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