array

Note: This project is currently WIP, no guarantees are made until an 0.1 release.

This library is all about arrays — contiguous blocks of memory. It basically provides a customizable std::vector<T> and containers built on top of it like flat sets and maps.

The equivalent of std::vector<T> — array<T> — does not take an Allocator. Instead, it takes a BlockStorage. This policy is responsible for completely managing the underlying memory block: It stores the current address and size and has full control over reserve() and shrink_to_fit(). This makes it possible to have a fixed sized array, an array with small buffer optimization and much more just by swapping the policy.

Features

Block Storage Implementations

• block_storage_embedded: uses a member array of char for memory allocations

• block_storage_heap<Heap, GrowthPolicy>: dynamic memory allocation. The Heap controls how memory is allocated — policy with allocate() and deallocate(), the GrowthPolicy how much memory is allocated.

  Heaps:

  • new_heap: uses ::operator new
  • allocator_heap<Allocator>: uses the given Allocator for allocation

  GrowthPolicys:

  • no_extra_growth: only allocates the minimum amount of memory necessary
  • factor_growth<Num, Den>: grows by factor Num / Den

  block_storage_new<GrowthPolicy> is a convenience typedef, uses the new_heap and a custom GrowthPolicy.

• block_storage_sbo: first uses block_storage_embedded, then another BlockStorage (small buffer optimization)

• block_storage_heap_sbo: alias for block_storage_sbo that uses the given Heap for allocation

• block_storage_default: the default BlockStorage (block_storage_new right now)

Containers

• array<T>: the std::vector<T> of this library
• small_array<T>: convenience alias for array<T> with SBO
• bag<T>: an array<T> where order of elements isn't important, allows an O(1) erase
• small_bag<T>: convenience alias for bag<T> with SBO
• variant_bag<BlockStorage, T1, T2, ...>: an SOA optimized bag<std::variant<T1, T2, ...>>
• flat_(multi)set<Key>: a sorted array<Key> with O(log n) lookup & co plus a superior interface to std::set
• flat_(multi)map<Key, Value>: a flat_set<Key> and an array<Value> for key-value-storage, again with superior interface compared to std::map

Views

• block_view<T>: a pointer + size pair that doesn't provide indexing (bag<T> provides only a block_view<T>)
• array_view<T>: a block_view<T> with indexing
• sorted_view<T, Compare>: an array_view<T> that is sorted according to Compare
• input_view<T, BlockStorage>: similar to std::initializer_list but allows stealing the memory
• byte_view<T>(): converts an array_view<T> into an array_view<std:byte> to allow byte-wise interpretation of the memory

Misc

• low-level memory manipulation utilities and algorithms
• pointer_iterator<Tag, T> utility to create distinct iterator types on top of pointers
• ContiguousIterator facilities

FAQ

Q: There are too many containers/policies, which do I use?

A: Take a look at the decision tree.

Q: Does size() return a signed or unsigned?

A: Unsigned, but I might change my mind.

Q: Is array_view<T> mutable or immutable?

A: Mutable because I don't have a better word right now.

Q: Maybe something like array_ref<T>?

A: FAQs are not really suited for bike shedding.

Q: It breaks when I do this!

A: Don't do that. And file an issue (or a PR, I have too many other projects...).

Q: This is awesome!

A: Thanks. I do have a Patreon page, so consider checking it out:

Documentation

Detailed reference documentation is WIP.

Building

Header-only (almost everything is a template anyway), no dependencies (currently).

It requires at least C++11, but works better with C++14 or 17. Compilers that are being tested on CI:

• Linux:
  • GCC 4.9 to 8
  • clang 3.9 to 7
• MacOS:
  • XCode 8, 9 and 10
• Windows:
  • Visual Studio 2015 and 2017

Newer compilers should work too.

BlockStorage concept

The core concept of this library is the BlockStorage — the type that controls memory block allocation:

class BlockStorage
{
public:
    /// Whether or not the block storage may embed some objects inside.
    /// If this is true, the move and swap operations must actually move objects.
    /// If this is false, they will never physically move the objects.
    ///
    /// It is optional: if it is not provided, it defaults to [std::false_type]().
    using embedded_storage = std::integral_constant<bool, ...>;

    /// The arguments required to create the block storage.
    ///
    /// These can be runtime parameters or references to allocators.
    /// It must be a type that is nothrow (and cheaply) copyable.
    ///
    /// It is optional: If it is not provided, the empty type [array::default_argument_type]() is used.
    struct argument_type;

    //=== constructors/destructors ===//
    /// \effects Creates a block storage with the maximal block size possible without dynamic allocation.
    /// \notes If there is not `argument_type` typedef, it must take [array::default_argument_type]() directly.
    explicit BlockStorage(argument_type arg) noexcept;

    BlockStorage(const BlockStorage&) = delete;
    BlockStorage& operator=(const BlockStorage&) = delete;

    /// Releases the memory of the block, if it is not empty.
    /// \notes It does not need to destroy any elements.
    ~BlockStorage() noexcept;

    /// Swap.
    /// \effects Exchanges ownership over the allocated memory blocks.
    /// When possible this is done without moving the already constructed objects.
    /// If that is not possible, they shall be moved to the beginning of the new location
    /// as if [array::uninitialized_destructive_move]() was used.
    /// The views are updated to view the new location of the constructed objects, if necessary.
    /// \throws Anything thrown by `T`s copy or move constructor.
    /// \notes This function must not allocate dynamic memory.
    template <typename T>
    static void swap(BlockStorage& lhs, block_view<T>& lhs_constructed,
                     BlockStorage& rhs, block_view<T>& rhs_constructed)
                        noexcept(!embedded_storage::value || std::is_nothrow_move_constructible<T>::value);

    //=== reserve/shrink_to_fit ===//
    /// \effects Increases the allocated memory block by at least `min_additional_bytes`.
    /// The range of already created objects is passed as well,
    /// they shall be moved to the beginning of the new location
    /// as if [array::uninitialized_destructive_move]() was used.
    /// \throws Anything thrown by the allocation function, or the copy/move constructor of `T`.
    /// If an exception is thrown, nothing must have changed.
    /// \notes Use [array::uninitialized_destructive_move]() to move the objects over,
    /// it already provides the strong exception safety for you.
    template <typename T>
    void reserve(size_type min_additional_bytes, const block_view<T>& constructed_objects);

    /// \effects Non-binding request to decrease the currently allocated memory block to the minimum needed.
    /// The range of already created objects is passed, those must be moved to the new location like with `reserve()`.
    /// \throws Anything thrown by the allocation function, or the copy/move constructor of `T`.
    /// If an exception is thrown, nothing must have changed.
    /// \notes Use [array::uninitialized_destructive_move]() to move the objects over,
    /// it already provides the strong exception safety for you.
    template <typename T>
    void shrink_to_fit(const block_view<T>& constructed_objects);

    //=== accessors ===//
    /// \returns The currently allocated memory block.
    memory_block block() const noexcept;

    /// \returns The arguments passed to the constructor.
    ///
    /// This is optional: if `argument_type` isn't provided, it can be left out.
    /// If `argument_type` is provided, it is not optional.
    argument_type argument() const noexcept;

    /// \returns The maximum size of a memory block managed by a storage created with the given arguments,
    /// or `memory_block::max_size()` if there is no limitation by the storage itself.
    ///
    /// This function is optional: if it isn't provided, `memory_block::max_size()` is used instead.
    /// \param 0
    /// This parameter can be left out if the information doesn't depend on any arguments.
    static size_type max_size(argument_type) noexcept;
};

Making some of the member functions optional is planned.

You can plug it into any container type of this library and fully control it.

Customizing only Allocation

If you just want to change the memory allocation and not the reserve policy, you can just use the Heap and GrowthPolicy concepts of block_storage_heap.

Heap is a simple Allocator concept:

struct Heap
{
    /// The handle for that particular heap.
    /// It must be cheaply and nothrow copyable.
    struct handle_type;

    /// Allocates a memory block of the given size and alignment or throws an exception if it is unable to do so.
    /// Doesn't need to handle size `0`.
    static memory_block allocate(handle_type& handle, size_type size, size_type alignment);

    /// Deallocates a memory block.
    /// Doesn't need to handle empty blocks.
    static void deallocate(handle_type& handle, memory_block&& block) noexcept;

    /// Returns the maximum size of a memory block, or [array::memory_block::max_size()]() if it isn't limited by the allocator.
    ///
    /// This function is optional: if it isn't provided, `memory_block::max_size()` is returned.
    static size_type max_size(const handle_type& handle) noexcept;
};

It is implemented by new_heap, for example, which simply forwards to new and delete.

The GrowthPolicy controls the growth factor of reserve() and shrink_to_fit():

struct GrowthPolicy
{
    /// \returns The new size of the memory block based on the current size,
    /// the minimal size it needs to increase, and the maximum size supported by the heap.
    /// \requires It must return at least `cur_size + additional_needed`.
    static size_type growth_size(size_type cur_size, size_type additional_needed,
                                 size_type max_size) noexcept;

    /// \returns The new size of the  memory block based on the current size,
    /// the minimal total size required, and the maximum size supported by the heap.
    /// \requires It must return at least `size_needed`.
    static size_type shrink_size(size_type cur_size, size_type size_needed,
                                 size_type max_size) noexcept;
};

You probably don't need to write a GrowthPolicy yourself as the library provides no_extra_growth and factor_growth<Num, Den>.

The two policy combined are used in block_storage_heap<Heap, GrowthPolicy>. If you use block_storage_new<default_growth> (which is block_storage_heap<new_heap, default_growth>), you have the behavior std::vector has today.

Small Buffer Optimization

If you want a small buffer optimization, simply combine your big buffer policy of choice with block_storage_sbo<SmallBuffer, BigBlockStorage>.

Using the Array

array<T> is this library's std::vector<T> but with a customizable BlockStorage. The interface is similarly enough so you don't need a tutorial, but note that it is not a drop-in replacement.

It also has some nice additional stuff like append_range() or view support (see below).

bag<T> is like array<T> but doesn't provide an index operator, as the position of elements is not guaranteed. As such it can have a set-like interface with just insert(element), but doesn't provide lookup. It is used if you just want a collection of elements and later need to iterator over them in any order, for example. Losing ordering guarantees means it can provide an O(1) erase(iter) (it swaps with the last element and does a pop_back()).

I heavily suggest seeing whether it is applicable for your use case.

Using the Set and Map

If you use flat_set or flat_map you have to provide a comparison predicate. This is not something like std::less but instead a three way comparison modelling KeyCompare:

/// The ordering of a key in relation to some other value - provided by the library.
enum class key_ordering
{
    less,       //< Other value is less than the key, i.e. sorted before it.
    equivalent, //< Other value is equivalent to the key, i.e. a duplicate.
    greater,    //< Other value is greater than key, i.e. sorted after it.
};

struct KeyCompare
{
    /// Compares the key with some other type.
    ///
    /// It must define a strict total ordering of the keys.
    /// `TransparentKey` may be restricted to certain types, or just the key type itself.
    template <typename Key, typename TransparentKey>
    static key_ordering compare(const Key& key, const TransparentKey& other) noexcept;
};

key_compare_default works for all types that provide a .compare() member function or a less-than operator, but it can also be specialized for your own types:

template <>
struct key_compare_default::customize_for<my_type>
{
    static key_ordering compare(const my_type& key, const my_type& other) noexcept
    {
        return ...;
    }

    // can be compared with strings which is less expensive than constructing the object first
    static key_ordering compare(const my_type& key, std::string_view other) noexcept
    {
        return ...;
    }
};

This should only be done if you want to put a type in a set that doesn't provide a compare function. If you want to change the order of elements (e.g. reverse them for some reason), provide a different KeyCompare altogether.

flat_set<T> stores the keys as one sorted array and provides an interface similar to std::set, but with more functionality (it has a .contains(), for example)!

If you want to have a mapping between keys and values, use flat_map<Key, Value>. Keys and values are stored in separate arrays linked implicitly by having the same index. This is the proposed design of std::flat_map as well.

The interface is similar to it as well, but better: It does not use std::pair, so no more insert(...).first->second, instead you have .iter()->value, for example. It also provides separate iterator over just the keys or just the values.

If you want to store the keys and values together in memory (maybe because the value is small), use flat_set<key_value_pair<Key, Value>>. You lose a little bit of convenience — there is no insert_or_assign() and lookup() gives you a key value pair as result (but only needs a key as input!), but might gain the extra performance.

The multi- variants behave just like you would expect.

Using the Block Views

The library provides a hierarchy of view types, i.e. pointer plus size pairs. They can be used just like std::string_view but for arbitrary types and are mutable. All containers can be constructed and convert to a matching view:

• block_view<T> — the most basic view, doesn't provide array access. bag<T> gives you a block_view<T>.

• array_view<T> — a block_view<T> with array access. array<T> gives you an array_view<T>.

• sorted_view<T, Compare> — an array_view<T> that is sorted. flat_set<Key> gives you a sorted_view<T, Compare>.

The views are essential because two array<T> with different BlockStorage policies are different types. So the containers are not really suited for use in interfaces, just as implementation details. Interfaces taking non-owning data should use the views instead.

For interfaces taking ownership, the special view input_view<T, BlockStorage> can be used instead. It can be used to create a container as efficient as possible. It either copies all elements, moves all elements, or takes ownership over a suitable memory block. This allows moving memory between different containers using the same BlockStorage.

Planned Features

Block Storage Implementations

• pmr_heap: uses a std::pmr::memory_resource for the allocation, for use with block_storage_heap
• A BlockStorage using an external fixed sized buffer
• A BlockStorage (adapter) that limits the maximum size

Containers

• unsized_array<T>: an array<T> that doesn't know its size, low-level building block
• ring_buffer<T>: a ring buffer of elements
• multi dimensional stuff?
• hash table?

Misc

• Better docs
• Better tests, switch to doc test for templated test cases?
• Better (and documented) exception safety
• Some unified precondition checking
• Simplifying BlockStorage concepts
• Destructive move support
• Stable pointer that don't get invalidated on reserve (i.e. indices)