3.2. Basic Array Handles
Chapter 2.4 (Data Sets) describes the basic data sets used by VTK‑m.
This chapter dives deeper into how VTK‑m represents data.
Ultimately, data structures like vtkm::cont::DataSet
can be broken down into arrays of numbers.
Arrays in VTK‑m are managed by a unit called an array handle.
An array handle, which is implemented with the vtkm::cont::ArrayHandle
class, manages an array of data that can be accessed or manipulated by VTK‑m algorithms.
It is typical to construct an array handle in the control environment to pass data to an algorithm running in the execution environment.
It is also typical for an algorithm running in the execution environment to populate an array handle, which can then be read back in the control environment.
It is also possible for an array handle to manage data created by one VTK‑m algorithm and passed to another, remaining in the execution environment the whole time and never copied to the control environment.
Did You Know?
The array handle may have multiple copies of the array, one for the control environment and one for each device. However, depending on the device and how the array is being used, the array handle will only have one copy when possible. Copies between the environments are implicit and lazy. They are copied only when an operation needs data in an environment where the data are not.
vtkm::cont::ArrayHandle
behaves like a shared smart pointer in that when the C++ object is copied, each copy holds a reference to the same array.
These copies are reference counted so that when all copies of the vtkm::cont::ArrayHandle
are destroyed, any allocated memory is released.
-
template<typename T, typename StorageTag_ = ::vtkm::cont::StorageTagBasic>
class ArrayHandle : public vtkm::cont::internal::ArrayHandleBase Manages an array-worth of data.
ArrayHandle
manages as array of data that can be manipulated by VTKm algorithms. TheArrayHandle
may have up to two copies of the array, one for the control environment and one for the execution environment, although depending on the device and how the array is being used, theArrayHandle
will only have one copy when possible.An
ArrayHandle
is often constructed by instantiating one of theArrayHandle
subclasses. Several basicArrayHandle
types can also be constructed directly and then allocated. TheArrayHandleBasic
subclass provides mechanisms for importing user arrays into anArrayHandle
.ArrayHandle
behaves like a shared smart pointer in that when it is copied each copy holds a reference to the same array. These copies are reference counted so that when all copies of theArrayHandle
are destroyed, any allocated memory is released.Subclassed by vtkm::cont::ArrayHandleImplicit< detail::PhiloxFunctor >, vtkm::cont::ArrayHandleRuntimeVec< vtkm::Float32 >, vtkm::cont::ArrayHandleTransform< vtkm::cont::ArrayHandleRandomUniformBits, detail::CanonicalFunctor< vtkm::Float64 > >, vtkm::cont::ArrayHandleTransform< vtkm::cont::ArrayHandleZip< vtkm::cont::ArrayHandleRandomUniformReal< vtkm::Float64 >, vtkm::cont::ArrayHandleRandomUniformReal< vtkm::Float64 > >, detail::BoxMuller >, vtkm::cont::ArrayHandleCartesianProduct< FirstHandleType, SecondHandleType, ThirdHandleType >, vtkm::cont::ArrayHandleDecorator< DecoratorImplT, ArrayTs >, vtkm::cont::ArrayHandleDiscard< ValueType_ >, vtkm::cont::ArrayHandleImplicit< FunctorType >, vtkm::cont::ArrayHandleZip< FirstHandleType, SecondHandleType >
Public Functions
-
inline ArrayHandle()
Constructs an empty ArrayHandle.
-
inline ArrayHandle(const vtkm::cont::ArrayHandle<ValueType, StorageTag> &src)
Copy constructor.
Implemented so that it is defined exclusively in the control environment. If there is a separate device for the execution environment (for example, with CUDA), then the automatically generated copy constructor could be created for all devices, and it would not be valid for all devices.
-
inline ArrayHandle(vtkm::cont::ArrayHandle<ValueType, StorageTag> &&src) noexcept
Move constructor.
Implemented so that it is defined exclusively in the control environment. If there is a separate device for the execution environment (for example, with CUDA), then the automatically generated move constructor could be created for all devices, and it would not be valid for all devices.
-
inline explicit ArrayHandle(const std::vector<vtkm::cont::internal::Buffer> &buffers)
Special constructor for subclass specializations that need to set the initial state array.
Used when pulling data from other sources.
-
inline explicit ArrayHandle(std::vector<vtkm::cont::internal::Buffer> &&buffers) noexcept
Special constructor for subclass specializations that need to set the initial state array.
Used when pulling data from other sources.
-
inline ~ArrayHandle()
Destructs an empty ArrayHandle.
Implemented so that it is defined exclusively in the control environment. If there is a separate device for the execution environment (for example, with CUDA), then the automatically generated destructor could be created for all devices, and it would not be valid for all devices.
-
inline vtkm::cont::ArrayHandle<ValueType, StorageTag> &operator=(const vtkm::cont::ArrayHandle<ValueType, StorageTag> &src)
Shallow copies an ArrayHandle.
-
inline vtkm::cont::ArrayHandle<ValueType, StorageTag> &operator=(vtkm::cont::ArrayHandle<ValueType, StorageTag> &&src) noexcept
Move and Assignment of an ArrayHandle.
-
inline bool operator==(const ArrayHandle<ValueType, StorageTag> &rhs) const
Like a pointer, two
ArrayHandle
s are considered equal if they point to the same location in memory.
-
inline StorageType GetStorage() const
Get the storage.
-
inline ReadPortalType ReadPortal() const
Get an array portal that can be used in the control environment.
The returned array can be used in the control environment to read values from the array. (It is not possible to write to the returned portal. That is
Get
will work on the portal, butSet
will not.)Note: The returned portal cannot be used in the execution environment. This is because the portal will not work on some devices like GPUs. To get a portal that will work in the execution environment, use
PrepareForInput
.
-
inline WritePortalType WritePortal() const
Get an array portal that can be used in the control environment.
The returned array can be used in the control environment to reand and write values to the array.
Note: The returned portal cannot be used in the execution environment. This is because the portal will not work on some devices like GPUs. To get a portal that will work in the execution environment, use
PrepareForInput
.
-
inline WritePortalType WritePortal(vtkm::cont::Token &token) const
Get an array portal that can be used in the control environment.
The returned array can be used in the control environment to reand and write values to the array.
Note: The returned portal cannot be used in the execution environment. This is because the portal will not work on some devices like GPUs. To get a portal that will work in the execution environment, use
PrepareForInput
.
-
inline vtkm::IdComponent GetNumberOfComponentsFlat() const
Returns the total number of components for each value in the array.
If the array holds
vtkm::Vec
objects, this will return the total number of components in each value assuming the object is flattened out to one level ofVec
objects. If the array holds a basic C type (such asfloat
), this will return 1. If the array holds a simpleVec
(such asvtkm::Vec3f
), this will return the number of components (in this case 3). If the array holds a hierarchy ofVec
s (such asvtkm::Vec<vtkm::Vec3f, 2>
), this will return the total number of vecs (in this case 6).If this object is holding an array where the number of components can be selected at runtime (for example,
vtkm::cont::ArrayHandleRuntimeVec
), this method will still return the correct number of components. However, if each value in the array can be aVec
of a different size (such asvtkm::cont::ArrayHandleGroupVecVariable
), this method will return 0 (because there is no consistent answer).
-
inline void Allocate(vtkm::Id numberOfValues, vtkm::CopyFlag preserve, vtkm::cont::Token &token) const
Allocates an array large enough to hold the given number of values.
The allocation may be done on an already existing array. If so, then the data are preserved as best as possible if the preserve flag is set to
vtkm::CopyFlag::On
. If the preserve flag is set tovtkm::CopyFlag::Off
(the default), any existing data could be wiped out.This method can throw
vtkm::cont::ErrorBadAllocation
if the array cannot be allocated orvtkm::cont::ErrorBadValue
if the allocation is not feasible (for example, the array storage is read-only).
-
inline void Allocate(vtkm::Id numberOfValues, vtkm::CopyFlag preserve = vtkm::CopyFlag::Off) const
Allocates an array large enough to hold the given number of values.
The allocation may be done on an already existing array. If so, then the data are preserved as best as possible if the preserve flag is set to
vtkm::CopyFlag::On
. If the preserve flag is set tovtkm::CopyFlag::Off
(the default), any existing data could be wiped out.This method can throw
vtkm::cont::ErrorBadAllocation
if the array cannot be allocated orvtkm::cont::ErrorBadValue
if the allocation is not feasible (for example, the array storage is read-only).
-
inline void AllocateAndFill(vtkm::Id numberOfValues, const ValueType &fillValue, vtkm::CopyFlag preserve, vtkm::cont::Token &token) const
Allocates an array and fills it with an initial value.
AllocateAndFill
behaves similar toAllocate
except that after allocation it fills the array with a givenfillValue
. This method is convenient when you wish to initialize the array.If the
preserve
flag isvtkm::CopyFlag::On
, then any data that existed before the call toAllocateAndFill
will remain after the call (assuming the new array size is large enough). If the array size is expanded, then the new values at the end will be filled.If the
preserve
flag isvtkm::CopyFlag::Off
(the default), the entire array is filled with the givenfillValue
.
-
inline void AllocateAndFill(vtkm::Id numberOfValues, const ValueType &fillValue, vtkm::CopyFlag preserve = vtkm::CopyFlag::Off) const
Allocates an array and fills it with an initial value.
AllocateAndFill
behaves similar toAllocate
except that after allocation it fills the array with a givenfillValue
. This method is convenient when you wish to initialize the array.If the
preserve
flag isvtkm::CopyFlag::On
, then any data that existed before the call toAllocateAndFill
will remain after the call (assuming the new array size is large enough). If the array size is expanded, then the new values at the end will be filled.If the
preserve
flag isvtkm::CopyFlag::Off
(the default), the entire array is filled with the givenfillValue
.
-
inline void Fill(const ValueType &fillValue, vtkm::Id startIndex, vtkm::Id endIndex, vtkm::cont::Token &token) const
Fills the array with a given value.
After calling this method, every entry in the array from
startIndex
(inclusive) toendIndex
(exclusive) of the array is set tofillValue
. IfstartIndex
orendIndex
is not specified, then the fill happens from the begining or end, respectively.
-
inline void Fill(const ValueType &fillValue, vtkm::Id startIndex, vtkm::Id endIndex) const
Fills the array with a given value.
After calling this method, every entry in the array from
startIndex
(inclusive) toendIndex
(exclusive) of the array is set tofillValue
. IfstartIndex
orendIndex
is not specified, then the fill happens from the begining or end, respectively.
-
inline void Fill(const ValueType &fillValue, vtkm::Id startIndex = 0) const
Fills the array with a given value.
After calling this method, every entry in the array from
startIndex
(inclusive) toendIndex
(exclusive) of the array is set tofillValue
. IfstartIndex
orendIndex
is not specified, then the fill happens from the begining or end, respectively.
-
inline void ReleaseResourcesExecution() const
Releases any resources being used in the execution environment (that are not being shared by the control environment).
-
inline void ReleaseResources() const
Releases all resources in both the control and execution environments.
-
inline ReadPortalType PrepareForInput(vtkm::cont::DeviceAdapterId device, vtkm::cont::Token &token) const
Prepares this array to be used as an input to an operation in the execution environment.
If necessary, copies data to the execution environment. Can throw an exception if this array does not yet contain any data. Returns a portal that can be used in code running in the execution environment.
The
Token
object provided will be attached to thisArrayHandle
. The returned portal is guaranteed to be valid while theToken
is still attached and in scope. Other operations on thisArrayHandle
that would invalidate the returned portal will block until theToken
is released. Likewise, this method will block if anotherToken
is already attached. This can potentially lead to deadlocks.
-
inline WritePortalType PrepareForInPlace(vtkm::cont::DeviceAdapterId device, vtkm::cont::Token &token) const
Prepares this array to be used in an in-place operation (both as input and output) in the execution environment.
If necessary, copies data to the execution environment. Can throw an exception if this array does not yet contain any data. Returns a portal that can be used in code running in the execution environment.
The
Token
object provided will be attached to thisArrayHandle
. The returned portal is guaranteed to be valid while theToken
is still attached and in scope. Other operations on thisArrayHandle
that would invalidate the returned portal will block until theToken
is released. Likewise, this method will block if anotherToken
is already attached. This can potentially lead to deadlocks.
-
inline WritePortalType PrepareForOutput(vtkm::Id numberOfValues, vtkm::cont::DeviceAdapterId device, vtkm::cont::Token &token) const
Prepares (allocates) this array to be used as an output from an operation in the execution environment.
The internal state of this class is set to have valid data in the execution array with the assumption that the array will be filled soon (i.e. before any other methods of this object are called). Returns a portal that can be used in code running in the execution environment.
The
Token
object provided will be attached to thisArrayHandle
. The returned portal is guaranteed to be valid while theToken
is still attached and in scope. Other operations on thisArrayHandle
that would invalidate the returned portal will block until theToken
is released. Likewise, this method will block if anotherToken
is already attached. This can potentially lead to deadlocks.
-
inline bool IsOnDevice(vtkm::cont::DeviceAdapterId device) const
Returns true if the ArrayHandle’s data is on the given device.
If the data are on the given device, then preparing for that device should not require any data movement.
-
inline bool IsOnHost() const
Returns true if the ArrayHandle’s data is on the host.
If the data are on the given device, then calling
ReadPortal
orWritePortal
should not require any data movement.
-
inline void SyncControlArray() const
Synchronizes the control array with the execution array.
If either the user array or control array is already valid, this method does nothing (because the data is already available in the control environment). Although the internal state of this class can change, the method is declared const because logically the data does not.
-
inline void Enqueue(const vtkm::cont::Token &token) const
Enqueue a token for access to this ArrayHandle.
This method places the given
Token
into the queue ofToken
s waiting for access to thisArrayHandle
and then returns immediately. When this token is later used to get data from thisArrayHandle
(for example, in a call toPrepareForInput
), it will use this place in the queue while waiting for access.This method is to be used to ensure that a set of accesses to an
ArrayHandle
that happen on multiple threads occur in a specified order. For example, if you spawn of a job to modify data in anArrayHandle
and then spawn off a job that reads that same data, you need to make sure that the first job gets access to theArrayHandle
before the second. If they both just attempt to call their respectivePrepare
methods, there is no guarantee which order they will occur. Having the spawning thread first call this method will ensure the order.Warning
After calling this method it is required to subsequently call a method like one of the
Prepare
methods that attaches the token to thisArrayHandle
. Otherwise, the enqueued token will block any subsequent access to theArrayHandle
, even if theToken
is destroyed.
-
inline void DeepCopyFrom(const vtkm::cont::ArrayHandle<ValueType, StorageTag> &source) const
Deep copies the data in the array.
Takes the data that is in source and copies that data into this array.
-
inline const std::vector<vtkm::cont::internal::Buffer> &GetBuffers() const
Returns the internal
Buffer
structures that hold the data.Note that great care should be taken when modifying buffers outside of the ArrayHandle.
-
inline ArrayHandle()
3.2.1. Creating Array Handles
vtkm::cont::ArrayHandle
is templated on the type of values being stored in the array.
There are multiple ways to create and populate an array handle.
The default vtkm::cont::ArrayHandle
constructor will create an empty array with nothing allocated in either the control or execution environment.
This is convenient for creating arrays used as the output for algorithms.
1 vtkm::cont::ArrayHandle<vtkm::Float32> outputArray;
Chapter ref{chap:AccessingAllocatingArrays} describes in detail how to allocate memory and access data in an vtkm::cont::ArrayHandle
.
However, you can use the vtkm::cont::make_ArrayHandle()
function for a simplified way to create an vtkm::cont::ArrayHandle
with data.
vtkm::cont::make_ArrayHandle()
has many forms.
An easy form to use takes an initializer list and creates a basic vtkm::cont::ArrayHandle
with it.
This allows you to create a short vtkm::cont::ArrayHandle
from literals.
-
template<typename T>
vtkm::cont::ArrayHandleBasic<T> vtkm::cont::make_ArrayHandle(std::initializer_list<T> &&values) Create an ArrayHandle directly from an initializer list of values.
1 auto fibonacciArray = vtkm::cont::make_ArrayHandle({ 0, 1, 1, 2, 3, 5, 8, 13 });
One problem with creating an array from an initializer list like this is that it can be tricky to specify the exact value type of the vtkm::cont::ArrayHandle
.
The value type of the vtkm::cont::ArrayHandle
will be the same types as the literals in the initializer list, but that might not match the type you actually need.
This is particularly true for types like vtkm::Id
and vtkm::FloatDefault
, which can change depending on compile options.
To specify the exact value type to use, give that type as a template argument to the vtkm::cont::make_ArrayHandle()
function.
1 vtkm::cont::ArrayHandle<vtkm::FloatDefault> inputArray =
2 vtkm::cont::make_ArrayHandle<vtkm::FloatDefault>({ 1.4142f, 2.7183f, 3.1416f });
Constructing an vtkm::cont::ArrayHandle
that points to a provided C array is also straightforward.
To do this, call vtkm::cont::make_ArrayHandle()
with the array pointer, the number of values in the C array, and a vtkm::CopyFlag
.
This last argument can be either vtkm::CopyFlag::On
to copy the array or vtkm::CopyFlag::Off
to share the provided buffer.
-
template<typename T>
vtkm::cont::ArrayHandleBasic<T> vtkm::cont::make_ArrayHandle(const T *array, vtkm::Id numberOfValues, vtkm::CopyFlag copy) A convenience function for creating an ArrayHandle from a standard C array.
-
enum class vtkm::CopyFlag
Identifier used to specify whether a function should deep copy data.
Values:
-
enumerator Off
-
enumerator On
-
enumerator Off
1 vtkm::Float32 dataBuffer[50];
2 // Populate dataBuffer with meaningful data. Perhaps read data from a file.
3
4 vtkm::cont::ArrayHandle<vtkm::Float32> inputArray =
5 vtkm::cont::make_ArrayHandle(dataBuffer, 50, vtkm::CopyFlag::On);
Likewise, you can use vtkm::cont::make_ArrayHandle()
to transfer data from a std::vector
to an vtkm::cont::ArrayHandle
.
This form of vtkm::cont::make_ArrayHandle()
takes the std::vector
as the first argument and a vtkm::CopyFlag
as the second argument.
-
template<typename T, typename Allocator>
vtkm::cont::ArrayHandleBasic<T> vtkm::cont::make_ArrayHandle(const std::vector<T, Allocator> &array, vtkm::CopyFlag copy) A convenience function for creating an ArrayHandle from an std::vector.
1 std::vector<vtkm::Float32> dataBuffer;
2 // Populate dataBuffer with meaningful data. Perhaps read data from a file.
3
4 vtkm::cont::ArrayHandle<vtkm::Float32> inputArray =
5 vtkm::cont::make_ArrayHandle(dataBuffer, vtkm::CopyFlag::On);
As hinted at earlier, it is possible to send vtkm::CopyFlag::On
to vtkm::cont::make_ArrayHandle()
to wrap an vtkm::cont::ArrayHandle
around an existing C array or std::vector
.
Doing so allows you to send the data to the vtkm::cont::ArrayHandle
without copying it.
It also provides a mechanism for VTK‑m to write directly into your array.
However, be aware that if you change or delete the data provided, the internal state of vtkm::cont::ArrayHandle
becomes invalid and undefined behavior can ensue.
A common manifestation of this error happens when a std::vector
goes out of scope.
This subtle interaction will cause the vtkm::cont::ArrayHandle
to point to an unallocated portion of the memory heap.
The following example provides an erroneous use of vtkm::cont::ArrayHandle
and some ways to fix it.
1VTKM_CONT vtkm::cont::ArrayHandle<vtkm::Float32> BadDataLoad()
2{
3 std::vector<vtkm::Float32> dataBuffer;
4 // Populate dataBuffer with meaningful data. Perhaps read data from a file.
5
6 vtkm::cont::ArrayHandle<vtkm::Float32> inputArray =
7 vtkm::cont::make_ArrayHandle(dataBuffer, vtkm::CopyFlag::Off);
8
9 return inputArray;
10 // THIS IS WRONG! At this point dataBuffer goes out of scope and deletes its
11 // memory. However, inputArray has a pointer to that memory, which becomes an
12 // invalid pointer in the returned object. Bad things will happen when the
13 // ArrayHandle is used.
14}
15
16VTKM_CONT vtkm::cont::ArrayHandle<vtkm::Float32> SafeDataLoad1()
17{
18 std::vector<vtkm::Float32> dataBuffer;
19 // Populate dataBuffer with meaningful data. Perhaps read data from a file.
20
21 vtkm::cont::ArrayHandle<vtkm::Float32> inputArray =
22 vtkm::cont::make_ArrayHandle(dataBuffer, vtkm::CopyFlag::On);
23
24 return inputArray;
25 // This is safe.
26}
27
28VTKM_CONT vtkm::cont::ArrayHandle<vtkm::Float32> SafeDataLoad2()
29{
30 std::vector<vtkm::Float32> dataBuffer;
31 // Populate dataBuffer with meaningful data. Perhaps read data from a file.
32
33 vtkm::cont::ArrayHandle<vtkm::Float32> inputArray =
34 vtkm::cont::make_ArrayHandleMove(std::move(dataBuffer));
35
36 return inputArray;
37 // This is safe.
38}
An easy way around the problem of having an vtkm::cont::ArrayHandle
’s data going out of scope is to copy the data into the vtkm::cont::ArrayHandle
.
Simply make the vtkm::CopyFlag
argument be vtkm::CopyFlag::On
to copy the data.
This solution is shown in Example 3.8, line 22.
What if you have a std::vector
that you want to pass to an vtkm::cont::ArrayHandle
and then want to only use in the vtkm::cont::ArrayHandle
?
In this case, it is wasteful to have to copy the data, but you also do not want to be responsible for keeping the std::vector
in scope.
To handle this, there is a special vtkm::cont::make_ArrayHandleMove()
that will move the memory out of the std::vector
and into the vtkm::cont::ArrayHandle
.
vtkm::cont::make_ArrayHandleMove()
takes an “rvalue” version of a std::vector
.
To create an “rvalue”, use the std::move
function provided by C++.
Once vtkm::cont::make_ArrayHandleMove()
is called, the provided std::vector
becomes invalid and any further access to it is undefined.
This solution is shown in :exlineref:ex:ArrayOutOfScope:MoveVector`.
-
template<typename T, typename Allocator>
vtkm::cont::ArrayHandleBasic<T> vtkm::cont::make_ArrayHandleMove(std::vector<T, Allocator> &&array) Move an std::vector into an ArrayHandle.
-
template<typename T, typename Allocator>
vtkm::cont::ArrayHandleBasic<T> vtkm::cont::make_ArrayHandle(std::vector<T, Allocator> &&array, vtkm::CopyFlag) Move an std::vector into an ArrayHandle.
3.2.2. Deep Array Copies
As stated previously, an vtkm::cont::ArrayHandle
object behaves as a smart pointer that copies references to the data without copying the data itself.
This is clearly faster and more memory efficient than making copies of the data itself and usually the behavior desired.
However, it is sometimes the case that you need to make a separate copy of the data.
The easiest way to copy an vtkm::cont::ArrayHandle
is to use the vtkm::cont::ArrayHandle::DeepCopyFrom()
method.
1 destArray.DeepCopyFrom(srcArray);
However, the vtkm::cont::ArrayHandle::DeepCopyFrom()
method only works if the two vtkm::cont::ArrayHandle
objects are the exact same type.
To simplify copying the data between vtkm::cont::ArrayHandle
objects of different types, VTK‑m comes with the vtkm::cont::ArrayCopy()
convenience function defined in vtkm/cont/ArrayCopy.h
.
vtkm::cont::ArrayCopy()
takes the array to copy from (the source) as its first argument and the array to copy to (the destination) as its second argument.
The destination array will be properly reallocated to the correct size.
1 vtkm::cont::ArrayCopy(srcArray, destArray);
-
template<typename SourceArrayType, typename DestArrayType>
inline void vtkm::cont::ArrayCopy(const SourceArrayType &source, DestArrayType &destination) Does a deep copy from one array to another array.
Given a source
ArrayHandle
and a destinationArrayHandle
, this function allocates the destinationArrayHandle
to the correct size and deeply copies all the values from the source to the destination.This method will attempt to copy the data using the device that the input data is already valid on. If the input data is only valid in the control environment, the runtime device tracker is used to try to find another device.
This should work on some non-writable array handles as well, as long as both source and destination are the same type.
This version of array copy uses a precompiled version of copy that is efficient for most standard memory layouts. However, there are some types of fancy
ArrayHandle
that cannot be handled directly, and the fallback for these arrays can be slow. If you see a warning in the log about an inefficient memory copy when extracting a component, pay heed and look for a different way to copy the data (perhaps usingArrayCopyDevice
).
-
template<typename SourceArrayType>
inline void vtkm::cont::ArrayCopy(const SourceArrayType &source, vtkm::cont::UnknownArrayHandle &destination) Does a deep copy from one array to another array.
Given a source
ArrayHandle
and a destinationArrayHandle
, this function allocates the destinationArrayHandle
to the correct size and deeply copies all the values from the source to the destination.This method will attempt to copy the data using the device that the input data is already valid on. If the input data is only valid in the control environment, the runtime device tracker is used to try to find another device.
This should work on some non-writable array handles as well, as long as both source and destination are the same type.
This version of array copy uses a precompiled version of copy that is efficient for most standard memory layouts. However, there are some types of fancy
ArrayHandle
that cannot be handled directly, and the fallback for these arrays can be slow. If you see a warning in the log about an inefficient memory copy when extracting a component, pay heed and look for a different way to copy the data (perhaps usingArrayCopyDevice
).
3.2.4. Mutability
One subtle feature of vtkm::cont::ArrayHandle
is that the class is, in principle, a pointer to an array pointer.
This means that the data in an vtkm::cont::ArrayHandle
is always mutable even if the class is declared const
.
You can change the contents of “constant” arrays via methods like vtkm::cont::ArrayHandle::WritePortal()
and vtkm::cont::ArrayHandle::PrepareForOutput()
.
It is even possible to change the underlying array allocation with methods like vtkm::cont::ArrayHandle::Allocate()
and vtkm::cont::ArrayHandle::ReleaseResources()
.
The upshot is that you can (sometimes) pass output arrays as constant vtkm::cont::ArrayHandle
references.
So if a constant vtkm::cont::ArrayHandle
can have its contents modified, what is the difference between a constant reference and a non-constant reference?
The difference is that the constant reference can change the array’s content, but not the array itself.
Basically, this means that you cannot perform shallow copies into a const
vtkm::cont::ArrayHandle
.
This can be a pretty big limitation, and many of VTK‑m’s internal device algorithms still require non-constant references for outputs.