28 Numerics library [numerics]

28.9 Basic linear algebra algorithms [linalg]

28.9.5 Tag classes [linalg.tags]

28.9.5.1 Storage order tags [linalg.tags.order]

The storage order tags describe the order of elements in an mdspan with layout_blas_packed ([linalg.layout.packed]) layout.

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struct column_major_t {
  explicit column_major_t() = default;
};
inline constexpr column_major_t column_major{};

struct row_major_t {
  explicit row_major_t() = default;
};
inline constexpr row_major_t row_major{};

column_major_t indicates a column-major order, and row_major_t indicates a row-major order.

28.9.5.2 Triangle tags [linalg.tags.triangle]

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struct upper_triangle_t {
  explicit upper_triangle_t() = default;
};
inline constexpr upper_triangle_t upper_triangle{};

struct lower_triangle_t {
  explicit lower_triangle_t() = default;
};
inline constexpr lower_triangle_t lower_triangle{};

These tag classes specify whether algorithms and other users of a matrix (represented as an mdspan) access the upper triangle (upper_triangle_t) or lower triangle (lower_triangle_t) of the matrix (see also [linalg.general]).

This is also subject to the restrictions of implicit_unit_diagonal_t if that tag is also used as a function argument; see below.

28.9.5.3 Diagonal tags [linalg.tags.diagonal]

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struct implicit_unit_diagonal_t {
  explicit implicit_unit_diagonal_t() = default;
};
inline constexpr implicit_unit_diagonal_t
  implicit_unit_diagonal{};

struct explicit_diagonal_t {
  explicit explicit_diagonal_t() = default;
};
inline constexpr explicit_diagonal_t explicit_diagonal{};

These tag classes specify whether algorithms access the matrix's diagonal entries, and if not, then how algorithms interpret the matrix's implicitly represented diagonal values.

The implicit_unit_diagonal_t tag indicates that an implicit unit diagonal is to be assumed ([linalg.general]).

The explicit_diagonal_t tag indicates that an explicit diagonal is used ([linalg.general]).