XPCOM Collections

`nsTArray` and `AutoTArray`

nsTArray<T> is a typesafe array for holding various objects, similar to std::vector<T>. (note that nsTArray<T> is dynamically-sized, unlike std::array<T>) Here’s an incomplete list of mappings between the two:

std::vector<T>	nsTArray<T>
`size()`	`Length()`
`empty()`	`IsEmpty()`
`resize()`	`SetLength()` or `SetLengthAndRetainStorage()`
`capacity()`	`Capacity()`
`reserve()`	`SetCapacity()`
`push_back()`	`AppendElement()`
`insert()`	`AppendElements()`
`emplace_back()`	`EmplaceBack()`
`clear()`	`Clear()` or `ClearAndRetainStorage()`
`data()`	`Elements()`
`at()`	`ElementAt()`
`back()`	`LastElement()`

Rust Bindings

When the thin_vec crate is built in Gecko, thin_vec::ThinVec<T> is guaranteed to have the same memory layout and allocation strategy as nsTArray, meaning that the two types may be used interchangeably across FFI boundaries. The type is not safe to pass by-value over FFI boundaries, due to Rust and C++ differing in when they run destructors.

The element type T must be memory-compatible with both Rust and C++ code to use over FFI.

`nsTHashMap` and `nsTHashSet`

These types are the recommended interface for writing new XPCOM hashmaps and hashsets in XPCOM code.

Supported Hash Keys

The following types are supported as the key parameter to nsTHashMap and nsTHashSet.

Type	Hash Key
`T*`	`nsPtrHashKey<T>`
`T*`	`nsPtrHashKey<T>`
`nsCString`	`nsCStringHashKey`
`nsString`	`nsStringHashKey`
`uint32_t`	`nsUint32HashKey`
`uint64_t`	`nsUint64HashKey`
`intptr_t`	`IntPtrHashKey`
`nsCOMPtr<nsISupports>`	`nsISupportsHashKey`
`RefPtr<T>`	`nsRefPtrHashKey<T>`
`nsID`	`nsIDHashKey`

Any key not in this list must inherit from the PLDHashEntryHdr class to implement manual hashing behaviour.

Class Reference

Note

The nsTHashMap and nsTHashSet types are not declared exactly like this in code. This is intended largely as a practical reference.

template<K, V> class nsTHashMap<K, V>

The nsTHashMap<K, V> class is currently defined as a thin type alias around nsBaseHashtable. See the methods defined on that class for more detailed documentation.

https://searchfox.org/mozilla-central/source/xpcom/ds/nsBaseHashtable.h

uint32_t Count() const

bool IsEmpty() const

bool Get(KeyType aKey, V *aData) const: Get the value, returning a flag indicating the presence of the entry in the table.

V Get(KeyType aKey) const: Get the value, returning a default-initialized object if the entry is not present in the table.

Maybe<V> MaybeGet(KeyType aKey) const: Get the value, returning Nothing if the entry is not present in the table.

V &LookupOrInsert(KeyType aKey, Args&&... aArgs) const

V &LookupOrInsertWith(KeyType aKey, F &&aFunc) const

template<K> class nsTHashSet<K>

The nsTHashSet<K> class is currently defined as a thin type alias around nsTBaseHashSet. See the methods defined on that class for more detailed documentation.

https://searchfox.org/mozilla-central/source/xpcom/ds/nsTHashSet.h