Qt Container Classes

Remarks#

Qt provides its own template container classes. They are all implicitly shared. They provide two kinds of iterators (Java style and STL style.)

Qt sequential containers include: QVector, QList, QLinkedList, QStack, QQueue.

Qt associative containers include: QMap, QMultiMap, QHash, QMultiHash, QSet.

QStack usage

QStack<T> is a template Qt class providing stack. Its analogue in STL is std::stack. It is last in, first out structure (LIFO).

QStack<QString> stack;
stack.push("First");
stack.push("Second");
stack.push("Third");
while (!stack.isEmpty())
{
    cout << stack.pop() << endl;
}

It will output: Third, Second, First.

QStack inherits from QVector so its implementation is quite different from STL. In STL std::stack is implemented as a wrapper to type passed as a template argument (deque by default). Still main operations are the same for QStack and for std::stack.

QVector usage

QVector<T> provides dynamic array template class. It provides better performance in most cases than QList<T> so it should be first choice.

It can be initialized in various ways:

QVector<int> vect;
vect << 1 << 2 << 3;

QVector<int> v {1, 2, 3, 4};

The latest involves initialization list.

QVector<QString> stringsVector;
stringsVector.append("First");
stringsVector.append("Second");

You can get i-th element of vector this way:

v[i] or at[i]

Make sure that i is valid position, even at(i) doesn’t make a check, this is a difference from std::vector.

QLinkedList usage

In Qt you should use QLinkedList in case you need to implement linked list.

It is fast to append, prepend, insert elements into QLinkedList - O(1), but index lookup is slower than in QList or QVector - O(n). This is normal taking into attention you have to iterate through nodes to find something in linked list.

Full algorithmic compexity table can be found here.

Just to insert some elements into QLinkedList you can use operator <<():

QLinkedList<QString> list;
list << "string1" << "string2" << "string3";

To insert elements in the middle of QLinkedList or modify all or some of its elements you can use Java style or STL style iterators. Here is a simple example how we multiply all the elements of QLinkedList by 2:

QLinkedList<int> integerList {1, 2, 3};
QLinkedList<int>::iterator it;
for (it = integerList.begin(); it != integerList.end(); ++it)
{
    *it *= 2;
}

QList

The QList class is a template class that provides lists. It stores items in a list that provides fast index-based access and index-based insertions and removals.

To insert items into the list, you can use operator<<(), insert(), append() or prepend(). For example:

operator<<()

QList<QString> list;
list << "one" << "two" << "three";

insert()

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

append()

QList<QString> list;
list.append("one");
list.append("two");
list.append("three");

prepend()

QList<QString> list;
list.prepend("one");
list.prepend("two");
list.prepend("three");

To access the item at a particular index position, you can use operator[]() or at(). at() may be faster than operator[](), it never causes deep copy of container and should work in constant-time. Neither of them does argument-check. Examples:

if (list[0] == "mystring")
    cout << "mystring found" << endl;

Or

if (list.at(i) == "mystring")
    cout << "mystring found at position " << i << endl;

To remove items, there are functions such as removeAt(), takeAt(), takeFirst(), takeLast(), removeFirst(), removeLast(), or removeOne(). Examples:

takeFirst()

// takeFirst() removes the first item in the list and returns it
QList<QWidget *> list;
...
while (!list.isEmpty())
    delete list.takeFirst();

removeOne()

// removeOne() removes the first occurrence of value in the list
QList<QString> list;
list << "sun" << "cloud" << "sun" << "rain";
list.removeOne("sun");

To find all occurrences of a particular value in a list, you can use indexOf() or lastIndexOf(). Example:

indexOf()

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