[#mp_units-vector_axes]
= xref:mp_units.adoc[mp_units]::vector_axes
:relfileprefix: ../
:mrdocs:

Ordered list of the 1D‐vector component axes a vector quantity decomposes into

== Synopsis

Declared in `<mp‐units/framework/vector_components.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<xref:mp_units/QuantitySpec.adoc[QuantitySpec] auto Axes...>
requires detail::ValidVectorAxes<Axes...>
struct vector_axes;
----

== Description

A user opts a vector whole into decomposition by specializing `vector_components` (the customization point in `customization_points.h`) and inheriting from this template, listing the per‐axis quantity specs in coordinate order:

[,cpp]
----
template<>
struct mp_units::vector_components<flight_velocity> :
    mp_units::vector_axes<forward_velocity, wind_drift, sink_rate> {};
----
The axis&hyphen;intrinsic preconditions are enforced here&colon; every axis must be a vector quantity, the axes must share a common hierarchy root (be the same kind of quantity), and they must be pairwise&hyphen;distinct kinds (declare each with `is&lowbar;kind`)&period; A specialization that violates them is ill&hyphen;formed at the point of definition&period; The relational preconditions (axes share the whole&apos;s hierarchy root, differ from the whole&apos;s kind, and fit the representation) need the whole and the representation, so they are enforced by the `Decomposable` concept&period;

== Static Data Members

[cols="1"]
|===
| Name
| xref:mp_units/vector_axes/axis.adoc[`axis`] 
| xref:mp_units/vector_axes/size.adoc[`size`] 
|===



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