- Qt 6.8
- Qt Core
- C++ Classes
- QContiguousCache
- Qt 6.8.3 Reference Documentation

# QContiguousCache Class

template <typename T> class QContiguousCache

The QContiguousCache class is a template class that provides a contiguous cache. More...

Header: | `#include <QContiguousCache>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`

- List of all members, including inherited members
- QContiguousCache is part of Implicitly Shared Classes.

Note: All functions in this class are reentrant.

## Public Functions

| QContiguousCache(qsizetype capacity = 0)
| QContiguousCache(const QContiguousCache<T> &other)
| ~QContiguousCache()
void | append(const T &value)
bool | areIndexesValid() const
const T & | at(qsizetype i) const
qsizetype | available() const
qsizetype | capacity() const
void | clear()
bool | containsIndex(qsizetype i) const
qsizetype | count() const
T & | first()
const T & | first() const
qsizetype | firstIndex() const
void | insert(qsizetype i, const T &value)
bool | isEmpty() const
bool | isFull() const
T & | last()
const T & | last() const
qsizetype | lastIndex() const
void | normalizeIndexes()
void | prepend(const T &value)
void | removeFirst()
void | removeLast()
void | setCapacity(qsizetype size)
qsizetype | size() const
void | swap(QContiguousCache<T> &other)
T | takeFirst()
T | takeLast()
bool | operator!=(const QContiguousCache<T> &other) const
QContiguousCache<T> & | operator=(QContiguousCache<T> &&other)
QContiguousCache<T> & | operator=(const QContiguousCache<T> &other)
bool | operator==(const QContiguousCache<T> &other) const
T & | operator[](qsizetype i)
const T & | operator[](qsizetype i) const

## Detailed Description

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.

QContiguousCache operates on a fixed capacity, set with setCapacity() or passed as a parameter to the constructor. This capacity is the upper bound on memory usage by the cache itself, not including the memory allocated by the elements themselves. Note that a cache with a capacity of zero (the default) means no items will be stored: the insert(), append() and prepend() operations will effectively be no-ops. Therefore, it's important to set the capacity to a reasonable value before adding items to the cache.

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 the Contiguous Cache example.

## Member Function Documentation

### `[explicit]` QContiguousCache::QContiguousCache(qsizetype capacity = 0)

Constructs a cache with the given capacity.

See also setCapacity().

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

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 (copy-on-write), and that takes linear time.

See also operator=().

### QContiguousCache::~QContiguousCache()

Destroys the cache.

### void QContiguousCache::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(), and isFull().

### bool QContiguousCache::areIndexesValid() const

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 normalizeIndexes().

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

### const T &QContiguousCache::at(qsizetype i) const

Returns the item at index position i in the cache. i 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 149. This allows inserting and retrieving items into the cache based on a theoretical infinite list

See also firstIndex(), lastIndex(), insert(), and operator[]().

### qsizetype QContiguousCache::available() const

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

See also size(), capacity(), and isFull().

### qsizetype QContiguousCache::capacity() const

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() and size().

### void QContiguousCache::clear()

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

### bool QContiguousCache::containsIndex(qsizetype i) const

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

See also firstIndex() and lastIndex().

### qsizetype QContiguousCache::count() const

Same as size().

### T &QContiguousCache::first()

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

See also last() and isEmpty().

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

This is an overloaded function.

### qsizetype QContiguousCache::firstIndex() const

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

See also capacity(), size(), and lastIndex().

### void QContiguousCache::insert(qsizetype i, const T &value)

Inserts the value at the index position i. If the cache already contains an item at i then that value is replaced. If i 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 i 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(), and lastIndex().

### bool QContiguousCache::isEmpty() const

Returns `true` if no items are stored within the cache.

See also size() and capacity().

### bool QContiguousCache::isFull() const

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

See also size() and capacity().

### T &QContiguousCache::last()

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

See also first() and isEmpty().

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

This is an overloaded function.

### qsizetype QContiguousCache::lastIndex() const

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

See also capacity(), size(), and firstIndex().

### void QContiguousCache::normalizeIndexes()

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(), and prepend().

### void QContiguousCache::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(), and isFull().

### void QContiguousCache::removeFirst()

Removes the first item from the cache. This function assumes that the cache isn't empty.

See also removeLast().

### void QContiguousCache::removeLast()

Removes the last item from the cache. This function assumes that the cache isn't empty.

See also removeFirst().

### void QContiguousCache::setCapacity(qsizetype 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() and isFull().

### qsizetype QContiguousCache::size() const

Returns the number of items contained within the cache.

See also capacity().

### `[noexcept]` void QContiguousCache::swap(QContiguousCache<T> &other)

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

### T QContiguousCache::takeFirst()

Removes the first item in the cache and returns it. This function assumes that the cache isn't empty.

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

See also takeLast() and removeFirst().

### T QContiguousCache::takeLast()

Removes the last item in the cache and returns it. This function assumes that the cache isn't empty.

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

See also takeFirst() and removeLast().

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

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

See also operator==().

### `[noexcept]` QContiguousCache<T> &QContiguousCache::operator=(QContiguousCache<T> &&other)

Move-assigns other to this QContiguousCache instance.

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

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

### bool QContiguousCache::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 function requires the value type to implement the `operator==()`.

See also operator!=().

### T &QContiguousCache::operator[](qsizetype i)

Returns the item at index position i as a modifiable reference. If the cache does not contain an item at the given index position i 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() and at().

### const T &QContiguousCache::operator[](qsizetype i) const

This is an overloaded function.

Same as at(i).