HistoryBuffer


`HistoryBuffer`
Prev	Chapter 14. Classes from `HistorianSupport.g`	Next

`HistoryBuffer`

HistoryBuffer — a set of HistoryValues returned from a Historian query.

Synopsis

class HistoryBuffer
{
    npoints;
}

Instance Variables

npoints

The number of values (of type HistoryValue) in this data set.

Methods

Result sets from historical queries can be large. Consequently the access methods for a result set come in two types. The first type, named getSomething, will return a reference to the particular data array or item, without making a copy. This reference is only valid until the HistoryBuffer instance is destroyed, which may happen at any time after all references to the HistoryBuffer instance go out of scope.


	If the `HistoryValue` or array returned from the `get` method is used after the `HistoryBuffer` is destroyed, the DataHub instance could crash.

Alternately, you can use the copySomething methods to create duplicates of the data requested. These instances will persist after the HistoryBuffer is destroyed. The copy methods are more robust, but increase the CPU usage and memory usage of large queries. In general you should use the copy methods when you are unsure whether the lifetimes of the HistoryValues will exceed the lifetime of the HistoryBuffer.

copyToArray()

Copy the internal data representation to an array of HistoryValue instances. This method returns the new array. This method is similar to toArray, except that it makes a copy of each HistoryValue instance.

copyValue(index)

Returns a copy of the HistoryValue at the given index within the data set. The index is a number starting at 0 and less then npoints. This is the safe version of the getValue method.

getCount()

Returns the number of values in this data set.

getValue(index)

Returns the HistoryValue at the given index in the data set. This is the unsafe version of copyValue. It will return a reference to the HistoryValue within the HistoryBuffer. This reference will only be valid until the HistoryBuffer is destroyed.

setValue(index, histvalue)

Modifies the original data in the HistoryBuffer at the given index. This method will result in changes to the original data, and to any HistoryValue instances that have been acquired by reference using getValue or toArray. The histvalue argument is an instance of HistoryValue.

toArray()

Returns an array of HistoryValue instances as references to the original data. This is equivalent to creating an array by repeatedly calling getValue. The resulting data is only valid until the HistoryBuffer is destroyed.