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Merge topic 'arrayhandle-mutable-const'

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a2a8dcdfd Allow a `const ArrayHandle` to be reallocated

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Li-Ta Lo <ollie@lanl.gov>
Merge-request: !2570
This commit is contained in:
Kenneth Moreland 2021-09-02 23:39:30 +00:00 committed by Kitware Robot
commit c494963626
2 changed files with 47 additions and 4 deletions
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42
docs/changelog/arrayhandle-mutable.const.md Normal file
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@ -0,0 +1,42 @@
# Allow a `const ArrayHandle` to be reallocated
Previously, the `Allocate` method of `ArrayHandle` was _not_ declared as
`const`. Likewise, the methods that depended on `Allocate`, namely
`ReleaseResources` and `PrepareForOutput` were also not declared `const`.
The main consequence of this was that if an `ArrayHandle` were passed as a
constant reference argument to a method (e.g. `const ArrayHandle<T>& arg`),
then the array could not be reallocated.
This seems right at first blush. However, we have changed these methods to
be `const` so that you can in fact reallocate the `ArrayHandle`. This is
because the `ArrayHandle` is in principle a pointer to an array pointer.
Such a structure in C will allow you to change the pointer to the array,
and so in this context it makes sense for `ArrayHandle` to support that as
well.
Although this distinction will certainly be confusing to users, we think
this change is correct for a variety of reasons.
1. This change makes the behavior of `ArrayHandle` consistent with the
behavior of `UnknownArrayHandle`. The latter needed this behavior to
allow `ArrayHandle`s to be passed as output arguments to methods that
get automatically converted to `UnknownArrayHandle`.
2. Before this change, a `const ArrayHandle&` was still multible is many
way. In particular, it was possible to change the data in the array
even if the array could not be resized. You could still call things
like `WritePortal` and `PrepareForInOut`. The fact that you could
change it for some things and not others was confusing. The fact that
you could call `PrepareForInOut` but not `PrepareForOutput` was doubly
confusing.
3. Passing a value by constant reference should be the same, from the
calling code's perspective, as passing by value. Although the function
can change an argument passed by value, that change is not propogated
back to the calling code. However, in the case of `ArrayHandle`,
calling by value would allow the array to be reallocated from the
calling side whereas a constant reference would prevent that. This
change makes the two behaviors consistent.
4. The supposed assurance that the `ArrayHandle` would not be reallocated
was easy to break even accidentally. If the `ArrayHandle` was assigned
to another `ArrayHandle` (for example as a class' member or wrapped
inside of an `UnknownArrayHandle`), then the array was free to be
reallocated.

9
vtkm/cont/ArrayHandle.h
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@ -515,12 +515,13 @@ public:
///
VTKM_CONT void Allocate(vtkm::Id numberOfValues,
vtkm::CopyFlag preserve,
vtkm::cont::Token& token)
vtkm::cont::Token& token) const
{
StorageType::ResizeBuffers(numberOfValues, this->GetBuffers(), preserve, token);
}
VTKM_CONT void Allocate(vtkm::Id numberOfValues, vtkm::CopyFlag preserve = vtkm::CopyFlag::Off)
VTKM_CONT void Allocate(vtkm::Id numberOfValues,
vtkm::CopyFlag preserve = vtkm::CopyFlag::Off) const
{
vtkm::cont::Token token;
this->Allocate(numberOfValues, preserve, token);
@ -543,7 +544,7 @@ public:
/// Releases all resources in both the control and execution environments.
///
VTKM_CONT void ReleaseResources() { this->Allocate(0); }
VTKM_CONT void ReleaseResources() const { this->Allocate(0); }
/// Prepares this array to be used as an input to an operation in the
/// execution environment. If necessary, copies data to the execution
@ -599,7 +600,7 @@ public:
///
VTKM_CONT WritePortalType PrepareForOutput(vtkm::Id numberOfValues,
vtkm::cont::DeviceAdapterId device,
vtkm::cont::Token& token)
vtkm::cont::Token& token) const
{
this->Allocate(numberOfValues, vtkm::CopyFlag::Off, token);
return StorageType::CreateWritePortal(this->GetBuffers(), device, token);