23 Containers library [containers]

The submdspan facilities create a new mdspan viewing a subset of elements of an existing input mdspan.

The subset viewed by the created mdspan is determined by the SliceSpecifier arguments.

Given a signed or unsigned integer type IndexType, a type S is a submdspan slice type for IndexType if at least one of the following holds:

• (2.1)
  is_convertible_v<S, full_extent_t> is true;
• (2.2)
  is_convertible_v<S, IndexType> is true;
• (2.3)
  S is a specialization of strided_slice and is_convertible_v<X, IndexType> is true for X denoting S​::​offset_type, S​::​extent_type, and S​::​stride_type; or
• (2.4)
  all of the following hold:
  • (2.4.1)
    the declaration auto [...ls] = std​::​move(s); is well-formed for some object s of type S,
  • (2.4.2)
    sizeof...(ls) is equal to 2, and
  • (2.4.3)
    (is_convertible_v<decltype(std​::​move(ls)), IndexType> && ...) is true.

Given a signed or unsigned integer type IndexType, a type S is a canonical submdspan index type for IndexType if S is either IndexType or constant_wrapper<v> for some value v of type IndexType, such that v is greater than or equal to zero.

Given a signed or unsigned integer type IndexType, a type S is a canonical submdspan slice type for IndexType if exactly one of the following is true:

• (4.1)
  S is full_extent_t;
• (4.2)
  S is a canonical submdspan index type for IndexType; or
• (4.3)
  S is a specialization of strided_slice where all of the following hold:
  • (4.3.1)
    S​::​offset_type, S​::​extent_type, and S​::​stride_type are all canonical submdspan index types for IndexType; and
  • (4.3.2)
    if S​::​stride_type and S​::​extent_type are both specializations of constant_wrapper, then S​::​stride_type​::​value is greater than zero.

A type S is a collapsing slice type if it is neither full_extent_t nor a specialization of strided_slice.

A type S is a unit-stride slice type if

• (6.1)
  S is a specialization of strided_slice where S​::​stride_type is a specialization of constant_wrapper and S​::​stride_type​::​value is equal to 1, or
• (6.2)
  S denotes full_extent_t.

Given an object e of type E that is a specialization of extents, and an object s of type S that is a canonical submdspan slice type for E​::​index_type, the submdspan slice range of s for the $k^{\text{th}}$ extent of e is:

• (7.1)
  [0, e.extent(k)), if S is full_extent_t;
• (7.2)
  [E​::​index_type(s.offset), E​::​index_type(s.offset + s.extent)), if S is a specialization of strided_slice; otherwise
• (7.3)
  [E​::​index_type(s), $\text{E::index\_type(s)}+1$)

Given a type E that is a specialization of extents, a type S is a valid submdspan slice type for the $k^{\text{th}}$ extent of E if S is a canonical slice type for E​::​index_type, and for x equal to E​::​static_extent(k), either x is equal to dynamic_extent; or

• (8.1)
  if S is a specialization of strided_slice:
  • (8.1.1)
    if S​::​offset_type is a specialization of constant_wrapper, then S​::​offset_type​::​value is less than or equal to x;
  • (8.1.2)
    if S​::​offset_type is a specialization of constant_wrapper, then S​::​extent_type​::​value is less than or equal to x; and
  • (8.1.3)
    if both S​::​offset_type and S​::​extent_type are specializations of constant_wrapper, then S​::​offset_type​::​value + S​::​extent_type​::​value is less than or equal to x; or
• (8.2)
  if S is a specialization of constant_wrapper, then S​::​value is less than x.

Given an object e of type E that is a specialization of extents and an object s of type S, s is a valid submdspan slice for the $k^{\text{th}}$ extent of e if

• (9.1)
  S is a valid submdspan slice type for the $k^{\text{th}}$ extent of E;
• (9.2)
  the $k^{\text{th}}$ interval of e contains the submdspan slice range of s for the $k^{\text{th}}$ extent of e; and
• (9.3)
  if S is a specialization of strided_slice, then:
  • (9.3.1)
    s.extent is greater than or equal to zero, and
  • (9.3.2)
    either s.extent equals zero or s.stride is greater than zero.