Make DataReader::read() const

[t0ny-peng]

I have a const class function that checks if a DataReader member has any data ready to be received. The problem is, DataReader::read() itself is not const.

https://github.com/eclipse-cyclonedds/cyclonedds-cxx/blob/master/src/ddscxx/include/dds/sub/detail/DataReader.hpp#L74

I understand that take will modify the internal state of the buffer, but since read returns a view and won't affect the counter, would it make sense to mark it as const?

Thanks

[eboasson]

Hi @t0ny-peng, unfortunately read too modifies the state of the history cache: it changes the "sample state" and "view state" for the samples/instances in the result. I think that's enough to not be able to mark it const.

It did remind me that I've wanted a peek operation for a long time, one that has no side-effects on the reader or its history cache whatsoever. I've added it in the C API (see cyclonedds#1842); adding it to the C++ API should be trivial (just another set of functions like for read/take) but not something I can do right now, but maybe someone else (or you yourself) can do it.