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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
// See the comment at the top of mfbt/HashTable.h for a comparison between
// PLDHashTable and mozilla::HashTable.
#ifndef PLDHashTable_h
#define PLDHashTable_h
#include <utility>
#include "mozilla/Assertions.h"
#include "mozilla/Atomics.h"
#include "mozilla/HashFunctions.h"
#include "mozilla/Maybe.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/fallible.h"
#include "nscore.h"
using PLDHashNumber = mozilla::HashNumber;
static const uint32_t kPLDHashNumberBits = mozilla::kHashNumberBits;
#if defined(DEBUG) || defined(FUZZING)
class PLDHashTable;
struct PLDHashTableOps;
// Table entry header structure.
// In order to allow in-line allocation of key and value, we do not declare
// either here. Instead, the API uses const void *key as a formal parameter.
// The key need not be stored in the entry; it may be part of the value, but
// need not be stored at all.
// Callback types are defined below and grouped into the PLDHashTableOps
// structure, for single static initialization per hash table sub-type.
// Each hash table sub-type should make its entry type a subclass of
// PLDHashEntryHdr. PLDHashEntryHdr is merely a common superclass to present a
// uniform interface to PLDHashTable clients. The zero-sized base class
// optimization, employed by all of our supported C++ compilers, will ensure
// that this abstraction does not make objects needlessly larger.
struct PLDHashEntryHdr {
PLDHashEntryHdr() = default;
PLDHashEntryHdr(const PLDHashEntryHdr&) = delete;
PLDHashEntryHdr& operator=(const PLDHashEntryHdr&) = delete;
PLDHashEntryHdr(PLDHashEntryHdr&&) = default;
PLDHashEntryHdr& operator=(PLDHashEntryHdr&&) = default;
friend class PLDHashTable;
// This class does three kinds of checking:
// - that calls to one of |mOps| or to an enumerator do not cause re-entry into
// the table in an unsafe way;
// - that multiple threads do not access the table in an unsafe way;
// - that a table marked as immutable is not modified.
// "Safe" here means that multiple concurrent read operations are ok, but a
// write operation (i.e. one that can cause the entry storage to be reallocated
// or destroyed) cannot safely run concurrently with another read or write
// operation. This meaning of "safe" is only partial; for example, it does not
// cover whether a single entry in the table is modified by two separate
// threads. (Doing such checking would be much harder.)
// It does this with two variables:
// - mState, which embodies a tri-stage tagged union with the following
// variants:
// - Idle
// - Read(n), where 'n' is the number of concurrent read operations
// - Write
// - mIsWritable, which indicates if the table is mutable.
class Checker {
constexpr Checker() : mState(kIdle), mIsWritable(true) {}
Checker& operator=(Checker&& aOther) {
// Atomic<> doesn't have an |operator=(Atomic<>&&)|.
mState = uint32_t(aOther.mState);
mIsWritable = bool(aOther.mIsWritable);
aOther.mState = kIdle;
// XXX Shouldn't we set mIsWritable to true here for consistency?
return *this;
static bool IsIdle(uint32_t aState) { return aState == kIdle; }
static bool IsRead(uint32_t aState) {
return kRead1 <= aState && aState <= kReadMax;
static bool IsRead1(uint32_t aState) { return aState == kRead1; }
static bool IsWrite(uint32_t aState) { return aState == kWrite; }
bool IsIdle() const { return mState == kIdle; }
bool IsWritable() const { return mIsWritable; }
void SetNonWritable() { mIsWritable = false; }
// NOTE: the obvious way to implement these functions is to (a) check
// |mState| is reasonable, and then (b) update |mState|. But the lack of
// atomicity in such an implementation can cause problems if we get unlucky
// thread interleaving between (a) and (b).
// So instead for |mState| we are careful to (a) first get |mState|'s old
// value and assign it a new value in single atomic operation, and only then
// (b) check the old value was reasonable. This ensures we don't have
// interleaving problems.
// For |mIsWritable| we don't need to be as careful because it can only in
// transition in one direction (from writable to non-writable).
void StartReadOp() {
uint32_t oldState = mState++; // this is an atomic increment
MOZ_RELEASE_ASSERT(IsIdle(oldState) || IsRead(oldState));
MOZ_RELEASE_ASSERT(oldState < kReadMax); // check for overflow
void EndReadOp() {
uint32_t oldState = mState--; // this is an atomic decrement
void StartWriteOp() {
uint32_t oldState =;
void EndWriteOp() {
// Check again that the table is writable, in case it was marked as
// non-writable just after the IsWritable() assertion in StartWriteOp()
// occurred.
uint32_t oldState =;
void StartIteratorRemovalOp() {
// When doing removals at the end of iteration, we go from Read1 state to
// Write and then back.
uint32_t oldState =;
void EndIteratorRemovalOp() {
// Check again that the table is writable, in case it was marked as
// non-writable just after the IsWritable() assertion in
// StartIteratorRemovalOp() occurred.
uint32_t oldState =;
void StartDestructorOp() {
// A destructor op is like a write, but the table doesn't need to be
// writable.
uint32_t oldState =;
void EndDestructorOp() {
uint32_t oldState =;
// Things of note about the representation of |mState|.
// - The values between kRead1..kReadMax represent valid Read(n) values.
// - kIdle and kRead1 are deliberately chosen so that incrementing the -
// former gives the latter.
// - 9999 concurrent readers should be enough for anybody.
static const uint32_t kIdle = 0;
static const uint32_t kRead1 = 1;
static const uint32_t kReadMax = 9999;
static const uint32_t kWrite = 10000;
mozilla::Atomic<uint32_t, mozilla::SequentiallyConsistent> mState;
mozilla::Atomic<bool, mozilla::SequentiallyConsistent> mIsWritable;
// A PLDHashTable may be allocated on the stack or within another structure or
// class. No entry storage is allocated until the first element is added. This
// means that empty hash tables are cheap, which is good because they are
// common.
// There used to be a long, math-heavy comment here about the merits of
// double hashing vs. chaining; it was removed in bug 1058335. In short, double
// hashing is more space-efficient unless the element size gets large (in which
// case you should keep using double hashing but switch to using pointer
// elements). Also, with double hashing, you can't safely hold an entry pointer
// and use it after an add or remove operation, unless you sample Generation()
// before adding or removing, and compare the sample after, dereferencing the
// entry pointer only if Generation() has not changed.
class PLDHashTable {
// A slot represents a cached hash value and its associated entry stored in
// the hash table. The hash value and the entry are not stored contiguously.
struct Slot {
Slot(PLDHashEntryHdr* aEntry, PLDHashNumber* aKeyHash)
: mEntry(aEntry), mKeyHash(aKeyHash) {}
Slot(const Slot&) = default;
Slot(Slot&& aOther) = default;
Slot& operator=(Slot&& aOther) = default;
bool operator==(const Slot& aOther) { return mEntry == aOther.mEntry; }
PLDHashNumber KeyHash() const { return *HashPtr(); }
void SetKeyHash(PLDHashNumber aHash) { *HashPtr() = aHash; }
PLDHashEntryHdr* ToEntry() const { return mEntry; }
bool IsFree() const { return KeyHash() == 0; }
bool IsRemoved() const { return KeyHash() == 1; }
bool IsLive() const { return IsLiveHash(KeyHash()); }
static bool IsLiveHash(uint32_t aHash) { return aHash >= 2; }
void MarkFree() { *HashPtr() = 0; }
void MarkRemoved() { *HashPtr() = 1; }
void MarkColliding() { *HashPtr() |= kCollisionFlag; }
void Next(uint32_t aEntrySize) {
char* p = reinterpret_cast<char*>(mEntry);
p += aEntrySize;
mEntry = reinterpret_cast<PLDHashEntryHdr*>(p);
PLDHashNumber* HashPtr() const { return mKeyHash; }
PLDHashEntryHdr* mEntry;
PLDHashNumber* mKeyHash;
// This class maintains the invariant that every time the entry store is
// changed, the generation is updated.
// The data layout separates the cached hashes of entries and the entries
// themselves to save space. We could store the entries thusly:
// +--------+--------+---------+
// | entry0 | entry1 | ... |
// +--------+--------+---------+
// where the entries themselves contain the cached hash stored as their
// first member. PLDHashTable did this for a long time, with entries looking
// like:
// class PLDHashEntryHdr
// {
// PLDHashNumber mKeyHash;
// };
// class MyEntry : public PLDHashEntryHdr
// {
// ...
// };
// The problem with this setup is that, depending on the layout of
// `MyEntry`, there may be platform ABI-mandated padding between `mKeyHash`
// and the first member of `MyEntry`. This ABI-mandated padding is wasted
// space, and was surprisingly common, e.g. when MyEntry contained a single
// pointer on 64-bit platforms.
// As previously alluded to, the current setup stores things thusly:
// +-------+-------+-------+-------+--------+--------+---------+
// | hash0 | hash1 | ..... | hashN | entry0 | entry1 | ... |
// +-------+-------+-------+-------+--------+--------+---------+
// which contains no wasted space between the hashes themselves, and no
// wasted space between the entries themselves. malloc is guaranteed to
// return blocks of memory with at least word alignment on all of our major
// platforms. PLDHashTable mandates that the size of the hash table is
// always a power of two, so the alignment of the memory containing the
// first entry is always at least the alignment of the entire entry store.
// That means the alignment of `entry0` should be its natural alignment.
// Entries may have problems if they contain over-aligned members such as
// SIMD vector types, but this has not been a problem in practice.
// Note: It would be natural to store the generation within this class, but
// we can't do that without bloating sizeof(PLDHashTable) on 64-bit machines.
// So instead we store it outside this class, and Set() takes a pointer to it
// and ensures it is updated as necessary.
class EntryStore {
char* mEntryStore;
static char* Entries(char* aStore, uint32_t aCapacity) {
return aStore + aCapacity * sizeof(PLDHashNumber);
char* Entries(uint32_t aCapacity) const {
return Entries(Get(), aCapacity);
EntryStore() : mEntryStore(nullptr) {}
~EntryStore() {
mEntryStore = nullptr;
char* Get() const { return mEntryStore; }
bool IsAllocated() const { return !!mEntryStore; }
Slot SlotForIndex(uint32_t aIndex, uint32_t aEntrySize,
uint32_t aCapacity) const {
char* entries = Entries(aCapacity);
auto entry =
reinterpret_cast<PLDHashEntryHdr*>(entries + aIndex * aEntrySize);
auto hashes = reinterpret_cast<PLDHashNumber*>(Get());
return Slot(entry, &hashes[aIndex]);
Slot SlotForPLDHashEntry(PLDHashEntryHdr* aEntry, uint32_t aCapacity,
uint32_t aEntrySize) {
char* entries = Entries(aCapacity);
char* entry = reinterpret_cast<char*>(aEntry);
uint32_t entryOffset = entry - entries;
uint32_t slotIndex = entryOffset / aEntrySize;
return SlotForIndex(slotIndex, aEntrySize, aCapacity);
template <typename F>
void ForEachSlot(uint32_t aCapacity, uint32_t aEntrySize, F&& aFunc) {
ForEachSlot(Get(), aCapacity, aEntrySize, std::move(aFunc));
template <typename F>
static void ForEachSlot(char* aStore, uint32_t aCapacity,
uint32_t aEntrySize, F&& aFunc) {
char* entries = Entries(aStore, aCapacity);
Slot slot(reinterpret_cast<PLDHashEntryHdr*>(entries),
for (size_t i = 0; i < aCapacity; ++i) {
void Set(char* aEntryStore, uint16_t* aGeneration) {
mEntryStore = aEntryStore;
*aGeneration += 1;
// These fields are packed carefully. On 32-bit platforms,
// sizeof(PLDHashTable) is 20. On 64-bit platforms, sizeof(PLDHashTable) is
// 32; 28 bytes of data followed by 4 bytes of padding for alignment.
const PLDHashTableOps* const mOps; // Virtual operations; see below.
EntryStore mEntryStore; // (Lazy) entry storage and generation.
uint16_t mGeneration; // The storage generation.
uint8_t mHashShift; // Multiplicative hash shift.
const uint8_t mEntrySize; // Number of bytes in an entry.
uint32_t mEntryCount; // Number of entries in table.
uint32_t mRemovedCount; // Removed entry sentinels in table.
mutable Checker mChecker;
// Table capacity limit; do not exceed. The max capacity used to be 1<<23 but
// that occasionally that wasn't enough. Making it much bigger than 1<<26
// probably isn't worthwhile -- tables that big are kind of ridiculous.
// Also, the growth operation will (deliberately) fail if |capacity *
// mEntrySize| overflows a uint32_t, and mEntrySize is always at least 8
// bytes.
static const uint32_t kMaxCapacity = ((uint32_t)1 << 26);
static const uint32_t kMinCapacity = 8;
// Making this half of kMaxCapacity ensures it'll fit. Nobody should need an
// initial length anywhere nearly this large, anyway.
static const uint32_t kMaxInitialLength = kMaxCapacity / 2;
// This gives a default initial capacity of 8.
static const uint32_t kDefaultInitialLength = 4;
// Initialize the table with |aOps| and |aEntrySize|. The table's initial
// capacity is chosen such that |aLength| elements can be inserted without
// rehashing; if |aLength| is a power-of-two, this capacity will be
// |2*length|. However, because entry storage is allocated lazily, this
// initial capacity won't be relevant until the first element is added; prior
// to that the capacity will be zero.
// This will crash if |aEntrySize| and/or |aLength| are too large.
PLDHashTable(const PLDHashTableOps* aOps, uint32_t aEntrySize,
uint32_t aLength = kDefaultInitialLength);
PLDHashTable(PLDHashTable&& aOther)
// Initialize fields which are checked by the move assignment operator
// and the destructor (which the move assignment operator calls).
: mOps(nullptr), mEntryStore(), mGeneration(0), mEntrySize(0) {
*this = std::move(aOther);
PLDHashTable& operator=(PLDHashTable&& aOther);
// This should be used rarely.
const PLDHashTableOps* Ops() const { return mOps; }
// Size in entries (gross, not net of free and removed sentinels) for table.
// This can be zero if no elements have been added yet, in which case the
// entry storage will not have yet been allocated.
uint32_t Capacity() const {
return mEntryStore.IsAllocated() ? CapacityFromHashShift() : 0;
uint32_t EntrySize() const { return mEntrySize; }
uint32_t EntryCount() const { return mEntryCount; }
uint32_t Generation() const { return mGeneration; }
// To search for a |key| in |table|, call:
// entry = table.Search(key);
// If |entry| is non-null, |key| was found. If |entry| is null, key was not
// found.
PLDHashEntryHdr* Search(const void* aKey) const;
// To add an entry identified by |key| to table, call:
// entry = table.Add(key, mozilla::fallible);
// If |entry| is null upon return, then the table is severely overloaded and
// memory can't be allocated for entry storage.
// Otherwise, if the initEntry hook was provided, |entry| will be
// initialized. If the initEntry hook was not provided, the caller
// should initialize |entry| as appropriate.
PLDHashEntryHdr* Add(const void* aKey, const mozilla::fallible_t&);
// This is like the other Add() function, but infallible, and so never
// returns null.
PLDHashEntryHdr* Add(const void* aKey);
// To remove an entry identified by |key| from table, call:
// table.Remove(key);
// If |key|'s entry is found, it is cleared (via table->mOps->clearEntry).
// The table's capacity may be reduced afterwards.
void Remove(const void* aKey);
// To remove an entry found by a prior search, call:
// table.RemoveEntry(entry);
// The entry, which must be present and in use, is cleared (via
// table->mOps->clearEntry). The table's capacity may be reduced afterwards.
void RemoveEntry(PLDHashEntryHdr* aEntry);
// Remove an entry already accessed via Search() or Add().
// NB: this is a "raw" or low-level method. It does not shrink the table if
// it is underloaded. Don't use it unless necessary and you know what you are
// doing, and if so, please explain in a comment why it is necessary instead
// of RemoveEntry().
void RawRemove(PLDHashEntryHdr* aEntry);
// This function is equivalent to
// ClearAndPrepareForLength(kDefaultInitialLength).
void Clear();
// This function clears the table's contents and frees its entry storage,
// leaving it in a empty state ready to be used again. Afterwards, when the
// first element is added the entry storage that gets allocated will have a
// capacity large enough to fit |aLength| elements without rehashing.
// It's conceptually the same as calling the destructor and then re-calling
// the constructor with the original |aOps| and |aEntrySize| arguments, and
// a new |aLength| argument.
void ClearAndPrepareForLength(uint32_t aLength);
// Measure the size of the table's entry storage. If the entries contain
// pointers to other heap blocks, you have to iterate over the table and
// measure those separately; hence the "Shallow" prefix.
size_t ShallowSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
// Like ShallowSizeOfExcludingThis(), but includes sizeof(*this).
size_t ShallowSizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
// Mark a table as immutable for the remainder of its lifetime. This
// changes the implementation from asserting one set of invariants to
// asserting a different set.
void MarkImmutable() {
// If you use PLDHashEntryStub or a subclass of it as your entry struct, and
// if your entries move via memcpy and clear via memset(0), you can use these
// stub operations.
static const PLDHashTableOps* StubOps();
// The individual stub operations in StubOps().
static PLDHashNumber HashVoidPtrKeyStub(const void* aKey);
static bool MatchEntryStub(const PLDHashEntryHdr* aEntry, const void* aKey);
static void MoveEntryStub(PLDHashTable* aTable, const PLDHashEntryHdr* aFrom,
PLDHashEntryHdr* aTo);
static void ClearEntryStub(PLDHashTable* aTable, PLDHashEntryHdr* aEntry);
// Hash/match operations for tables holding C strings.
static PLDHashNumber HashStringKey(const void* aKey);
static bool MatchStringKey(const PLDHashEntryHdr* aEntry, const void* aKey);
class EntryHandle {
EntryHandle(EntryHandle&& aOther) noexcept;
EntryHandle(const EntryHandle&) = delete;
EntryHandle& operator=(const EntryHandle&) = delete;
EntryHandle& operator=(EntryHandle&& aOther) = delete;
// Is this slot currently occupied?
bool HasEntry() const { return mSlot.IsLive(); }
explicit operator bool() const { return HasEntry(); }
// Get the entry stored in this slot. May not be called unless the slot is
// currently occupied.
PLDHashEntryHdr* Entry() {
return mSlot.ToEntry();
template <class F>
void Insert(F&& aInitEntry) {
// If the slot is currently vacant, the slot is occupied and `initEntry` is
// invoked to initialize the entry. Returns the entry stored in now-occupied
// slot.
template <class F>
PLDHashEntryHdr* OrInsert(F&& aInitEntry) {
if (!HasEntry()) {
return Entry();
/** Removes the entry. Note that the table won't shrink on destruction of
* the EntryHandle.
* \pre HasEntry()
* \post !HasEntry()
void Remove();
/** Removes the entry, if it exists. Note that the table won't shrink on
* destruction of the EntryHandle.
* \post !HasEntry()
void OrRemove();
friend class PLDHashTable;
EntryHandle(PLDHashTable* aTable, PLDHashNumber aKeyHash, Slot aSlot);
void OccupySlot();
PLDHashTable* mTable;
PLDHashNumber mKeyHash;
Slot mSlot;
template <class F>
auto WithEntryHandle(const void* aKey, F&& aFunc)
-> std::invoke_result_t<F, EntryHandle&&> {
return std::forward<F>(aFunc)(MakeEntryHandle(aKey));
template <class F>
auto WithEntryHandle(const void* aKey, const mozilla::fallible_t& aFallible,
F&& aFunc)
-> std::invoke_result_t<F, mozilla::Maybe<EntryHandle>&&> {
return std::forward<F>(aFunc)(MakeEntryHandle(aKey, aFallible));
// This is an iterator for PLDHashtable. Assertions will detect some, but not
// all, mid-iteration table modifications that might invalidate (e.g.
// reallocate) the entry storage.
// Any element can be removed during iteration using Remove(). If any
// elements are removed, the table may be resized once iteration ends.
// Example usage:
// for (auto iter = table.Iter(); !iter.Done(); iter.Next()) {
// auto entry = static_cast<FooEntry*>(iter.Get());
// // ... do stuff with |entry| ...
// // ... possibly call iter.Remove() once ...
// }
// or:
// for (PLDHashTable::Iterator iter(&table); !iter.Done(); iter.Next()) {
// auto entry = static_cast<FooEntry*>(iter.Get());
// // ... do stuff with |entry| ...
// // ... possibly call iter.Remove() once ...
// }
// The latter form is more verbose but is easier to work with when
// making subclasses of Iterator.
class Iterator {
explicit Iterator(PLDHashTable* aTable);
struct EndIteratorTag {};
Iterator(PLDHashTable* aTable, EndIteratorTag aTag);
Iterator(Iterator&& aOther);
// Have we finished?
bool Done() const { return mNexts == mNextsLimit; }
// Get the current entry.
PLDHashEntryHdr* Get() const {
return mCurrent.ToEntry();
// Advance to the next entry.
void Next();
// Remove the current entry. Must only be called once per entry, and Get()
// must not be called on that entry afterwards.
void Remove();
bool operator==(const Iterator& aOther) const {
MOZ_ASSERT(mTable == aOther.mTable);
return mNexts == aOther.mNexts;
Iterator Clone() const { return {*this}; }
PLDHashTable* mTable; // Main table pointer.
Slot mCurrent; // Pointer to the current entry.
uint32_t mNexts; // Number of Next() calls.
uint32_t mNextsLimit; // Next() call limit.
bool mHaveRemoved; // Have any elements been removed?
uint8_t mEntrySize; // Size of entries.
bool IsOnNonLiveEntry() const;
void MoveToNextLiveEntry();
Iterator() = delete;
Iterator(const Iterator&);
Iterator& operator=(const Iterator&) = delete;
Iterator& operator=(const Iterator&&) = delete;
Iterator Iter() { return Iterator(this); }
// Use this if you need to initialize an Iterator in a const method. If you
// use this case, you should not call Remove() on the iterator.
Iterator ConstIter() const {
return Iterator(const_cast<PLDHashTable*>(this));
static uint32_t HashShift(uint32_t aEntrySize, uint32_t aLength);
static const PLDHashNumber kCollisionFlag = 1;
PLDHashNumber Hash1(PLDHashNumber aHash0) const;
void Hash2(PLDHashNumber aHash, uint32_t& aHash2Out,
uint32_t& aSizeMaskOut) const;
static bool MatchSlotKeyhash(Slot& aSlot, const PLDHashNumber aHash);
Slot SlotForIndex(uint32_t aIndex) const;
// We store mHashShift rather than sizeLog2 to optimize the collision-free
// case in SearchTable.
uint32_t CapacityFromHashShift() const {
return ((uint32_t)1 << (kPLDHashNumberBits - mHashShift));
PLDHashNumber ComputeKeyHash(const void* aKey) const;
enum SearchReason { ForSearchOrRemove, ForAdd };
// Avoid using bare `Success` and `Failure`, as those names are commonly
// defined as macros.
template <SearchReason Reason, typename PLDSuccess, typename PLDFailure>
auto SearchTable(const void* aKey, PLDHashNumber aKeyHash,
PLDSuccess&& aSucess, PLDFailure&& aFailure) const;
Slot FindFreeSlot(PLDHashNumber aKeyHash) const;
bool ChangeTable(int aDeltaLog2);
void RawRemove(Slot& aSlot);
void ShrinkIfAppropriate();
mozilla::Maybe<EntryHandle> MakeEntryHandle(const void* aKey,
const mozilla::fallible_t&);
EntryHandle MakeEntryHandle(const void* aKey);
PLDHashTable(const PLDHashTable& aOther) = delete;
PLDHashTable& operator=(const PLDHashTable& aOther) = delete;
// Compute the hash code for a given key to be looked up, added, or removed.
// A hash code may have any PLDHashNumber value.
typedef PLDHashNumber (*PLDHashHashKey)(const void* aKey);
// Compare the key identifying aEntry with the provided key parameter. Return
// true if keys match, false otherwise.
typedef bool (*PLDHashMatchEntry)(const PLDHashEntryHdr* aEntry,
const void* aKey);
// Copy the data starting at aFrom to the new entry storage at aTo. Do not add
// reference counts for any strong references in the entry, however, as this
// is a "move" operation: the old entry storage at from will be freed without
// any reference-decrementing callback shortly.
typedef void (*PLDHashMoveEntry)(PLDHashTable* aTable,
const PLDHashEntryHdr* aFrom,
PLDHashEntryHdr* aTo);
// Clear the entry and drop any strong references it holds. This callback is
// invoked by Remove(), but only if the given key is found in the table.
typedef void (*PLDHashClearEntry)(PLDHashTable* aTable,
PLDHashEntryHdr* aEntry);
// Initialize a new entry. This function is called when
// Add() finds no existing entry for the given key, and must add a new one.
typedef void (*PLDHashInitEntry)(PLDHashEntryHdr* aEntry, const void* aKey);
// Finally, the "vtable" structure for PLDHashTable. The first four hooks
// must be provided by implementations; they're called unconditionally by the
// generic PLDHashTable.cpp code. Hooks after these may be null.
// Summary of allocation-related hook usage with C++ placement new emphasis:
// initEntry Call placement new using default key-based ctor.
// moveEntry Call placement new using copy ctor, run dtor on old
// entry storage.
// clearEntry Run dtor on entry.
// Note the reason why initEntry is optional: the default hooks (stubs) clear
// entry storage. On a successful Add(tbl, key), the returned entry pointer
// addresses an entry struct whose entry members are still clear (null). Add()
// callers can test such members to see whether the entry was newly created by
// the Add() call that just succeeded. If placement new or similar
// initialization is required, define an |initEntry| hook. Of course, the
// |clearEntry| hook must zero or null appropriately.
// XXX assumes 0 is null for pointer types.
struct PLDHashTableOps {
// Mandatory hooks. All implementations must provide these.
PLDHashHashKey hashKey;
PLDHashMatchEntry matchEntry;
PLDHashMoveEntry moveEntry;
// Optional hooks start here. If null, these are not called.
PLDHashClearEntry clearEntry;
PLDHashInitEntry initEntry;
// A minimal entry is a subclass of PLDHashEntryHdr and has a void* key pointer.
struct PLDHashEntryStub : public PLDHashEntryHdr {
const void* key;
#endif /* PLDHashTable_h */