QVarLengthArray Class

The QVarLengthArray class provides a low-level variable-length array. More...

Header: #include <QVarLengthArray>
qmake: QT += core

Note: All functions in this class are reentrant.

Public Types

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

Detailed Description

The C++ language doesn't support variable-length arrays on the stack. For example, the following code won't compile:


  int myfunc(int n)
  {
      int table[n + 1];  // WRONG
      ...
      return table[n];
  }

The alternative is to allocate the array on the heap (with new):


  int myfunc(int n)
  {
      int *table = new int[n + 1];
      ...
      int ret = table[n];
      delete[] table;
      return ret;
  }

However, if myfunc() is called very frequently from the application's inner loop, heap allocation can be a major source of slowdown.

QVarLengthArray is an attempt to work around this gap in the C++ language. It allocates a certain number of elements on the stack, and if you resize the array to a larger size, it automatically uses the heap instead. Stack allocation has the advantage that it is much faster than heap allocation.

Example:


  int myfunc(int n)
  {
      QVarLengthArray<int, 1024> array(n + 1);
      ...
      return array[n];
  }

In the example above, QVarLengthArray will preallocate 1024 elements on the stack and use them unless n + 1 is greater than 1024. If you omit the second template argument, QVarLengthArray's default of 256 is used.

QVarLengthArray'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 *.

QVarLengthArray, like QVector, provides a resizable array data structure. The main differences between the two classes are:

In summary, QVarLengthArray is a low-level optimization class that only makes sense in very specific cases. It is used a few places inside Qt and was added to Qt's public API for the convenience of advanced users.

See also QVector, QList, and QLinkedList.

Member Type Documentation

typedef QVarLengthArray::const_iterator

Typedef for const T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

typedef QVarLengthArray::const_pointer

Typedef for const T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

typedef QVarLengthArray::const_reference

Typedef for const T &. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

typedef QVarLengthArray::const_reverse_iterator

Typedef for std::reverse_iterator<const T*>. Provided for STL compatibility.

This typedef was introduced in Qt 5.6.

typedef QVarLengthArray::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

typedef QVarLengthArray::iterator

Typedef for T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

typedef QVarLengthArray::pointer

Typedef for T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

typedef QVarLengthArray::reference

Typedef for T &. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

typedef QVarLengthArray::reverse_iterator

Typedef for std::reverse_iterator<T*>. Provided for STL compatibility.

This typedef was introduced in Qt 5.6.

typedef QVarLengthArray::size_type

Typedef for int. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

typedef QVarLengthArray::value_type

Typedef for T. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.