QList Class Reference
[QtCore module]

Detailed Description

The QList class is a template class that provides lists.

QList<T> is one of Qt's generic container classes. It stores a list of values and provides fast index-based access as well as fast insertions and removals.

QList<T>, QLinkedList<T>, and QVector<T> provide similar functionality. Here's an overview:

• For most purposes, QList is the right class to use. Its index-based API is more convenient than QLinkedList's iterator-based API, and it is usually faster than QVector because of the way it stores its items in memory. It also expands to less code in your executable.
• If you need a real linked list, with guarantees of constant time insertions in the middle of the list and iterators to items rather than indexes, use QLinkedList.
• If you want the items to occupy adjacent memory positions, use QVector.

Internally, QList<T> is represented as an array of pointers to items of type T. If T is itself a pointer type or a basic type that is no larger than a pointer, or if T is one of Qt's shared classes, then QList<T> stores the items directly in the pointer array. For lists under a thousand items, this array representation allows for very fast insertions in the middle, and it allows index-based access. Furthermore, operations like prepend() and append() are very fast, because QList preallocates memory at both ends of its internal array. (See Algorithmic Complexity for details.) Note, however, that for unshared list items that are larger than a pointer, each append or insert of a new item requires allocating the new item on the heap, and this per item allocation might make QVector a better choice in cases that do lots of appending or inserting, since QVector allocates memory for its items in a single heap allocation.

Note that the internal array only ever gets bigger over the life of the list. It never shrinks. The internal array is deallocated by the destructor and by the assignment operator, when one list is assigned to another.

Here's an example of a QList that stores integers and a QList that stores QDate values:

 QList<int> integerList;
 QList<QDate> dateList;

Qt includes a QStringList class that inherits QList<QString> and adds a few convenience functions, such as QStringList::join() and QStringList::find(). (QString::split() creates QStringLists from strings.)

QList stores a list of items. The default constructor creates an empty list. To insert items into the list, you can use operator<<():

 QList<QString> list;
 list << "one" << "two" << "three";
 // list: ["one", "two", "three"]

QList provides these basic functions to add, move, and remove items: insert(), replace(), removeAt(), move(), and swap(). In addition, it provides the following convenience functions: append(), prepend(), removeFirst(), and removeLast().

QList uses 0-based indexes, just like C++ arrays. To access the item at a particular index position, you can use operator[](). On non-const lists, operator[]() returns a reference to the item and can be used on the left side of an assignment:

 if (list[0] == "Bob")
     list[0] = "Robert";

Because QList is implemented as an array of pointers, this operation is very fast (constant time). For read-only access, an alternative syntax is to use at():

 for (int i = 0; i < list.size(); ++i) {
     if (list.at(i) == "Jane")
         cout << "Found Jane at position " << i << endl;
 }

at() can be faster than operator[](), because it never causes a deep copy to occur.

A common requirement is to remove an item from a list and do something with it. For this, QList provides takeAt(), takeFirst(), and takeLast(). Here's a loop that removes the items from a list one at a time and calls delete on them:

 QList<QWidget *> list;
 ...
 while (!list.isEmpty())
     delete list.takeFirst();

Inserting and removing items at either ends of the list is very fast (constant time in most cases), because QList preallocates extra space on both sides of its internal buffer to allow for fast growth at both ends of the list.

If you want to find all occurrences of a particular value in a list, use indexOf() or lastIndexOf(). The former searches forward starting from a given index position, the latter searches backward. Both return the index of a matching item if they find it; otherwise, they return -1. For example:

 int i = list.indexOf("Jane");
 if (i != -1)
     cout << "First occurrence of Jane is at position " << i << endl;

If you simply want to check whether a list contains a particular value, use contains(). If you want to find out how many times a particular value occurs in the list, use count(). If you want to replace all occurrences of a particular value with another, use replace().

QList's value type must be an assignable data type. This covers most data types that are commonly used, but the compiler won't let you, for example, store a QWidget as a value; instead, store a QWidget *. A few functions have additional requirements; for example, indexOf() and lastIndexOf() expect the value type to support operator==(). These requirements are documented on a per-function basis.

Like the other container classes, QList provides Java-style iterators (QListIterator and QMutableListIterator) and STL-style iterators (QList::const_iterator and QList::iterator). In practice, these are rarely used, because you can use indexes into the QList. QList is implemented in such a way that direct index-based access is just as fast as using iterators.

QList does not support inserting, prepending, appending or replacing with references to its own values. Doing so will cause your application to abort with an error message.

To make QList as efficient as possible, its member functions don't validate their input before using it. Except for isEmpty(), member functions always assume the list is not empty. Member functions that take index values as parameters always assume their index value parameters are in the valid range. This means QList member functions can fail. If you define QT_NO_DEBUG when you compile, failures will not be detected. If you don't define QT_NO_DEBUG, failures will be detected using Q_ASSERT() or Q_ASSERT_X() with an appropriate message.

To avoid failures when your list can be empty, call isEmpty() before calling other member functions. If you must pass an index value that might not be in the valid range, check that it is less than the value returned by size() but not less than 0.

See also QListIterator, QMutableListIterator, QLinkedList, and QVector.

Member Type Documentation

 QList<QString> list;
 list.append("one");
 list.append("two");
 list.append("three");
 // list: ["one", "two", "three"]

 QSet<int> set;
 set << 20 << 30 << 40 << ... << 70;

 QList<int> list = QList<int>::fromSet(set);
 qSort(list);

 std::list<double> stdlist;
 list.push_back(1.2);
 list.push_back(0.5);
 list.push_back(3.14);

 QList<double> list = QList<double>::fromStdList(stdlist);

 QVector<double> vect;
 vect << 20.0 << 30.0 << 40.0 << 50.0;

 QList<double> list = QVector<T>::fromVector(vect);
 // list: [20.0, 30.0, 40.0, 50.0]

 QList<QString> list;
 list << "A" << "B" << "C" << "B" << "A";
 list.indexOf("B");          // returns 1
 list.indexOf("B", 1);       // returns 1
 list.indexOf("B", 2);       // returns 3
 list.indexOf("X");          // returns -1

 QList<QString> list;
 list << "alpha" << "beta" << "delta";
 list.insert(2, "gamma");
 // list: ["alpha", "beta", "gamma", "delta"]

 QList<QString> list;
 list << "A" << "B" << "C" << "B" << "A";
 list.lastIndexOf("B");      // returns 3
 list.lastIndexOf("B", 3);   // returns 3
 list.lastIndexOf("B", 2);   // returns 1
 list.lastIndexOf("X");      // returns -1

 QList<QString> list;
 list << "A" << "B" << "C" << "D" << "E" << "F";
 list.move(1, 4);
 // list: ["A", "C", "D", "E", "B", "F"]

 QList<QString> list;
 list.prepend("one");
 list.prepend("two");
 list.prepend("three");
 // list: ["three", "two", "one"]

 QList<QString> list;
 list << "sun" << "cloud" << "sun" << "rain";
 list.removeAll("sun");
 // list: ["cloud", "rain"]

 QList<QString> list;
 list << "sun" << "cloud" << "sun" << "rain";
 list.removeOne("sun");
 // list: ["cloud", ,"sun", "rain"]

 QList<QString> list;
 list << "A" << "B" << "C" << "D" << "E" << "F";
 list.swap(1, 4);
 // list: ["A", "E", "C", "D", "B", "F"]

 QStringList list;
 list << "Julia" << "Mike" << "Mike" << "Julia" << "Julia";

 QSet<QString> set = list.toSet();
 set.contains("Julia");  // returns true
 set.contains("Mike");   // returns true
 set.size();             // returns 2

 QList<double> list;
 list << 1.2 << 0.5 << 3.14;

 std::list<double> stdlist = list.toStdList();

 QStringList list;
 list << "Sven" << "Kim" << "Ola";

 QVector<QString> vect = list.toVector();
 // vect: ["Sven", "Kim", "Ola"]