# Implementation classes¶

## Requirements¶

An implementation class in xtensor is a final class that models a specific kind of expression. It must inherit (either directly or indirectly) from xexpression and define (or inherit from classes that define) the following types:

container types

value_type;
reference;
const_reference;
pointer;
const_pointer;
size_type;
difference_type;
shape_type;


iterator types

iterator;
const_iterator;
reverse_iterator;
const_reverse_iterator;

template <class S, layout_type L>
template <class S, layout_type L>
template <class S, layout_type L>
template <class S, layout_type L>

storage_iterator;
const_storage_iterator;
reverse_storage_iterator;
const_reverse_storage_iterator;


layout data

static layout_type static_layout;
static bool contiguous_layout;


It must also provide the following methods, either by defining them itself, or by inheriting from classes that define them, partially or totally:

shape methods

size_type size() const noexcept;
size_type dimension() const noexcept;
const inner_shpae_type& shape() const noexcept;


template <class S>

template <class S>


data access methods

template <class... Args>
const_reference operator()(Args... args) const;

template <class... Args>
const_reference at(Args... args) const;

template <class S>
disable_integral_t<S, const_reference> operator[](const S& index) const;

template <class I>
const_reference operator[](std::initialize_list<I> index) const;

template <class It>
const_reference element(It first, It last) const;

const storage_type& storage() const;


iteration methods

These methods are usually provided by inheriting from xconst_iterable or xiterable. See Iterating over expressions for more details.

If the expression is mutable, it must also define the non-const counterparts of the data access methods, and inherits from a semantic class to provide assignment operators.

## xarray and xtensor¶

Although they represent different concepts, xarray and xtensor have really similar implementations so only xarray will be covered.

xarray is a strided array expression that can be assigned to. Everything xarray needs is already defined in classes modeling Concepts, so xarray only has to inherit from these classes and define constructors and assignment operators:

Besides implementing the methods that define value semantic, xarray and xtensor hold the data container. Since the xcontainer base class implements all the logic for accessing the data, it must me able to access the data container. This is achieved by requiring that every class inheriting from xcontainer provides the following methods:

storage_type& storage_impl() noexcept;
const storage_type& storage_impl() const noexcept;


These are the implementation methods of the storage() interface methods defined in xcontainer, and thus are defined in the private section of xarray and xtensor. In order to grant access to xcontainer, this last one is declared as friend:

template <class EC, layout_type L, class SC, class Tag>
class xarray : public xstrided_container<xarray<EC, L, SC, Tag>,
public xcontainer_semantic<xarray<EC, L, SC, Tag>>
{
public:

// ....

private:

storage_type m_storage;
storage_type& storage() noexcept;
const storage_type& storage() const noexcept;

friend class xcontainer<xarray<EC, L, SC, Tag>>;
};


This pattern is similar to the template method pattern used in hierarchy of classes with entity semantic (see virtuality).

## Inner types definition¶

Although the base classes use the types defined in the Requirement section, they cannot define them; first because different base classes may need the same types and we want to avoid duplication of type definitions. The second reason is that most of the types may rely on other types specific to the implementation classes. For instance, value_type, reference, etc, of xarray are simply the types defined in the container type hold by xarray:

using value_type = typename storage_type::value_type;
using reference = typename storage_type::reference;
using const_reference = typename storage_type::const_reference;
...


Moreover, CRTP base classes cannot access inner types defined in CRTP leaf classes, because a CRTP leaf class is only declared, not defined, when the CRTP base class is being defined.

The solution is to define those types in an external structure that is specialized for each CRTP leaf class:

// Declaration only, no generic definition
template <class C>
struct xcontainer_inner_types;


In xarray.hpp

template <class EC, layout_type L, class SC, class Tag>
struct xcontainer_inner_types<xarray<EC, L,SC, Tag>>
{
// Definition of types required by CRTP bases
};


In order to avoid a lot of boilerplate, the CRTP base classes expect only a few types to be defined in this structure, and then compute the other types, based on these former definitions. The requirements on types definition regarding the base classes is detailed below.

xsemantic

The semantic classes only expect the following type: temporary_type.

xcontainer

xcontainer and xstrided_container expect the following types to be defined:

storage_type;
shape_type;
strides_type;
backstrides_type;
inner_shape_type;
inner_strides_type;
inner_backstrides_type;
layout_type;


xiterable

Since many expressions are not containers, the definition of types required by the iterable concept is done in a dedicated structure following the same pattern as xcontainer_inner_types, i.e. a sturcture declared and specialized for each final class:

template <class C>
struct xiterable_inner_types;


The following types must be defined in each specialization:

inner_shape_type;
const_stepper;
stepper;


More detail about the stepper types is given in Iterating over expressions.