[vtkusers] Accessing keys in vtkInformation without having key pointers at first

[Sunrise]

I would like to find /search for keys appended in vtkInformation object
and then access their values. But, it seems that if we do not have key
pointers, we can not inquiry the vtkInformaton that which key-pairs it
contains!

Assume I have vtkInformation *inputInfo object in Filter2, that coming
from previous filter (Filter1) in a pipeline. Filter1 added some
key/value pairs of type vtkInformationDoubleVectorkey into outputInfo
object. Now, in the new filter (Filter2) I want to access DoubleVector key.

In Filter1 header, a key is defined with a static function:
static vtkInformationDoubleVectorKey * MyKey();

key-value pair is appended to outputInfo with:
outputInfo->Set(Filter1::MyKey(),someValues,someLength);

So in Filter2 by calling Filter1::MyKey(), I can access the key values via:
values = inputInfo->Get(Filter1::MyKey());

But, the problem is that Filter1 could be one of more general filters
that create their own key. Filter2 is not restricted to be connected
just to Filter1, downstream of Filter2 can be FilterX. So I do not know
which FilterX generated key for DoubleVector type in inputInfo. "All the
Filter2 should know is that in inputInfo there is a key-value pair with
DoubleVector type".

Also I don't want to create a superclass for my filters and generate
keys at there (something that vtkStreamingDemandDrivenPipeline does for
its keys).

I looked at this thread

http://vtk.1045678.n5.nabble.com/vtkInformation-access-with-equivalent-keys-td5545920.html

to figure out how can I access keys with other similar key-types. The
answer suggests to use CopyEntries method. If you look at source code
for vtkInformation::CopyEntries(...), you will see that keys in info
should be know again in order to use CopyEntries.

Any idea would be appreciated.