A variant of EvictingCacheMap that tracks weights for entries and evicts entries in LRU order to ensure the total weight of all entries stays below some set maximum. Weights are stored as a size_t with each entry.
Synopsis
Declared in <folly/container/WeightedEvictingCacheMap.h>
template<
class _TKey,
class _TValue,
class _THash = HeterogeneousAccessHash<TKey>,
class _TKeyEqual = HeterogeneousAccessEqualTo<TKey>>
class WeightedEvictingCacheMap;Description
Example usage: if TKey is std::string and TValue is some large, complex object type, the weight could be the estimated memory size of the key and complex object. Tracking the weight explicitly minimizes costly recomputation of the estimated memory size. Thus, the total weight of all entries approximates the total memory usage.
TValue must be either movable or copyable. TKey must be copyable.
IMPORTANT NOTES: * Returned references, pointers, or iterators are potentially invalid after any pruning operation (set, insert, etc. that might increase total weight), or any set/insert on the same key (which are allowed to create a new entry or modify an existing entry becoming obsolete). * This is NOT a thread‐safe structure. * For simplicity, functions taking a key implicitly inherit type constraints from EvictingCacheMap. (Must either match TKey or EligibleForHeterogeneousFind/Insert.)
This implementation has not been highly optimized.
Types
Name |
Description |
A stored value together with its tracked weight. |
Type Aliases
Name |
Description |
Callback type invoked on eviction with the key, value, and weight. |
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Const iterator over key and ValueAndWeight pairs. |
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Const reverse iterator over key and ValueAndWeight pairs. |
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Mutable iterator that dereferences to a key and ValueAndWeight pair. |
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Reverse mutable iterator over key and ValueAndWeight pairs. |
Member Functions
Name |
Description |
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Constructors |
Assignment operators |
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Returns a const iterator to the most recently used entry. |
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Returns a const iterator past the least recently used entry. |
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Clear the cache to an empty state. |
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Returns a const reverse iterator to the least recently used entry. |
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Returns a const reverse iterator past the most recently used entry. |
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Typical empty function |
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Erases any entry with given key or iterator. |
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Check for existence of a specific key in the map. This operation has no effect on LRU order. |
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Get the iterator associated with a specific key. This function always promotes a found value to the head of the LRU. Although values can be modified through iterators, weights are const. See updateWeight(). |
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Get the value associated with a specific key. This function always promotes a found value to the head of the LRU. The TValue can be modified in place through the reference, keeping in mind the reference can easily be invalidated (IMPORTANT NOTES above). |
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Returns total weight of all entries currently in the cache. |
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Returns the maximum allowed total weight of all entries in the cache. |
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Sets the maximum allowed total weight of all entries in the cache, evicting entries as needed for the new limit. |
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Set the prune hook, which is the function invoked on the key and value on each eviction. An operation will throw if the pruneHook throws. Note that this prune hook is not automatically called on entries explicitly erase()ed nor on remaining entries at destruction time. |
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Get the number of elements in the dictionary |
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Static Data Members
Name |
Description |
Approximate memory used by each cache entry. |
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