Qt 5.6 | Qt Core | C++ Classes | QVector

Qt 5.6.2 Reference Documentation

# QVector Class

The QVector class is a template class that provides a dynamic array. More...

Header: | #include <QVector>
qmake: | QT += core
Inherited By: | QStack and QXmlStreamAttributes

- List of all members, including inherited members

Note: All functions in this class are reentrant.

## Public Types

typedef | ConstIterator
typedef | Iterator
typedef | const_iterator
typedef | const_pointer
typedef | const_reference
typedef | const_reverse_iterator
typedef | difference_type
typedef | iterator
typedef | pointer
typedef | reference
typedef | reverse_iterator
typedef | size_type
typedef | value_type

## Public Functions

| QVector()
| QVector(int size)
| QVector(int size, const T &value)
| QVector(const QVector<T> &other)
| QVector(QVector<T> &&other)
| QVector(std::initializer_list<T> args)
| ~QVector()
void | append(const T &value)
void | append(T &&value)
void | append(const QVector<T> &value)
const T & | at(int i) const
reference | back()
const_reference | back() const
iterator | begin()
const_iterator | begin() const
int | capacity() const
const_iterator | cbegin() const
const_iterator | cend() const
void | clear()
const_iterator | constBegin() const
const T * | constData() const
const_iterator | constEnd() const
const T & | constFirst() const
const T & | constLast() const
bool | contains(const T &value) const
int | count(const T &value) const
int | count() const
const_reverse_iterator | crbegin() const
const_reverse_iterator | crend() const
T * | data()
const T * | data() const
bool | empty() const
iterator | end()
const_iterator | end() const
bool | endsWith(const T &value) const
iterator | erase(iterator pos)
iterator | erase(iterator begin, iterator end)
QVector<T> & | fill(const T &value, int size = -1)
T & | first()
const T & | first() const
T & | front()
const_reference | front() const
int | indexOf(const T &value, int from = 0) const
void | insert(int i, const T &value)
void | insert(int i, int count, const T &value)
iterator | insert(iterator before, int count, const T &value)
iterator | insert(iterator before, const T &value)
bool | isEmpty() const
T & | last()
const T & | last() const
int | lastIndexOf(const T &value, int from = -1) const
int | length() const
QVector<T> | mid(int pos, int length = -1) const
void | move(int from, int to)
void | pop_back()
void | pop_front()
void | prepend(const T &value)
void | push_back(const T &value)
void | push_back(T &&value)
void | push_front(const T &value)
reverse_iterator | rbegin()
const_reverse_iterator | rbegin() const
void | remove(int i)
void | remove(int i, int count)
int | removeAll(const T &t)
void | removeAt(int i)
void | removeFirst()
void | removeLast()
bool | removeOne(const T &t)
reverse_iterator | rend()
const_reverse_iterator | rend() const
void | replace(int i, const T &value)
void | reserve(int size)
void | resize(int size)
int | size() const
void | squeeze()
bool | startsWith(const T &value) const
void | swap(QVector<T> &other)
T | takeAt(int i)
T | takeFirst()
T | takeLast()
QList<T> | toList() const
std::vector<T> | toStdVector() const
T | value(int i) const
T | value(int i, const T &defaultValue) const
bool | operator!=(const QVector<T> &other) const
QVector<T> | operator+(const QVector<T> &other) const
QVector<T> & | operator+=(const QVector<T> &other)
QVector<T> & | operator+=(const T &value)
QVector<T> & | operator<<(const T &value)
QVector<T> & | operator<<(const QVector<T> &other)
QVector<T> & | operator=(const QVector<T> &other)
QVector<T> & | operator=(QVector<T> &&other)
bool | operator==(const QVector<T> &other) const
T & | operator[](int i)
const T & | operator[](int i) const

## Static Public Members

QVector<T> | fromList(const QList<T> &list)
QVector<T> | fromStdVector(const std::vector<T> &vector)

## Related Non-Members

uint | qHash(const QVector<T> &key, uint seed = 0)
bool | operator<(const QVector<T> &lhs, const QVector<T> &rhs)
QDataStream & | operator<<(QDataStream &out, const QVector<T> &vector)
bool | operator<=(const QVector<T> &lhs, const QVector<T> &rhs)
bool | operator>(const QVector<T> &lhs, const QVector<T> &rhs)
bool | operator>=(const QVector<T> &lhs, const QVector<T> &rhs)
QDataStream & | operator>>(QDataStream &in, QVector<T> &vector)

## Detailed Description

The QVector class is a template class that provides a dynamic array.

QVector<T> is one of Qt's generic container classes. It stores its items in adjacent memory locations and provides fast index-based access.

QList<T>, QLinkedList<T>, QVector<T>, and QVarLengthArray<T> provide similar APIs and functionality. They are often interchangeable, but there are performance consequences. Here is an overview of use cases:

- QVector should be your default first choice. QVector<T> will usually give better performance than QList<T>, because QVector<T> always stores its items sequentially in memory, where QList<T> will allocate its items on the heap unless `sizeof(T) <= sizeof(void*)` and T has been declared to be either a `Q_MOVABLE_TYPE` or a `Q_PRIMITIVE_TYPE` using Q_DECLARE_TYPEINFO. See the Pros and Cons of Using QList for an explanation.
- However, QList is used throughout the Qt APIs for passing parameters and for returning values. Use QList to interface with those APIs.
- If you need a real linked list, which guarantees constant time insertions mid-list and uses iterators to items rather than indexes, use QLinkedList.

Note: QVector and QVarLengthArray both guarantee C-compatible array layout. QList does not. This might be important if your application must interface with a C API.

Note: Iterators into a QLinkedList and references into heap-allocating QLists remain valid long as the referenced items remain in the container. This is not true for iterators and references into a QVector and non-heap-allocating QLists.

Here's an example of a QVector that stores integers and a QVector that stores QString values:

      QVector<int> integerVector;
      QVector<QString> stringVector;

QVector stores its items in a vector (array). Typically, vectors are created with an initial size. For example, the following code constructs a QVector with 200 elements:

      QVector<QString> vector(200);

The elements are automatically initialized with a default-constructed value. If you want to initialize the vector with a different value, pass that value as the second argument to the constructor:

      QVector<QString> vector(200, "Pass");

You can also call fill() at any time to fill the vector with a value.

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

      if (vector[0] == "Liz")
          vector[0] = "Elizabeth";

For read-only access, an alternative syntax is to use at():

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

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

Another way to access the data stored in a QVector is to call data(). The function returns a pointer to the first item in the vector. You can use the pointer to directly access and modify the elements stored in the vector. The pointer is also useful if you need to pass a QVector to a function that accepts a plain C++ array.

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

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

If you simply want to check whether a vector contains a particular value, use contains(). If you want to find out how many times a particular value occurs in the vector, use count().

QVector provides these basic functions to add, move, and remove items: insert(), replace(), remove(), prepend(), append(). With the exception of append() and replace(), these functions can be slow (linear time) for large vectors, because they require moving many items in the vector by one position in memory. If you want a container class that provides fast insertion/removal in the middle, use QList or QLinkedList instead.

Unlike plain C++ arrays, QVectors can be resized at any time by calling resize(). If the new size is larger than the old size, QVector might need to reallocate the whole vector. QVector tries to reduce the number of reallocations by preallocating up to twice as much memory as the actual data needs.

If you know in advance approximately how many items the QVector will contain, you can call reserve(), asking QVector to preallocate a certain amount of memory. You can also call capacity() to find out how much memory QVector actually allocated.

Note that using non-const operators and functions can cause QVector to do a deep copy of the data. This is due to implicit sharing.

QVector'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, QVector provides Java-style iterators (QVectorIterator and QMutableVectorIterator) and STL-style iterators (QVector::const_iterator and QVector::iterator). In practice, these are rarely used, because you can use indexes into the QVector.

In addition to QVector, Qt also provides QVarLengthArray, a very low-level class with little functionality that is optimized for speed.

QVector 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.

#### More Information on Using Qt Containers

For a detailed discussion comparing Qt containers with each other and with STL containers, see Understand the Qt Containers.

See also QVectorIterator, QMutableVectorIterator, QList, and QLinkedList.

## Member Type Documentation

### typedef QVector::ConstIterator

Qt-style synonym for QVector::const_iterator.

### typedef QVector::Iterator

Qt-style synonym for QVector::iterator.

### typedef QVector::const_iterator

The QVector::const_iterator typedef provides an STL-style const iterator for QVector and QStack.

QVector provides both STL-style iterators and Java-style iterators. The STL-style const iterator is simply a typedef for "const T *" (pointer to const T).

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 QVector::constBegin(), QVector::constEnd(), QVector::iterator, and QVectorIterator.

### typedef QVector::const_pointer

Typedef for const T *. Provided for STL compatibility.

### typedef QVector::const_reference

Typedef for T &. Provided for STL compatibility.

### typedef QVector::const_reverse_iterator

The QVector::const_reverse_iterator typedef provides an STL-style const reverse iterator for QVector.

It is simply a typedef for `std::reverse_iterator<const T*>`.

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.

This typedef was introduced in Qt 5.6.

See also QVector::rbegin(), QVector::rend(), QVector::reverse_iterator, and QVector::const_iterator.

### typedef QVector::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

### typedef QVector::iterator

The QVector::iterator typedef provides an STL-style non-const iterator for QVector and QStack.

QVector provides both STL-style iterators and Java-style iterators. The STL-style non-const iterator is simply a typedef for "T *" (pointer to T).

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 QVector::begin(), QVector::end(), QVector::const_iterator, and QMutableVectorIterator.

### typedef QVector::pointer

Typedef for T *. Provided for STL compatibility.

### typedef QVector::reference

Typedef for T &. Provided for STL compatibility.

### typedef QVector::reverse_iterator

The QVector::reverse_iterator typedef provides an STL-style non-const reverse iterator for QVector.

It is simply a typedef for `std::reverse_iterator<T*>`.

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.

This typedef was introduced in Qt 5.6.

See also QVector::rbegin(), QVector::rend(), QVector::const_reverse_iterator, and QVector::iterator.

### typedef QVector::size_type

Typedef for int. Provided for STL compatibility.

### typedef QVector::value_type

Typedef for T. Provided for STL compatibility.

## Member Function Documentation

### QVector::QVector()

Constructs an empty vector.

See also resize().

### QVector::QVector(int size)

Constructs a vector with an initial size of size elements.

The elements are initialized with a default-constructed value.

See also resize().

### QVector::QVector(int size, const T &value)

Constructs a vector with an initial size of size elements. Each element is initialized with value.

See also resize() and fill().

### QVector::QVector(const QVector<T> &other)

Constructs a copy of other.

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

See also operator=().

### QVector::QVector(QVector<T> &&other)

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

This function was introduced in Qt 5.2.

### QVector::QVector(std::initializer_list<T> args)

Constructs a vector from the std::initializer_list given by args.

This constructor is only enabled if the compiler supports C++11 initializer lists.

This function was introduced in Qt 4.8.

### QVector::~QVector()

Destroys the vector.

### void QVector::append(const T &value)

Inserts value at the end of the vector.

Example:

      QVector<QString> vector;
      vector.append("one");
      vector.append("two");
      QString three = "three";
      vector.append(three);
      // vector: ["one", "two", "three"]
      // three: "three"

This is the same as calling resize(size() + 1) and assigning value to the new last element in the vector.

This operation is relatively fast, because QVector typically allocates more memory than necessary, so it can grow without reallocating the entire vector each time.

See also operator<<(), prepend(), and insert().

### void QVector::append(T &&value)

This is an overloaded function.

Example:

      QVector<QString> vector;
      vector.append("one");
      vector.append("two");
      QString three = "three";
      vector.append(std::move(three));
      // vector: ["one", "two", "three"]
      // three: ""

This function was introduced in Qt 5.6.

### void QVector::append(const QVector<T> &value)

This is an overloaded function.

Appends the items of the value vector to this vector.

This function was introduced in Qt 5.5.

See also operator<<() and operator+=().

### const T &QVector::at(int i) const

Returns the item at index position i in the vector.

i must be a valid index position in the vector (i.e., 0 <= i < size()).

See also value() and operator[]().

### reference QVector::back()

This function is provided for STL compatibility. It is equivalent to last().

### const_reference QVector::back() const

This is an overloaded function.

### iterator QVector::begin()

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

See also constBegin() and end().

### const_iterator QVector::begin() const

This is an overloaded function.

### int QVector::capacity() const

Returns the maximum number of items that can be stored in the vector without forcing a reallocation.

The sole purpose of this function is to provide a means of fine tuning QVector's memory usage. In general, you will rarely ever need to call this function. If you want to know how many items are in the vector, call size().

See also reserve() and squeeze().

### const_iterator QVector::cbegin() const

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

This function was introduced in Qt 5.0.

See also begin() and cend().

### const_iterator QVector::cend() const

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

This function was introduced in Qt 5.0.

See also cbegin() and end().

### void QVector::clear()

Removes all the elements from the vector and releases the memory used by the vector.

### const_iterator QVector::constBegin() const

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

See also begin() and constEnd().

### const T *QVector::constData() const

Returns a const pointer to the data stored in the vector. The pointer can be used to access the items in the vector. The pointer remains valid as long as the vector isn't reallocated.

This function is mostly useful to pass a vector to a function that accepts a plain C++ array.

See also data() and operator[]().

### const_iterator QVector::constEnd() const

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

See also constBegin() and end().

### const T &QVector::constFirst() const

Returns a const reference to the first item in the vector. This function assumes that the vector isn't empty.

This function was introduced in Qt 5.6.

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

### const T &QVector::constLast() const

Returns a const reference to the last item in the vector. This function assumes that the vector isn't empty.

This function was introduced in Qt 5.6.

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

### bool QVector::contains(const T &value) const

Returns `true` if the vector contains an occurrence of value; otherwise returns `false`.

This function requires the value type to have an implementation of `operator==()`.

See also indexOf() and count().

### int QVector::count(const T &value) const

Returns the number of occurrences of value in the vector.

This function requires the value type to have an implementation of `operator==()`.

See also contains() and indexOf().

### int QVector::count() const

This is an overloaded function.

Same as size().

### const_reverse_iterator QVector::crbegin() const

Returns a const STL-style reverse iterator pointing to the first item in the vector, in reverse order.

This function was introduced in Qt 5.6.

See also begin(), rbegin(), and rend().

### const_reverse_iterator QVector::crend() const

Returns a const STL-style reverse iterator pointing to one past the last item in the vector, in reverse order.

This function was introduced in Qt 5.6.

See also end(), rend(), and rbegin().

### T *QVector::data()

Returns a pointer to the data stored in the vector. The pointer can be used to access and modify the items in the vector.

Example:

      QVector<int> vector(10);
      int *data = vector.data();
      for (int i = 0; i < 10; ++i)
          data[i] = 2 * i;

The pointer remains valid as long as the vector isn't reallocated.

This function is mostly useful to pass a vector to a function that accepts a plain C++ array.

See also constData() and operator[]().

### const T *QVector::data() const

This is an overloaded function.

### bool QVector::empty() const

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

### iterator QVector::end()

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

See also begin() and constEnd().

### const_iterator QVector::end() const

This is an overloaded function.

### bool QVector::endsWith(const T &value) const

Returns `true` if this vector is not empty and its last item is equal to value; otherwise returns `false`.

This function was introduced in Qt 4.5.

See also isEmpty() and last().

### iterator QVector::erase(iterator pos)

Removes the item pointed to by the iterator pos from the vector, and returns an iterator to the next item in the vector (which may be end()).

See also insert() and remove().

### iterator QVector::erase(iterator begin, iterator end)

This is an overloaded function.

Removes all the items from begin up to (but not including) end. Returns an iterator to the same item that end referred to before the call.

### QVector<T> &QVector::fill(const T &value, int size = -1)

Assigns value to all items in the vector. If size is different from -1 (the default), the vector is resized to size size beforehand.

Example:

      QVector<QString> vector(3);
      vector.fill("Yes");
      // vector: ["Yes", "Yes", "Yes"]

      vector.fill("oh", 5);
      // vector: ["oh", "oh", "oh", "oh", "oh"]

See also resize().

### T &QVector::first()

Returns a reference to the first item in the vector. This function assumes that the vector isn't empty.

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

### const T &QVector::first() const

This is an overloaded function.

### `[static]` QVector<T> QVector::fromList(const QList<T> &list)

Returns a QVector object with the data contained in list.

Example:

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

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

See also toList() and QList::toVector().

### `[static]` QVector<T> QVector::fromStdVector(const std::vector<T> &vector)

Returns a QVector object with the data contained in vector. The order of the elements in the QVector is the same as in vector.

Example:

      std::vector<double> stdvector;
      vector.push_back(1.2);
      vector.push_back(0.5);
      vector.push_back(3.14);

      QVector<double> vector = QVector<double>::fromStdVector(stdvector);

See also toStdVector() and QList::fromStdList().

### T &QVector::front()

This function is provided for STL compatibility. It is equivalent to first().

### const_reference QVector::front() const

This is an overloaded function.

### int QVector::indexOf(const T &value, int from = 0) const

Returns the index position of the first occurrence of value in the vector, searching forward from index position from. Returns -1 if no item matched.

Example:

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

This function requires the value type to have an implementation of `operator==()`.

See also lastIndexOf() and contains().

### void QVector::insert(int i, const T &value)

Inserts value at index position i in the vector. If i is 0, the value is prepended to the vector. If i is size(), the value is appended to the vector.

Example:

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

For large vectors, this operation can be slow (linear time), because it requires moving all the items at indexes i and above by one position further in memory. If you want a container class that provides a fast insert() function, use QLinkedList instead.

See also append(), prepend(), and remove().

### void QVector::insert(int i, int count, const T &value)

This is an overloaded function.

Inserts count copies of value at index position i in the vector.

Example:

      QVector<double> vector;
      vector << 2.718 << 1.442 << 0.4342;
      vector.insert(1, 3, 9.9);
      // vector: [2.718, 9.9, 9.9, 9.9, 1.442, 0.4342]

### iterator QVector::insert(iterator before, int count, const T &value)

Inserts count copies of value in front of the item pointed to by the iterator before. Returns an iterator pointing at the first of the inserted items.

### iterator QVector::insert(iterator before, const T &value)

This is an overloaded function.

Inserts value in front of the item pointed to by the iterator before. Returns an iterator pointing at the inserted item.

### bool QVector::isEmpty() const

Returns `true` if the vector has size 0; otherwise returns `false`.

See also size() and resize().

### T &QVector::last()

Returns a reference to the last item in the vector. This function assumes that the vector isn't empty.

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

### const T &QVector::last() const

This is an overloaded function.

### int QVector::lastIndexOf(const T &value, int from = -1) const

Returns the index position of the last occurrence of the value value in the vector, searching backward from index position from. If from is -1 (the default), the search starts at the last item. Returns -1 if no item matched.

Example:

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

This function requires the value type to have an implementation of `operator==()`.

See also indexOf().

### int QVector::length() const

Same as size() and count().

Provided for compatibility with QList.

This function was introduced in Qt 5.2.

See also size(), count(), and QList::length().

### QVector<T> QVector::mid(int pos, int length = -1) const

Returns a sub-vector which contains elements from this vector, starting at position pos. If length is -1 (the default), all elements after pos are included; otherwise length elements (or all remaining elements if there are less than length elements) are included.

### void QVector::move(int from, int to)

Moves the item at index position from to index position to.

Provided for compatibility with QList.

This function was introduced in Qt 5.6.

See also QList::move().

### void QVector::pop_back()

This function is provided for STL compatibility. It is equivalent to removeLast().

### void QVector::pop_front()

This function is provided for STL compatibility. It is equivalent to removeFirst().

### void QVector::prepend(const T &value)

Inserts value at the beginning of the vector.

Example:

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

This is the same as vector.insert(0, value).

For large vectors, this operation can be slow (linear time), because it requires moving all the items in the vector by one position further in memory. If you want a container class that provides a fast prepend() function, use QList or QLinkedList instead.

See also append() and insert().

### void QVector::push_back(const T &value)

This function is provided for STL compatibility. It is equivalent to append(value).

### void QVector::push_back(T &&value)

This is an overloaded function.

This function was introduced in Qt 5.6.

### void QVector::push_front(const T &value)

This function is provided for STL compatibility. It is equivalent to prepend(value).

### reverse_iterator QVector::rbegin()

Returns a STL-style reverse iterator pointing to the first item in the vector, in reverse order.

This function was introduced in Qt 5.6.

See also begin(), crbegin(), and rend().

### const_reverse_iterator QVector::rbegin() const

This is an overloaded function.

This function was introduced in Qt 5.6.

### void QVector::remove(int i)

This is an overloaded function.

Removes the element at index position i.

See also insert(), replace(), and fill().

### void QVector::remove(int i, int count)

This is an overloaded function.

Removes count elements from the middle of the vector, starting at index position i.

See also insert(), replace(), and fill().

### int QVector::removeAll(const T &t)

Removes all elements that compare equal to t from the vector. Returns the number of elements removed, if any.

Provided for compatibility with QList.

This function was introduced in Qt 5.4.

See also removeOne() and QList::removeAll().

### void QVector::removeAt(int i)

Removes the element at index position i. Equivalent to

      remove(i);

Provided for compatibility with QList.

This function was introduced in Qt 5.2.

See also remove() and QList::removeAt().

### void QVector::removeFirst()

Removes the first item in the vector. Calling this function is equivalent to calling remove(0). The vector must not be empty. If the vector can be empty, call isEmpty() before calling this function.

This function was introduced in Qt 5.1.

See also remove(), takeFirst(), and isEmpty().

### void QVector::removeLast()

Removes the last item in the vector. Calling this function is equivalent to calling remove(size() - 1). The vector must not be empty. If the vector can be empty, call isEmpty() before calling this function.

This function was introduced in Qt 5.1.

See also remove(), takeLast(), removeFirst(), and isEmpty().

### bool QVector::removeOne(const T &t)

Removes the first element that compares equal to t from the vector. Returns whether an element was, in fact, removed.

Provided for compatibility with QList.

This function was introduced in Qt 5.4.

See also removeAll() and QList::removeOne().

### reverse_iterator QVector::rend()

Returns a STL-style reverse iterator pointing to one past the last item in the vector, in reverse order.

This function was introduced in Qt 5.6.

See also end(), crend(), and rbegin().

### const_reverse_iterator QVector::rend() const

This is an overloaded function.

This function was introduced in Qt 5.6.

### void QVector::replace(int i, const T &value)

Replaces the item at index position i with value.

i must be a valid index position in the vector (i.e., 0 <= i < size()).

See also operator[]() and remove().

### void QVector::reserve(int size)

Attempts to allocate memory for at least size elements. If you know in advance how large the vector will be, you should call this function to prevent reallocations and memory fragmentation.

If size is an underestimate, the worst that will happen is that the QVector will be a bit slower. If size is an overestimate, you may have used more memory than the normal QVector growth strategy would have allocated—or you may have used less.

An alternative to reserve() is calling resize(). Whether or not that is faster than reserve() depends on the element type, because resize() default-constructs all elements, and requires assignment to existing entries rather than calling append(), which copy- or move-constructs. For simple types, like `int` or `double`, resize() is typically faster, but for anything more complex, you should prefer reserve().

Warning: If the size passed to resize() was underestimated, you run out of allocated space and into undefined behavior. This problem does not exist with reserve(), because it treats the size as just a hint.

See also squeeze() and capacity().

### void QVector::resize(int size)

Sets the size of the vector to size. If size is greater than the current size, elements are added to the end; the new elements are initialized with a default-constructed value. If size is less than the current size, elements are removed from the end.

Since Qt 5.6, resize() doesn't shrink the capacity anymore. To shed excess capacity, use squeeze().

See also size().

### int QVector::size() const

Returns the number of items in the vector.

See also isEmpty() and resize().

### void QVector::squeeze()

Releases any memory not required to store the items.

The sole purpose of this function is to provide a means of fine tuning QVector's memory usage. In general, you will rarely ever need to call this function.

See also reserve() and capacity().

### bool QVector::startsWith(const T &value) const

Returns `true` if this vector is not empty and its first item is equal to value; otherwise returns `false`.

This function was introduced in Qt 4.5.

See also isEmpty() and first().

### void QVector::swap(QVector<T> &other)

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

This function was introduced in Qt 4.8.

### T QVector::takeAt(int i)

Removes the element at index position i and returns it.

Equivalent to

      T t = at(i);
      remove(i);
      return t;

Provided for compatibility with QList.

This function was introduced in Qt 5.2.

See also takeFirst(), takeLast(), and QList::takeAt().

### T QVector::takeFirst()

Removes the first item in the vector and returns it. This function assumes the vector is not empty. To avoid failure, call isEmpty() before calling this function.

This function was introduced in Qt 5.1.

See also takeLast() and removeFirst().

### T QVector::takeLast()

Removes the last item in the list and returns it. This function assumes the vector is not empty. To avoid failure, call isEmpty() before calling this function.

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

This function was introduced in Qt 5.1.

See also takeFirst() and removeLast().

### QList<T> QVector::toList() const

Returns a QList object with the data contained in this QVector.

Example:

      QVector<double> vect;
      vect << "red" << "green" << "blue" << "black";

      QList<double> list = vect.toList();
      // list: ["red", "green", "blue", "black"]

See also fromList() and QList::fromVector().

### std::vector<T> QVector::toStdVector() const

Returns a std::vector object with the data contained in this QVector. Example:

      QVector<double> vector;
      vector << 1.2 << 0.5 << 3.14;

      std::vector<double> stdvector = vector.toStdVector();

See also fromStdVector() and QList::toStdList().

### T QVector::value(int i) const

Returns the value at index position i in the vector.

If the index i is out of bounds, the function returns a default-constructed value. If you are certain that i is within bounds, you can use at() instead, which is slightly faster.

See also at() and operator[]().

### T QVector::value(int i, const T &defaultValue) const

This is an overloaded function.

If the index i is out of bounds, the function returns defaultValue.

### bool QVector::operator!=(const QVector<T> &other) const

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

Two vectors are considered equal if they contain the same values in the same order.

This function requires the value type to have an implementation of `operator==()`.

See also operator==().

### QVector<T> QVector::operator+(const QVector<T> &other) const

Returns a vector that contains all the items in this vector followed by all the items in the other vector.

See also operator+=().

### QVector<T> &QVector::operator+=(const QVector<T> &other)

Appends the items of the other vector to this vector and returns a reference to this vector.

See also operator+() and append().

### QVector<T> &QVector::operator+=(const T &value)

This is an overloaded function.

Appends value to the vector.

See also append() and operator<<().

### QVector<T> &QVector::operator<<(const T &value)

Appends value to the vector and returns a reference to this vector.

See also append() and operator+=().

### QVector<T> &QVector::operator<<(const QVector<T> &other)

Appends other to the vector and returns a reference to the vector.

### QVector<T> &QVector::operator=(const QVector<T> &other)

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

### QVector<T> &QVector::operator=(QVector<T> &&other)

Move-assigns other to this QVector instance.

This function was introduced in Qt 5.2.

### bool QVector::operator==(const QVector<T> &other) const

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

Two vectors are considered equal if they contain the same values in the same order.

This function requires the value type to have an implementation of `operator==()`.

See also operator!=().

### T &QVector::operator[](int i)

Returns the item at index position i as a modifiable reference.

i must be a valid index position in the vector (i.e., 0 <= i < size()).

Note that using non-const operators can cause QVector to do a deep copy.

See also at() and value().

### const T &QVector::operator[](int i) const

This is an overloaded function.

Same as at(i).

## Related Non-Members

### uint qHash(const QVector<T> &key, uint seed = 0)

Returns the hash value for key, using seed to seed the calculation.

This function requires qHash() to be overloaded for the value type `T`.

This function was introduced in Qt 5.6.

### bool operator<(const QVector<T> &lhs, const QVector<T> &rhs)

Returns `true` if vector lhs is lexicographically less than rhs; otherwise returns `false`.

This function requires the value type to have an implementation of `operator<()`.

This function was introduced in Qt 5.6.

### QDataStream &operator<<(QDataStream &out, const QVector<T> &vector)

Writes the vector vector to stream out.

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

See also Format of the QDataStream operators.

### bool operator<=(const QVector<T> &lhs, const QVector<T> &rhs)

Returns `true` if vector lhs is lexicographically less than or equal to rhs; otherwise returns `false`.

This function requires the value type to have an implementation of `operator<()`.

This function was introduced in Qt 5.6.

### bool operator>(const QVector<T> &lhs, const QVector<T> &rhs)

Returns `true` if vector lhs is lexicographically greater than rhs; otherwise returns `false`.

This function requires the value type to have an implementation of `operator<()`.

This function was introduced in Qt 5.6.

### bool operator>=(const QVector<T> &lhs, const QVector<T> &rhs)

Returns `true` if vector lhs is lexicographically greater than or equal to rhs; otherwise returns `false`.

This function requires the value type to have an implementation of `operator<()`.

This function was introduced in Qt 5.6.

### QDataStream &operator>>(QDataStream &in, QVector<T> &vector)

Reads a vector from stream in into vector.

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

See also Format of the QDataStream operators.