to store items of a
+ specified type. For example, if you need a resizable array of
+ \l{QString}s, use QVector<QString>.
+ These container classes are designed to be lighter, safer, and
+ easier to use than the STL containers. If you are unfamiliar with
+ the STL, or prefer to do things the "Qt way", you can use these
+ classes instead of the STL classes.
+ The container classes are \l{implicitly shared}, they are
+ \l{reentrant}, and they are optimized for speed, low memory
+ consumption, and minimal inline code expansion, resulting in
+ smaller executables. In addition, they are \l{thread-safe}
+ in situations where they are used as read-only containers
+ by all threads used to access them.
+ For traversing the items stored in a container, you can use one
+ of two types of iterators: \l{Java-style iterators} and
+ \l{STL-style iterators}. The Java-style iterators are easier to
+ use and provide high-level functionality, whereas the STL-style
+ iterators are slightly more efficient and can be used together
+ with Qt's and STL's \l{generic algorithms}.
+ Qt also offers a \l{foreach} keyword that make it very
+ easy to iterate over all the items stored in a container.
+ \section1 The Container Classes
+ Qt provides the following sequential containers: QList,
+ QLinkedList, QVector, QStack, and QQueue. For most
+ applications, QList is the best type to use. Although it is
+ implemented as an array-list, it provides very fast prepends and
+ appends. If you really need a linked-list, use QLinkedList; if you
+ want your items to occupy consecutive memory locations, use QVector.
+ QStack and QQueue are convenience classes that provide LIFO and
+ FIFO semantics.
+ Qt also provides these associative containers: QMap,
+ QMultiMap, QHash, QMultiHash, and QSet. The "Multi" containers
+ conveniently support multiple values associated with a single
+ key. The "Hash" containers provide faster lookup by using a hash
+ function instead of a binary search on a sorted set.
+ As special cases, the QCache and QContiguousCache classes provide
+ efficient hash-lookup of objects in a limited cache storage.
+ \table
+ \header \o Class \o Summary
+ \row \o \l{QList}<T>
+ \o This is by far the most commonly used container class. It
+ stores a list of values of a given type (T) that can be accessed
+ by index. Internally, the QList is implemented using an array,
+ ensuring that index-based access is very fast.
+ Items can be added at either end of the list using
+ QList::append() and QList::prepend(), or they can be inserted in
+ the middle using QList::insert(). More than any other container
+ class, QList is highly optimized to expand to as little code as
+ possible in the executable. QStringList inherits from
+ QList<QString>.
+ \row \o \l{QLinkedList}<T>
+ \o This is similar to QList, except that it uses
+ iterators rather than integer indexes to access items. It also
+ provides better performance than QList when inserting in the
+ middle of a huge list, and it has nicer iterator semantics.
+ (Iterators pointing to an item in a QLinkedList remain valid as
+ long as the item exists, whereas iterators to a QList can become
+ invalid after any insertion or removal.)
+ \row \o \l{QVector}<T>
+ \o This stores an array of values of a given type at adjacent
+ positions in memory. Inserting at the front or in the middle of
+ a vector can be quite slow, because it can lead to large numbers
+ of items having to be moved by one position in memory.
+ \row \o \l{QStack}<T>
+ \o This is a convenience subclass of QVector that provides
+ "last in, first out" (LIFO) semantics. It adds the following
+ functions to those already present in QVector:
+ \l{QStack::push()}{push()}, \l{QStack::pop()}{pop()},
+ and \l{QStack::top()}{top()}.
+ \row \o \l{QQueue}<T>
+ \o This is a convenience subclass of QList that provides
+ "first in, first out" (FIFO) semantics. It adds the following
+ functions to those already present in QList:
+ \l{QQueue::enqueue()}{enqueue()},
+ \l{QQueue::dequeue()}{dequeue()}, and \l{QQueue::head()}{head()}.
+ \row \o \l{QSet}<T>
+ \o This provides a single-valued mathematical set with fast
+ lookups.
+ \row \o \l{QMap}<Key, T>
+ \o This provides a dictionary (associative array) that maps keys
+ of type Key to values of type T. Normally each key is associated
+ with a single value. QMap stores its data in Key order; if order
+ doesn't matter QHash is a faster alternative.
+ \row \o \l{QMultiMap}<Key, T>
+ \o This is a convenience subclass of QMap that provides a nice
+ interface for multi-valued maps, i.e. maps where one key can be
+ associated with multiple values.
+ \row \o \l{QHash}<Key, T>
+ \o This has almost the same API as QMap, but provides
+ significantly faster lookups. QHash stores its data in an
+ arbitrary order.
+ \row \o \l{QMultiHash}<Key, T>
+ \o This is a convenience subclass of QHash that
+ provides a nice interface for multi-valued hashes.
+ \endtable
+ Containers can be nested. For example, it is perfectly possible
+ to use a QMap<QString, QList<int> >, where the key type is
+ QString and the value type QList<int>. The only pitfall is that
+ you must insert a space between the closing angle brackets (>);
+ otherwise the C++ compiler will misinterpret the two >'s as a
+ right-shift operator (>>) and report a syntax error.
+ The containers are defined in individual header files with the
+ same name as the container (e.g., \c <QLinkedList>). For
+ convenience, the containers are forward declared in \c
+ <QtContainerFwd>.
+ \keyword assignable data type
+ \keyword assignable data types
+ The values stored in the various containers can be of any
+ \e{assignable data type}. To qualify, a type must provide a
+ default constructor, a copy constructor, and an assignment
+ operator. This covers most data types you are likely to want to
+ store in a container, including basic types such as \c int and \c
+ double, pointer types, and Qt data types such as QString, QDate,
+ and QTime, but it doesn't cover QObject or any QObject subclass
+ (QWidget, QDialog, QTimer, etc.). If you attempt to instantiate a
+ QList<QWidget>, the compiler will complain that QWidget's copy
+ constructor and assignment operators are disabled. If you want to
+ store these kinds of objects in a container, store them as
+ pointers, for example as QList<QWidget *>.
+ Here's an example custom data type that meets the requirement of
+ an assignable data type:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 0
+ If we don't provide a copy constructor or an assignment operator,
+ C++ provides a default implementation that performs a
+ member-by-member copy. In the example above, that would have been
+ sufficient. Also, if you don't provide any constructors, C++
+ provides a default constructor that initializes its member using
+ default constructors. Although it doesn't provide any
+ explicit constructors or assignment operator, the following data
+ type can be stored in a container:
+ \snippet doc/src/snippets/streaming/main.cpp 0
+ Some containers have additional requirements for the data types
+ they can store. For example, the Key type of a QMap<Key, T> must
+ provide \c operator<(). Such special requirements are documented
+ in a class's detailed description. In some cases, specific
+ functions have special requirements; these are described on a
+ per-function basis. The compiler will always emit an error if a
+ requirement isn't met.
+ Qt's containers provide operator<<() and operator>>() so that they
+ can easily be read and written using a QDataStream. This means
+ that the data types stored in the container must also support
+ operator<<() and operator>>(). Providing such support is
+ straightforward; here's how we could do it for the Movie struct
+ above:
+ \snippet doc/src/snippets/streaming/main.cpp 1
+ \codeline
+ \snippet doc/src/snippets/streaming/main.cpp 2
+ \keyword default-constructed values
+ The documentation of certain container class functions refer to
+ \e{default-constructed values}; for example, QVector
+ automatically initializes its items with default-constructed
+ values, and QMap::value() returns a default-constructed value if
+ the specified key isn't in the map. For most value types, this
+ simply means that a value is created using the default
+ constructor (e.g. an empty string for QString). But for primitive
+ types like \c{int} and \c{double}, as well as for pointer types,
+ the C++ language doesn't specify any initialization; in those
+ cases, Qt's containers automatically initialize the value to 0.
+ \section1 The Iterator Classes
+ Iterators provide a uniform means to access items in a container.
+ Qt's container classes provide two types of iterators: Java-style
+ iterators and STL-style iterators. Iterators of both types are
+ invalidated when the data in the container is modified or detached
+ from \l{Implicit Sharing}{implicitly shared copies} due to a call
+ to a non-const member function.
+ \section2 Java-Style Iterators
+ The Java-style iterators are new in Qt 4 and are the standard
+ ones used in Qt applications. They are more convenient to use than
+ the STL-style iterators, at the price of being slightly less
+ efficient. Their API is modelled on Java's iterator classes.
+ For each container class, there are two Java-style iterator data
+ types: one that provides read-only access and one that provides
+ read-write access.
+ \table
+ \header \o Containers \o Read-only iterator
+ \o Read-write iterator
+ \row \o QList<T>, QQueue<T> \o QListIterator<T>
+ \o QMutableListIterator<T>
+ \row \o QLinkedList<T> \o QLinkedListIterator<T>
+ \o QMutableLinkedListIterator<T>
+ \row \o QVector<T>, QStack<T> \o QVectorIterator<T>
+ \o QMutableVectorIterator<T>
+ \row \o QSet<T> \o QSetIterator<T>
+ \o QMutableSetIterator<T>
+ \row \o QMap<Key, T>, QMultiMap<Key, T> \o QMapIterator<Key, T>
+ \o QMutableMapIterator<Key, T>
+ \row \o QHash<Key, T>, QMultiHash<Key, T> \o QHashIterator<Key, T>
+ \o QMutableHashIterator<Key, T>
+ \endtable
+ In this discussion, we will concentrate on QList and QMap. The
+ iterator types for QLinkedList, QVector, and QSet have exactly
+ the same interface as QList's iterators; similarly, the iterator
+ types for QHash have the same interface as QMap's iterators.
+ Unlike STL-style iterators (covered \l{STL-style
+ iterators}{below}), Java-style iterators point \e between items
+ rather than directly \e at items. For this reason, they are
+ either pointing to the very beginning of the container (before
+ the first item), at the very end of the container (after the last
+ item), or between two items. The diagram below shows the valid
+ iterator positions as red arrows for a list containing four
+ items:
+ \img javaiterators1.png
+ Here's a typical loop for iterating through all the elements of a
+ QList<QString> in order and printing them to the console:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 1
+ It works as follows: The QList to iterate over is passed to the
+ QListIterator constructor. At that point, the iterator is located
+ just in front of the first item in the list (before item "A").
+ Then we call \l{QListIterator::hasNext()}{hasNext()} to
+ check whether there is an item after the iterator. If there is, we
+ call \l{QListIterator::next()}{next()} to jump over that
+ item. The next() function returns the item that it jumps over. For
+ a QList<QString>, that item is of type QString.
+ Here's how to iterate backward in a QList:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 2
+ The code is symmetric with iterating forward, except that we
+ start by calling \l{QListIterator::toBack()}{toBack()}
+ to move the iterator after the last item in the list.
+ The diagram below illustrates the effect of calling
+ \l{QListIterator::next()}{next()} and
+ \l{QListIterator::previous()}{previous()} on an iterator:
+ \img javaiterators2.png
+ The following table summarizes the QListIterator API:
+ \table
+ \header \o Function \o Behavior
+ \row \o \l{QListIterator::toFront()}{toFront()}
+ \o Moves the iterator to the front of the list (before the first item)
+ \row \o \l{QListIterator::toBack()}{toBack()}
+ \o Moves the iterator to the back of the list (after the last item)
+ \row \o \l{QListIterator::hasNext()}{hasNext()}
+ \o Returns true if the iterator isn't at the back of the list
+ \row \o \l{QListIterator::next()}{next()}
+ \o Returns the next item and advances the iterator by one position
+ \row \o \l{QListIterator::peekNext()}{peekNext()}
+ \o Returns the next item without moving the iterator
+ \row \o \l{QListIterator::hasPrevious()}{hasPrevious()}
+ \o Returns true if the iterator isn't at the front of the list
+ \row \o \l{QListIterator::previous()}{previous()}
+ \o Returns the previous item and moves the iterator back by one position
+ \row \o \l{QListIterator::peekPrevious()}{peekPrevious()}
+ \o Returns the previous item without moving the iterator
+ \endtable
+ QListIterator provides no functions to insert or remove items
+ from the list as we iterate. To accomplish this, you must use
+ QMutableListIterator. Here's an example where we remove all
+ odd numbers from a QList<int> using QMutableListIterator:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 3
+ The next() call in the loop is made every time. It jumps over the
+ next item in the list. The
+ \l{QMutableListIterator::remove()}{remove()} function removes the
+ last item that we jumped over from the list. The call to
+ \l{QMutableListIterator::remove()}{remove()} does not invalidate
+ the iterator, so it is safe to continue using it. This works just
+ as well when iterating backward:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 4
+ If we just want to modify the value of an existing item, we can
+ use \l{QMutableListIterator::setValue()}{setValue()}. In the code
+ below, we replace any value larger than 128 with 128:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 5
+ Just like \l{QMutableListIterator::remove()}{remove()},
+ \l{QMutableListIterator::setValue()}{setValue()} operates on the
+ last item that we jumped over. If we iterate forward, this is the
+ item just before the iterator; if we iterate backward, this is
+ the item just after the iterator.
+ The \l{QMutableListIterator::next()}{next()} function returns a
+ non-const reference to the item in the list. For simple
+ operations, we don't even need
+ \l{QMutableListIterator::setValue()}{setValue()}:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 6
+ As mentioned above, QLinkedList's, QVector's, and QSet's iterator
+ classes have exactly the same API as QList's. We will now turn to
+ QMapIterator, which is somewhat different because it iterates on
+ (key, value) pairs.
+ Like QListIterator, QMapIterator provides
+ \l{QMapIterator::toFront()}{toFront()},
+ \l{QMapIterator::toBack()}{toBack()},
+ \l{QMapIterator::hasNext()}{hasNext()},
+ \l{QMapIterator::next()}{next()},
+ \l{QMapIterator::peekNext()}{peekNext()},
+ \l{QMapIterator::hasPrevious()}{hasPrevious()},
+ \l{QMapIterator::previous()}{previous()}, and
+ \l{QMapIterator::peekPrevious()}{peekPrevious()}. The key and
+ value components are extracted by calling key() and value() on
+ the object returned by next(), peekNext(), previous(), or
+ peekPrevious().
+ The following example removes all (capital, country) pairs where
+ the capital's name ends with "City":
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 7
+ QMapIterator also provides a key() and a value() function that
+ operate directly on the iterator and that return the key and
+ value of the last item that the iterator jumped above. For
+ example, the following code copies the contents of a QMap into a
+ QHash:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 8
+ If we want to iterate through all the items with the same
+ value, we can use \l{QMapIterator::findNext()}{findNext()}
+ or \l{QMapIterator::findPrevious()}{findPrevious()}.
+ Here's an example where we remove all the items with a particular
+ value:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 9
+ \section2 STL-Style Iterators
+ STL-style iterators have been available since the release of Qt
+ 2.0. They are compatible with Qt's and STL's \l{generic
+ algorithms} and are optimized for speed.
+ For each container class, there are two STL-style iterator types:
+ one that provides read-only access and one that provides
+ read-write access. Read-only iterators should be used wherever
+ possible because they are faster than read-write iterators.
+ \table
+ \header \o Containers \o Read-only iterator
+ \o Read-write iterator
+ \row \o QList<T>, QQueue<T> \o QList<T>::const_iterator
+ \o QList<T>::iterator
+ \row \o QLinkedList<T> \o QLinkedList<T>::const_iterator
+ \o QLinkedList<T>::iterator
+ \row \o QVector<T>, QStack<T> \o QVector<T>::const_iterator
+ \o QVector<T>::iterator
+ \row \o QSet<T> \o QSet<T>::const_iterator
+ \o QSet<T>::iterator
+ \row \o QMap<Key, T>, QMultiMap<Key, T> \o QMap<Key, T>::const_iterator
+ \o QMap<Key, T>::iterator
+ \row \o QHash<Key, T>, QMultiHash<Key, T> \o QHash<Key, T>::const_iterator
+ \o QHash<Key, T>::iterator
+ \endtable
+ The API of the STL iterators is modelled on pointers in an array.
+ For example, the \c ++ operator advances the iterator to the next
+ item, and the \c * operator returns the item that the iterator
+ points to. In fact, for QVector and QStack, which store their
+ items at adjacent memory positions, the
+ \l{QVector::iterator}{iterator} type is just a typedef for \c{T *},
+ and the \l{QVector::iterator}{const_iterator} type is
+ just a typedef for \c{const T *}.
+ In this discussion, we will concentrate on QList and QMap. The
+ iterator types for QLinkedList, QVector, and QSet have exactly
+ the same interface as QList's iterators; similarly, the iterator
+ types for QHash have the same interface as QMap's iterators.
+ Here's a typical loop for iterating through all the elements of a
+ QList<QString> in order and converting them to lowercase:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 10
+ Unlike \l{Java-style iterators}, STL-style iterators point
+ directly at items. The begin() function of a container returns an
+ iterator that points to the first item in the container. The
+ end() function of a container returns an iterator to the
+ imaginary item one position past the last item in the container.
+ end() marks an invalid position; it must never be dereferenced.
+ It is typically used in a loop's break condition. If the list is
+ empty, begin() equals end(), so we never execute the loop.
+ The diagram below shows the valid iterator positions as red
+ arrows for a vector containing four items:
+ \img stliterators1.png
+ Iterating backward with an STL-style iterator requires us to
+ decrement the iterator \e before we access the item. This
+ requires a \c while loop:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 11
+ In the code snippets so far, we used the unary \c * operator to
+ retrieve the item (of type QString) stored at a certain iterator
+ position, and we then called QString::toLower() on it. Most C++
+ compilers also allow us to write \c{i->toLower()}, but some
+ don't.
+ For read-only access, you can use const_iterator, constBegin(),
+ and constEnd(). For example:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 12
+ The following table summarizes the STL-style iterators' API:
+ \table
+ \header \o Expression \o Behavior
+ \row \o \c{*i} \o Returns the current item
+ \row \o \c{++i} \o Advances the iterator to the next item
+ \row \o \c{i += n} \o Advances the iterator by \c n items
+ \row \o \c{--i} \o Moves the iterator back by one item
+ \row \o \c{i -= n} \o Moves the iterator back by \c n items
+ \row \o \c{i - j} \o Returns the number of items between iterators \c i and \c j
+ \endtable
+ The \c{++} and \c{--} operators are available both as prefix
+ (\c{++i}, \c{--i}) and postfix (\c{i++}, \c{i--}) operators. The
+ prefix versions modify the iterators and return a reference to
+ the modified iterator; the postfix versions take a copy of the
+ iterator before they modify it, and return that copy. In
+ expressions where the return value is ignored, we recommend that
+ you use the prefix operators (\c{++i}, \c{--i}), as these are
+ slightly faster.
+ For non-const iterator types, the return value of the unary \c{*}
+ operator can be used on the left side of the assignment operator.
+ For QMap and QHash, the \c{*} operator returns the value
+ component of an item. If you want to retrieve the key, call key()
+ on the iterator. For symmetry, the iterator types also provide a
+ value() function to retrieve the value. For example, here's how
+ we would print all items in a QMap to the console:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 13
+ Thanks to \l{implicit sharing}, it is very inexpensive for a
+ function to return a container per value. The Qt API contains
+ dozens of functions that return a QList or QStringList per value
+ (e.g., QSplitter::sizes()). If you want to iterate over these
+ using an STL iterator, you should always take a copy of the
+ container and iterate over the copy. For example:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 14
+ This problem doesn't occur with functions that return a const or
+ non-const reference to a container.
+ \l{Implicit sharing} has another consequence on STL-style
+ iterators: You must not take a copy of a container while
+ non-const iterators are active on that container. Java-style
+ iterators don't suffer from that limitation.
+ \keyword foreach
+ \section1 The foreach Keyword
+ If you just want to iterate over all the items in a container
+ in order, you can use Qt's \c foreach keyword. The keyword is a
+ Qt-specific addition to the C++ language, and is implemented
+ using the preprocessor.
+ Its syntax is: \c foreach (\e variable, \e container) \e
+ statement. For example, here's how to use \c foreach to iterate
+ over a QLinkedList<QString>:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 15
+ The \c foreach code is significantly shorter than the equivalent
+ code that uses iterators:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 16
+ Unless the data type contains a comma (e.g., \c{QPair<int,
+ int>}), the variable used for iteration can be defined within the
+ \c foreach statement:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 17
+ And like any other C++ loop construct, you can use braces around
+ the body of a \c foreach loop, and you can use \c break to leave
+ the loop:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 18
+ With QMap and QHash, \c foreach accesses the value component of
+ the (key, value) pairs. If you want to iterate over both the keys
+ and the values, you can use iterators (which are fastest), or you
+ can write code like this:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 19
+ For a multi-valued map:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 20
+ Qt automatically takes a copy of the container when it enters a
+ \c foreach loop. If you modify the container as you are
+ iterating, that won't affect the loop. (If you do not modify the
+ container, the copy still takes place, but thanks to \l{implicit
+ sharing} copying a container is very fast.)
+ Since foreach creates a copy of the container, using a non-const
+ reference for the variable does not allow you to modify the original
+ container. It only affects the copy, which is probably not what you
+ want.
+ In addition to \c foreach, Qt also provides a \c forever
+ pseudo-keyword for infinite loops:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 21
+ If you're worried about namespace pollution, you can disable
+ these macros by adding the following line to your \c .pro file:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 22
+ \section1 Other Container-Like Classes
+ Qt includes three template classes that resemble containers in
+ some respects. These classes don't provide iterators and cannot
+ be used with the \c foreach keyword.
+ \list
+ \o QVarLengthArray<T, Prealloc> provides a low-level
+ variable-length array. It can be used instead of QVector in
+ places where speed is particularly important.
+ \o QCache<Key, T> provides a cache to store objects of a certain
+ type T associated with keys of type Key.
+ \o QContiguousCache<T> provides an efficient way of caching data
+ that is typically accessed in a contiguous way.
+ \o QPair<T1, T2> stores a pair of elements.
+ \endlist
+ Additional non-template types that compete with Qt's template
+ containers are QBitArray, QByteArray, QString, and QStringList.
+ \section1 Algorithmic Complexity
+ Algorithmic complexity is concerned about how fast (or slow) each
+ function is as the number of items in the container grow. For
+ example, inserting an item in the middle of a QLinkedList is an
+ extremely fast operation, irrespective of the number of items
+ stored in the QLinkedList. On the other hand, inserting an item
+ in the middle of a QVector is potentially very expensive if the
+ QVector contains many items, since half of the items must be
+ moved one position in memory.
+ To describe algorithmic complexity, we use the following
+ terminology, based on the "big Oh" notation:
+ \keyword constant time
+ \keyword logarithmic time
+ \keyword linear time
+ \keyword linear-logarithmic time
+ \keyword quadratic time
+ \list
+ \o \bold{Constant time:} O(1). A function is said to run in constant
+ time if it requires the same amount of time no matter how many
+ items are present in the container. One example is
+ QLinkedList::insert().
+ \o \bold{Logarithmic time:} O(log \e n). A function that runs in
+ logarithmic time is a function whose running time is
+ proportional to the logarithm of the number of items in the
+ container. One example is qBinaryFind().
+ \o \bold{Linear time:} O(\e n). A function that runs in linear time
+ will execute in a time directly proportional to the number of
+ items stored in the container. One example is
+ QVector::insert().
+ \o \bold{Linear-logarithmic time:} O(\e{n} log \e n). A function
+ that runs in linear-logarithmic time is asymptotically slower
+ than a linear-time function, but faster than a quadratic-time
+ function.
+ \o \bold{Quadratic time:} O(\e{n}\unicode{178}). A quadratic-time function
+ executes in a time that is proportional to the square of the
+ number of items stored in the container.
+ \endlist
+ The following table summarizes the algorithmic complexity of Qt's
+ sequential container classes:
+ \table
+ \header \o \o Index lookup \o Insertion \o Prepending \o Appending
+ \row \o QLinkedList<T> \o O(\e n) \o O(1) \o O(1) \o O(1)
+ \row \o QList<T> \o O(1) \o O(n) \o Amort. O(1) \o Amort. O(1)
+ \row \o QVector<T> \o O(1) \o O(n) \o O(n) \o Amort. O(1)
+ \endtable
+ In the table, "Amort." stands for "amortized behavior". For
+ example, "Amort. O(1)" means that if you call the function
+ only once, you might get O(\e n) behavior, but if you call it
+ multiple times (e.g., \e n times), the average behavior will be
+ O(1).
+ The following table summarizes the algorithmic complexity of Qt's
+ associative containers and sets:
+ \table
+ \header \o{1,2} \o{2,1} Key lookup \o{2,1} Insertion
+ \header \o Average \o Worst case \o Average \o Worst case
+ \row \o QMap<Key, T> \o O(log \e n) \o O(log \e n) \o O(log \e n) \o O(log \e n)
+ \row \o QMultiMap<Key, T> \o O(log \e n) \o O(log \e n) \o O(log \e n) \o O(log \e n)
+ \row \o QHash<Key, T> \o Amort. O(1) \o O(\e n) \o Amort. O(1) \o O(\e n)
+ \row \o QSet<Key> \o Amort. O(1) \o O(\e n) \o Amort. O(1) \o O(\e n)
+ \endtable
+ With QVector, QHash, and QSet, the performance of appending items
+ is amortized O(log \e n). It can be brought down to O(1) by
+ calling QVector::reserve(), QHash::reserve(), or QSet::reserve()
+ with the expected number of items before you insert the items.
+ The next section discusses this topic in more depth.
+ \section1 Growth Strategies
+ QVector<T>, QString, and QByteArray store their items
+ contiguously in memory; QList<T> maintains an array of pointers
+ to the items it stores to provide fast index-based access (unless
+ T is a pointer type or a basic type of the size of a pointer, in
+ which case the value itself is stored in the array); QHash<Key,
+ T> keeps a hash table whose size is proportional to the number
+ of items in the hash. To avoid reallocating the data every single
+ time an item is added at the end of the container, these classes
+ typically allocate more memory than necessary.
+ Consider the following code, which builds a QString from another
+ QString:
+ \snippet doc/src/snippets/code/doc_src_containers.cpp 23
+ We build the string \c out dynamically by appending one character
+ to it at a time. Let's assume that we append 15000 characters to
+ the QString string. Then the following 18 reallocations (out of a
+ possible 15000) occur when QString runs out of space: 4, 8, 12,
+ 16, 20, 52, 116, 244, 500, 1012, 2036, 4084, 6132, 8180, 10228,
+ 12276, 14324, 16372. At the end, the QString has 16372 Unicode
+ characters allocated, 15000 of which are occupied.
+ The values above may seem a bit strange, but here are the guiding
+ principles:
+ \list
+ \o QString allocates 4 characters at a time until it reaches size 20.
+ \o From 20 to 4084, it advances by doubling the size each time.
+ More precisely, it advances to the next power of two, minus
+ 12. (Some memory allocators perform worst when requested exact
+ powers of two, because they use a few bytes per block for
+ book-keeping.)
+ \o From 4084 on, it advances by blocks of 2048 characters (4096
+ bytes). This makes sense because modern operating systems
+ don't copy the entire data when reallocating a buffer; the
+ physical memory pages are simply reordered, and only the data
+ on the first and last pages actually needs to be copied.
+ \endlist
+ QByteArray and QList<T> use more or less the same algorithm as
+ QString.
+ QVector<T> also uses that algorithm for data types that can be
+ moved around in memory using memcpy() (including the basic C++
+ types, the pointer types, and Qt's \l{shared classes}) but uses a
+ different algorithm for data types that can only be moved by
+ calling the copy constructor and a destructor. Since the cost of
+ reallocating is higher in that case, QVector<T> reduces the
+ number of reallocations by always doubling the memory when
+ running out of space.
+ QHash<Key, T> is a totally different case. QHash's internal hash
+ table grows by powers of two, and each time it grows, the items
+ are relocated in a new bucket, computed as qHash(\e key) %
+ QHash::capacity() (the number of buckets). This remark applies to
+ QSet<T> and QCache<Key, T> as well.
+ For most applications, the default growing algorithm provided by
+ Qt does the trick. If you need more control, QVector<T>,
+ QHash<Key, T>, QSet<T>, QString, and QByteArray provide a trio of
+ functions that allow you to check and specify how much memory to
+ use to store the items:
+ \list
+ \o \l{QString::capacity()}{capacity()} returns the
+ number of items for which memory is allocated (for QHash and
+ QSet, the number of buckets in the hash table).
+ \o \l{QString::reserve()}{reserve}(\e size) explicitly
+ preallocates memory for \e size items.
+ \o \l{QString::squeeze()}{squeeze()} frees any memory
+ not required to store the items.
+ \endlist
+ If you know approximately how many items you will store in a
+ container, you can start by calling reserve(), and when you are
+ done populating the container, you can call squeeze() to release
+ the extra preallocated memory.