# std::mdspan<T,Extents,LayoutPolicy,AccessorPolicy>::mdspan

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< cpp‎ | container‎ | mdspan

constexpr mdspan(); | (1) | (since C++23)
template< class... OtherIndexTypes > constexpr explicit mdspan( data_handle_type p, OtherIndexTypes... exts ); | (2) | (since C++23)
template< class OtherIndexType, std::size_t N > constexpr explicit(N != rank_dynamic()) mdspan( data_handle_type p, std::span<OtherIndexType, N> exts ); | (3) | (since C++23)
template< class OtherIndexType, std::size_t N > constexpr explicit(N != rank_dynamic()) mdspan( data_handle_type p, const std::array<OtherIndexType, N>& exts ); | (4) | (since C++23)
constexpr mdspan( data_handle_type p, const extents_type& ext ); | (5) | (since C++23)
constexpr mdspan( data_handle_type p, const mapping_type& m ); | (6) | (since C++23)
constexpr mdspan( data_handle_type p, const mapping_type& m, const accessor_type& a ); | (7) | (since C++23)
template< class OtherElementType, class OtherExtents, class OtherLayoutPolicy, class OtherAccessor > constexpr explicit(/* see below */) mdspan( const mdspan<OtherElementType, OtherExtents, OtherLayoutPolicy, OtherAccessor>& other ); | (8) | (since C++23)
constexpr mdspan( const mdspan& rhs ) = default; | (9) | (since C++23)
constexpr mdspan( mdspan&& rhs ) = default; | (10) | (since C++23)

Construct an `mdspan`.

1) Construct an empty mdspan. Value-initializes `ptr_`, `map_`, and `acc_`.

- The behavior is undefined if `[`​0​`,` map_.required_span_size()`)` is not an accessible range of `ptr_` and `acc_` for the values of `map_` and `acc_` after the invocation of this constructor.
- This overload participates in overload resolution only if

  - rank_dynamic() > 0 is true,
  - std::is_default_constructible_v<data_handle_type> is true,
  - std::is_default_constructible_v<mapping_type> is true, and
  - std::is_default_constructible_v<accessor_type> is true.

2) Construct an mdspan over the underlying data referred by p with extents represented by `exts...`. Value-initializes `acc_`, direct-non-list-initializes `ptr_` with std::move(p) and `map_` with extents_type(static_cast<index_type>(std::move(exts))...).

- The behavior is undefined if `[`​0​`,` map_.required_span_size()`)` is not an accessible range of `ptr_` and `acc_` for the values of `map_` and `acc_` after the invocation of this constructor.
- Let N be sizeof...(OtherIndexTypes). This overload participates in overload resolution only if

  - (std::is_convertible_v<OtherIndexTypes, index_type> && ...) is true,
  - (std::is_nothrow_constructible<index_type, OtherIndexTypes> && ...) is true,
  - N == rank() || N == rank_dynamic() is true,
  - std::is_constructible_v<mapping_type, extents_type> is true, and
  - std::is_default_constructible_v<accessor_type> is true.

3,4) Construct an mdspan over the underlying data referred by p with extents represented by pack exts. Value-initializes `acc_`, direct-non-list-initializes `ptr_` with std::move(p) and `map_` with extents_type(exts).

- The behavior is undefined if `[`​0​`,` map_.required_span_size()`)` is not an accessible range of `ptr_` and `acc_` for the values of `map_` and `acc_` after the invocation of this constructor.
- This overload participates in overload resolution only if

  - std::is_convertible_v<const OtherIndexTypes&, index_type> is true,
  - std::is_nothrow_constructible<index_type, const OtherIndexTypes&> is true,
  - N == rank() || N == rank_dynamic() is true,
  - std::is_constructible_v<mapping_type, extents_type> is true, and
  - std::is_default_constructible_v<accessor_type> is true.

5) Construct an mdspan over the underlying data referred by p with extents represented by ext. Value-initializes `acc_`, direct-non-list-initializes `ptr_` with std::move(p) and `map_` with exts.

- The behavior is undefined if `[`​0​`,` map_.required_span_size()`)` is not an accessible range of p and `acc_` for the values of `map_` and `acc_` after the invocation of this constructor.
- This overload participates in overload resolution only if

  - std::is_constructible_v<mapping_type, const mapping_type&> is true, and
  - std::is_default_constructible_v<accessor_type> is true.

6) Construct an mdspan over the underlying data referred by p with layout mapping m. Value-initializes `acc_`, direct-non-list-initializes `ptr_` with std::move(p) and `map_` with m.

- The behavior is undefined if `[`​0​`,` m.required_span_size()`)` is not an accessible range of p and `acc_` for the values of `acc_` after the invocation of this constructor.
- This overload participates in overload resolution only if std::is_default_constructible_v<accessor_type> is true.

7) Construct an mdspan over the underlying data referred by p with layout mapping m and accessor a. Direct-non-list-initializes `ptr_` with std::move(p), `map_` with m and `acc_` with a.

- The behavior is undefined if `[`​0​`,` m.required_span_size()`)` is not an accessible range of p and a after the invocation of this constructor.

8) Converting constructor from another mdspan. Direct-non-list-initializes `ptr_` with other.ptr_, `map_` with other.map_ and `acc_` with other.acc_.

- The behavior is undefined if :

  - `[`​0​`,` map_.required_span_size()`)` is not an accessible range of `ptr_` and `acc_` for the values of `map_` and `acc_` after the invocation of this constructor, or
  - for each rank index `r` of extents_type, extents_type::static_extent(r) == std::dynamic_extent
    || extents_type::static_extent(r) == other.extent(r) is false.

- This overload participates in overload resolution only if

  - std::is_constructible_v<mapping_type, const OtherLayoutPolicy​::
    ​template mapping<OtherExtents>&> is true, and
  - std::is_constructible_v<accessor_type, const OtherAccessor&> is true.

- The program is ill-formed if:

  - std::is_constructible_v<data_handle_type,
    const OtherAccessor​::​data_handle_type&> is false, or
  - std::is_constructible_v<extents_type, OtherExtents> is false.

- The expression inside explicit is equivalent to: !std::is_convertible_v<const OtherLayoutPolicy::
  template mapping<OtherExtents>&, mapping_type>
  || !std::is_convertible_v<const OtherAccessor&, accessor_type>

9) Defaulted copy constructor.
10) Defaulted move constructor.

### Parameters

p | - | a handle to the underlying data
m | - | a layout mapping
a | - | an accessor
ext | - | represents a one-dimensional extents
exts | - | represents a multi-dimensional extents
other | - | another mdspan to convert from
rhs | - | another mdspan to copy or move from

### Example

| This section is incomplete Reason: no example

### References

- C++23 standard (ISO/IEC 14882:2023):

  - 24.7.3.6.2 Constructors [mdspan.mdspan.cons]

### See also

| This section is incomplete

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