vtk-m/docs/changelog/vec-traits-base-components.md

Recursive base component queries to VecTraits

This change adds a recursive BaseComponentType to VecTraits that recursively finds the base (non-Vec) type of a Vec. This is useful when dealing with potentially nested Vecs (e.g. Vec<Vec<T, M>, N>) and you want to know the precision of the math being defined.

using NestedVec = vtkm::Vec<vtkm::Vec<vtkm::Float32, 3>, 8>;

// ComponentType becomes vtkm::Vec<vtkm::Float32, 3>
using ComponentType = typename vtkm::VecTraits<NestedVec>::ComponentType;

// BaseComponentType becomes vtkm::Float32
using BaseComponentType = typename vtkm::VecTraits<NestedVec>::BaseComponentType;

Also added the ability to VecTraits to change the component type of a vector. The template RepalceComponentType resolves to a Vec of the same type with the component replaced with a new type. The template ReplaceBaseComponentType traverses down a nested type and replaces the base type.

using NestedVec = vtkm::Vec<vtkm::Vec<vtkm::Float32, 3>, 8>;

// NewVec1 becomes vtkm::Vec<vtkm::Float64, 8>
using NewVec1 =
  typename vtkm::VecTraits<NestedVec>::template ReplaceComponentType<vtkm::Float64>;

// NewVec2 becomes vtkm::Vec<vtkm::Vec<vtkm::Float64, 3>, 8>
using NewVec1 =
  typename vtkm::VecTraits<NestedVec>::template ReplaceBaseComponentType<vtkm::Float64>;

This functionality replaces the functionality in vtkm::BaseComponent. Unfortunately, vtkm::BaseComponent did not have the ability to replace the base component and there was no straightforward way to implement that outside of VecTraits.