The QContiguousCache class is a container providing a contiguous cache. More...

Public Typedefs
using	const_pointer = const value_type *

using	const_reference = const value_type &

using	difference_type = qptrdiff

using	pointer = value_type *

using	reference = value_type &

using	size_type = int

using	value_type = T

Public Methods
	QContiguousCache (const QContiguousCache< T > &other)

	QContiguousCache (int capacity=0)

	~QContiguousCache ()

void	append (const T &value)

bool	areIndexesValid () const

const T &	at (int pos) const

int	available () const

int	capacity () const

void	clear ()

bool	containsIndex (int pos) const

int	count () const

T &	first ()

const T &	first () const

int	firstIndex () const

void	insert (int pos, const T &value)

bool	isEmpty () const

bool	isFull () const

T &	last ()

const T &	last () const

int	lastIndex () const

void	normalizeIndexes ()

bool	operator!= (const QContiguousCache< T > &other) const

QContiguousCache< T > &	operator= (const QContiguousCache< T > &other)

QContiguousCache< T > &	operator= (QContiguousCache< T > &&other)

bool	operator== (const QContiguousCache< T > &other) const

T &	operator[] (int pos)

const T &	operator[] (int pos) const

void	prepend (const T &value)

void	removeFirst ()

void	removeLast ()

void	setCapacity (int size)

int	size () const

void	swap (QContiguousCache< T > &other)

T	takeFirst ()

T	takeLast ()

Detailed Description

template<typename T>
class QContiguousCache< T >

The QContiguousCache class provides an efficient way of caching items for display in a user interface view. Unlike QCache, it adds a restriction that elements within the cache are contiguous. This has the advantage of matching how user interface views most commonly request data, as a set of rows localized around the current scrolled position. This restriction allows the cache to consume less memory and processor cycles than QCache. The QContiguousCache class also can provide an upper bound on memory usage via setCapacity().

The simplest way of using a contiguous cache is to use the append() and prepend().

MyRecord record(int row) const {
    Q_ASSERT(row >= 0 && row < count());
 
    while(row > cache.lastIndex()) {
       cache.append(slowFetchRecord(cache.lastIndex()+1));
    }
 
    while(row < cache.firstIndex()) {
       cache.prepend(slowFetchRecord(cache.firstIndex()-1));
    }
 
    return cache.at(row);
}

If the cache is full then the item at the opposite end of the cache from where the new item is appended or prepended will be removed.

This usage can be further optimized by using the insert() function in the case where the requested row is a long way from the currently cached items. If there is a gap between where the new item is inserted and the currently cached items then the existing cached items are first removed to retain the contiguous nature of the cache. Hence it is important to take some care then when using insert() in order to avoid unwanted clearing of the cache.

The range of valid indexes for the QContiguousCache class are from 0 to INT_MAX. Calling prepend() such that the first index would become less than 0 or append() such that the last index would become greater than INT_MAX can result in the indexes of the cache being invalid. When the cache indexes are invalid it is important to call normalizeIndexes() before calling any of containsIndex(), firstIndex(), lastIndex(), at() or operator[](). Calling these functions when the cache has invalid indexes will result in undefined behavior. The indexes can be checked by using areIndexesValid()

In most cases the indexes will not exceed 0 to INT_MAX, and normalizeIndexes() will not need to be used.

See also: setCapacity()

Member Typedef Documentation

template<typename T >

QContiguousCache< T >::const_pointer

Equivalent to const T *.

template<typename T >

QContiguousCache< T >::const_reference

Equivalent to const T &.

template<typename T >

QContiguousCache< T >::difference_type

Equivalent to an integral type used to represent the distance between two elements.

template<typename T >

QContiguousCache< T >::pointer

Equivalent to T *.

template<typename T >

QContiguousCache< T >::reference

Equivalent to T &.

template<typename T >

QContiguousCache< T >::size_type

Equivalent to a signed integer of the appropriate size for your platform.

template<typename T >

QContiguousCache< T >::value_type

Equivalent to T.

Constructor & Destructor Documentation

template<typename T >

QContiguousCache< T >::QContiguousCache ( int capacity = 0 )

explicit

Constructs a cache with the given capacity.

See also: setCapacity()

template<typename T >

QContiguousCache< T >::QContiguousCache ( const QContiguousCache< T > & other )

inline

Constructs a copy of other.

This operation takes constant time, because QContiguousCache is implicitly shared. This makes returning a QContiguousCache from a function very fast. If a shared instance is modified it will be copied and that takes linear time.

See also: operator=()

template<typename T >

QContiguousCache< T >::~QContiguousCache ( )

inline

Destroys the QContiguousCache object.

Method Documentation

template<typename T >

void QContiguousCache< T >::append ( const T & value )

Inserts value at the end of the cache. If the cache is already full the item at the start of the cache will be removed.

See also: prepend(), insert(), isFull()

template<typename T >

bool QContiguousCache< T >::areIndexesValid ( ) const

inline

Returns whether the indexes for items stored in the cache are valid. Indexes can become invalid if items are appended after the index position INT_MAX or prepended before the index position 0. This is only expected to occur in very long lived circular buffer style usage of the contiguous cache. Indexes can be made valid again by calling normalizeIndexs().

See also: normalizeIndexes(), append(), prepend()

template<typename T >

const T & QContiguousCache< T >::at ( int pos ) const

inline

Returns the item at index position pos the cache. The value for pos must be a valid index position in the cache (i.e, firstIndex() <= i <= lastIndex()).

The indexes in the cache refer to the number of positions the item is from the first item appended into the cache. That is to say a cache with a capacity of 100, that has had 150 items appended will have a valid index range of 50 to

This allows inserting and retrieving items into the cache based on a theoretical infinite list.

See also: firstIndex(), lastIndex(), insert(), operator[]()

template<typename T >

int QContiguousCache< T >::available ( ) const

inline

Returns the number of items that can be added to the cache before it becomes full.

See also: size(), capacity(), isFull()

template<typename T >

int QContiguousCache< T >::capacity ( ) const

inline

Returns the number of items the cache can store before it is full. When a cache contains a number of items equal to its capacity, adding new items will cause items farthest from the added item to be removed.

See also: setCapacity(), size()

template<typename T >

void QContiguousCache< T >::clear ( )

Removes all items from the cache. The capacity is unchanged.

template<typename T >

bool QContiguousCache< T >::containsIndex ( int pos ) const

inline

Returns true if the cache's index range includes the given index pos.

See also: firstIndex(), lastIndex()

template<typename T >

int QContiguousCache< T >::count ( ) const

inline

Equivalent to calling size().

template<typename T >

T & QContiguousCache< T >::first ( )

inline

Returns a reference to the first item in the cache. This function assumes that the cache is not empty.

See also: last(), isEmpty()

template<typename T >

const T & QContiguousCache< T >::first ( ) const

inline

Returns a reference to the first item in the cache. This function assumes that the cache is not empty.

template<typename T >

int QContiguousCache< T >::firstIndex ( ) const

inline

Returns the first valid index in the cache. The index will be invalid if the cache is empty.

See also: capacity(), size(), lastIndex()

template<typename T >

void QContiguousCache< T >::insert	(	int	pos,
		const T &	value
	)

Inserts the value at the index position pos. If the cache already contains an item at pos then value will be replaced. If pos is either one more than lastIndex() or one less than firstIndex() it is the equivalent to an append() or a prepend().

If the given index pos is not within the current range of the cache nor adjacent to the bounds of the cache's index range, the cache is first cleared before inserting the item. At this point the cache will have a size of 1. It is worthwhile taking effort to insert items in an order that starts adjacent to the current index range for the cache.

The range of valid indexes for the QContiguousCache class are from 0 to INT_MAX. Inserting outside of this range has undefined behavior.

See also: prepend(), append(), isFull(), firstIndex(), lastIndex()

template<typename T >

bool QContiguousCache< T >::isEmpty ( ) const

inline

Returns true if no items are stored within the cache.

See also: size(), capacity()

template<typename T >

bool QContiguousCache< T >::isFull ( ) const

inline

Returns true if the number of items stored within the cache is equal to the capacity of the cache.

See also: size(), capacity()

template<typename T >

T & QContiguousCache< T >::last ( )

inline

Returns a reference to the last item in the cache. This function assumes that the cache is not empty.

See also: first(), isEmpty()

template<typename T >

const T & QContiguousCache< T >::last ( ) const

inline

Returns a reference to the last item in the cache. This function assumes that the cache is not empty.

template<typename T >

int QContiguousCache< T >::lastIndex ( ) const

inline

Returns the last valid index in the cache. The index will be invalid if the cache is empty.

See also: capacity(), size(), firstIndex()

template<typename T >

void QContiguousCache< T >::normalizeIndexes ( )

inline

Moves the first index and last index of the cache such that they point to valid indexes. The function does not modify the contents of the cache or the ordering of elements within the cache.

It is provided so that index overflows can be corrected when using the cache as a circular buffer.

QContiguousCache<int> cache(10);
cache.insert(INT_MAX, 1); // cache contains one value and has valid indexes, INT_MAX to INT_MAX
cache.append(2); // cache contains two values but does not have valid indexes.
cache.normalizeIndexes(); // cache has two values, 1 and 2.  New first index will be in the range of 0 to capacity().

See also: areIndexesValid(), append(), prepend()

template<typename T >

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

inline

Returns true if other is not equal to this cache, otherwise returns false. Two caches are considered equal if they contain the same values at the same indexes.

This method requires the type T to support operator==().

See also: operator==()

template<typename T >

QContiguousCache< T > & QContiguousCache< T >::operator= ( const QContiguousCache< T > & other )

Copy assigns from other and returns a reference to this object.

template<typename T >

QContiguousCache< T > & QContiguousCache< T >::operator= ( QContiguousCache< T > && other )

inline

Move assigns from other and returns a reference to this object.

template<typename T >

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

Returns true if other is equal to this cache, otherwise returns false. Two caches are considered equal if they contain the same values at the same indexes.

This method requires the type T to support operator==().

See also: operator!=()

template<typename T >

T & QContiguousCache< T >::operator[] ( int pos )

inline

Returns a reference to the data at index position pos. If the cache does not contain an item at the given index position then it will first insert an empty item at that position.

In most cases it is better to use either at() or insert().

Note: This non-const overload of operator[] requires QContiguousCache to make a deep copy. Use at() for read only access to a non-const QContiguousCache.

See also: insert(), at()

template<typename T >

const T & QContiguousCache< T >::operator[] ( int pos ) const

inline

Equivalent to calling at(pos).

template<typename T >

void QContiguousCache< T >::prepend ( const T & value )

Inserts value at the start of the cache. If the cache is already full the item at the end of the cache will be removed.

See also: append(), insert(), isFull()

template<typename T >

void QContiguousCache< T >::removeFirst ( )

inline

Removes the first item from the cache. This function assumes that the cache is not empty.

See also: removeLast()

template<typename T >

void QContiguousCache< T >::removeLast ( )

inline

Removes the last item from the cache. This method assumes that the cache is not empty.

See also: removeFirst()

template<typename T >

void QContiguousCache< T >::setCapacity ( int size )

Sets the capacity of the cache to the given size. A cache can hold a number of items equal to its capacity. When inserting, appending or prepending items to the cache, if the cache is already full then the item farthest from the added item will be removed.

If the given size is smaller than the current count of items in the cache then only the last size items from the cache will remain.

See also: capacity(), isFull()

template<typename T >

int QContiguousCache< T >::size ( ) const

inline

Returns the number of items contained within the cache.

See also: capacity()

template<typename T >

void QContiguousCache< T >::swap ( QContiguousCache< T > & other )

inline

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

template<typename T >

T QContiguousCache< T >::takeFirst ( )

inline

Removes the first item in the cache and returns it. This function assumes that the cache is not empty.

If you do not use the return value, removeFirst() is more efficient.

See also: takeLast(), removeFirst()

template<typename T >

T QContiguousCache< T >::takeLast ( )

inline

Removes the last item in the cache and returns it. This function assumes that the cache is not empty.

If you do not use the return value, removeLast() is more efficient.

See also: takeFirst(), removeLast()

Public Typedefs

Public Methods

Detailed Description

template<typename T> class QContiguousCache< T >

Member Typedef Documentation

Constructor & Destructor Documentation

Method Documentation

template<typename T>
class QContiguousCache< T >