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> broadcast_iterator<S, L>; template <class S, layout_type L> const_broadcast_iterator<S, L>; template <class S, layout_type L> reverse_broadcast_iterator<S, L>; template <class S, layout_type L> const_reverse_broadcast_iterator<S, 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_shape_type& shape() const noexcept;
broadcasting methods
template <class S> bool broadcast_shape(const S& shape) const;
data access methods
template <class... Args> const_reference operator()(Args... args) const; template <class... Args> const_reference at(Args... args) const; template <class... Args> const_reference unchecked(Args... args) const; template <class S> disable_integral_t<S, const_reference> operator[](const S& index) const; template <class I> const_reference operator[](std::initializer_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 :ref:`iterating-expression-label` 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.
xtensor provides the following expression classes:
Containers
xarray_container
: N-dimensional array with dynamic shapexarray_adaptor
: N-dimensional array adaptor for STL-like containers or C arraysxtensor_container
: N-dimensional array with static number of dimensionsxtensor_adaptor
: N-dimensional tensor adaptor for STL-like containers or C arraysxfixed_container
: N-dimensional array with static shapexfixed_adaptor
: N-dimensoinal fixed tensor adaptor for STL-like containers or C arrays
Most of the methods of these classes are implemented in the base class xcontainer
, the
inheriting classes only provide constructors and assignment operators for the value semantic.
The container classes are generally used through type aliases which set many of the template arguments:
xarray
xtensor
xfixed_tensor
The classes for adaptors can be instantiated through the many overloads of xt::adapt
function,
so that their templates parameters are deduced.
Scalar
xtensor provides the xscalar
class to adapt scalar values and give them the required API.
Optional containers
xoptional_assembly
: N-dimensional array holding optional values.xoptional_assembly_adaptor
: N-dimensional adaptor holding optional values.
Most of the mehtods of these classes are defined in their base class xoptional_assembly_base
.
Views
xview
: N-dimensional view with static number of slices, supporting all kind of slicesxstrided_view
: N-dimensional view with dynamic number of slices, supporting strided slices only (see below)xdynamic_view
: N-dimensional view with dynamic number of slices, supporting all kind of slicesxfunctor_view
: N-dimensional view applying a functor to its underlying elements (e.g.imag
,real
)xindex_view
: Flat (1D) view yielding the values at the indices of its index arrayxmasked_view
: View on optional expression hiding values depending on a mask
When the index of an element in the underlying expression of a view can be computed thanks to a strided scheme, the slice used in this view is said to be a strided slice. xtensor provides the following strided slices:
xrange
xstepped_range
xall
xnewaxis
The following slices are not strided, and thus incompatible with xstrided_view
:
xkeep_slice
xdrop_slice
Functional expressions
Contrary to containers and views, the functional expressions are immutable.
xbroadcast
: Broadcasts an expression to a specific shapexfunction
: N-dimensional function operating on tensor expressionsxgenerator
: N-dimensional function operating on indicesxreducer
: Reducing function operating over specified axes
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 :ref:`concepts-label`, 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).
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 :ref:`iterating-expression-label`.