2.0 KiB
Replaced vtkm::ListTag with vtkm::List
The original vtkm::ListTag was designed when we had to support compilers
that did not provide C++11's variadic templates. Thus, the design hides
type lists, which were complicated to support.
Now that we support C++11, variadic templates are trivial and we can easily
create templated type aliases with using. Thus, it is now simpler to deal
with a template that lists types directly.
Hence, vtkm::ListTag is deprecated and vtkm::List is now supported. The
main difference between the two is that whereas vtkm::ListTag allowed you
to create a list by subclassing another list, vtkm::List cannot be
subclassed. (Well, it can be subclassed, but the subclass ceases to be
considered a list.) Thus, where before you would declare a list like
struct MyList : vtkm::ListTagBase<Type1, Type2, Type3>
{
};
you now make an alias
using MyList = vtkm::List<Type1, Type2, Type3>;
If the compiler reports the MyList type in an error or warning, it
actually uses the fully qualified vtkm::List<Type1, Type2, Type3>.
Although this makes errors more verbose, it makes it easier to diagnose
problems because the types are explicitly listed.
The new vtkm::List comes with a list of utility templates to manipulate
lists that mostly mirrors those in vtkm::ListTag: VTKM_IS_LIST,
ListApply, ListSize, ListAt, ListIndexOf, ListHas, ListAppend,
ListIntersect, ListTransform, ListRemoveIf, and ListCross. All of
these utilities become vtkm::List<> types (where applicable), which makes
them more consistent than the old vtkm::ListTag versions.
Thus, if you have a declaration like
vtkm::ListAppend(vtkm::List<Type1a, Type2a>, vtkm::List<Type1b, Type2b>>
this gets changed automatically to
vtkm::List<Type1a, Type2a, Type1b, Type2b>
This is in contrast to the equivalent old version, which would create a new
type for vtkm::ListTagAppend in addition to the ultimate actual list it
constructs.