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ATLAS Offline Software
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ATLAS Offline Software
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a "variant" ContainerId More...
#include <VariantDef.h>
Public Types | |
| using | xAODObjectIdType = typename CIBase::xAODObjectIdType |
| the xAOD type to use with ObjectId More... | |
| using | xAODObjectRangeType = typename CIBase::xAODObjectRangeType |
| the xAOD type to use with ObjectRange More... | |
| using | xAODElementLinkType = typename CIBase::xAODElementLinkType |
| the xAOD type to use with ElementLink More... | |
| using | baseId = CIBase |
| using | mpCIList = boost::mp11::mp_list< CIList... > |
Static Public Member Functions | |
| template<ContainerIdConcept CI> | |
| static constexpr bool | isValidContainer () |
| template<ContainerIdConcept CI> | |
| static constexpr unsigned | getVariantIndex () |
| get the index of the given container in the list More... | |
Static Public Attributes | |
| static constexpr bool | isContainerId = true |
| identify this as a container id definition More... | |
| static constexpr bool | isMutable = CIBase::isMutable |
| whether this is a non-const container More... | |
| static constexpr bool | perEventRange = false |
| whether this can be retrieved as a range per event More... | |
| static constexpr bool | perEventId = false |
| whether this can be retrieved as a single object per event More... | |
| static constexpr std::size_t | numVariants = sizeof...(CIList) |
| static constexpr std::array< std::string_view, numVariants > | idNameArray = {CIList::idName...} |
a "variant" ContainerId
A "variant" ContainerId (CI) in this context means that an object id or link can refer to an object in one of several containers. This is contrasted with regular object ids or links, which are only ever associated with a single container. If you are wondering about the name, I named it after std::variant, as it seems like a similar concept.
While it may seem very convenient to be able to refer to more than a single container, it is actually a non-trivial thing to achieve in columnar mode, and should only be used when that capability is needed and can't be reasonably avoided. In practice that means use it when otherwise you would have to implement similar functionality in your own code.
This also only works with a fixed list of containers, not an open-ended list. As such you have to pass in a CIList... of container ids you intend to use with it. Further you need to pass a CIBase that isn't a possible variant container, but is used to define a common base class in xAOD mode. If you also want to use it as a possible "variant" container id, you need to list it twice.
Note that it is generally possible for variant links to refer to containers not in the list, but variant (non-optional) object ids are guaranteed to refer to a valid object in one of the containers.
It is possible to go from a variant id/link to a regular id, but due to the way it is implemented there are slight differences between modes: In xAOD mode it is based on dynamic_cast of underlying objects, whereas in columnar mode it is based on unique ids for each container. In practice that means that if you have e.g. multiple track containers in your variant you can (and have to) distinguish them in columnar mode, whereas in xAOD mode they will show up as belonging to each of the containers.
Note that in columnar mode if you are only interested in a single variant for a variant link, it is probably more efficient to query the link directly for the specific variant (as opposed to converting it to a variant object id first). However, the impact of this hasn't been benchmarked, so it is unclear whether it provides a significant performance benefit.
In xAOD mode the object ids are represented as pointers to the base class (as identified via CIBase), and links are represented by a pointer to the proper ElementLink. Implementation-wise this is mostly pass through in xAOD mode, with no overhead compared to "regular" containers.
In columnar mode the object ids are represented as a pair of integers (and a data pointer), as opposed to the single integer for the regular columnar object ids. That second integer identifies the container the object belongs to, and is essentially just the index in the variant list.
Object links in columnar mode are a bit more complex, but at the heart they are still a pair of integers. However, in the persistent format the integer identifying the container can not be expected to correspond to the index in the variant list. Instead it is assumed to be a key that is unique for each container, and then there is another data-vector that holds the unique key for each "variant" container in order.
Variant accessors in columnar mode essentially just create N accessors internally, one for each variant container. This is necessary because the accessors are linked to the specific container they operate on. However that also means that the number of accessors can quickly balloon if a variant container has a large number of variants and accessors.
Definition at line 97 of file VariantDef.h.
| using columnar::VariantContainerId< CIBase, CIList >::baseId = CIBase |
Definition at line 126 of file VariantDef.h.
| using columnar::VariantContainerId< CIBase, CIList >::mpCIList = boost::mp11::mp_list<CIList...> |
Definition at line 146 of file VariantDef.h.
| using columnar::VariantContainerId< CIBase, CIList >::xAODElementLinkType = typename CIBase::xAODElementLinkType |
the xAOD type to use with ElementLink
Definition at line 120 of file VariantDef.h.
| using columnar::VariantContainerId< CIBase, CIList >::xAODObjectIdType = typename CIBase::xAODObjectIdType |
the xAOD type to use with ObjectId
Definition at line 114 of file VariantDef.h.
| using columnar::VariantContainerId< CIBase, CIList >::xAODObjectRangeType = typename CIBase::xAODObjectRangeType |
the xAOD type to use with ObjectRange
Definition at line 117 of file VariantDef.h.
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inlinestaticconstexpr |
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inlinestaticconstexpr |
Definition at line 133 of file VariantDef.h.
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staticconstexpr |
Definition at line 130 of file VariantDef.h.
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staticconstexpr |
identify this as a container id definition
Definition at line 102 of file VariantDef.h.
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staticconstexpr |
whether this is a non-const container
Definition at line 105 of file VariantDef.h.
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staticconstexpr |
Definition at line 128 of file VariantDef.h.
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staticconstexpr |
whether this can be retrieved as a single object per event
Definition at line 111 of file VariantDef.h.
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staticconstexpr |
whether this can be retrieved as a range per event
Definition at line 108 of file VariantDef.h.
1.8.18