spir: > What is this "old" feature?
It's a basic DbC feature that's currently missing in D because of some implementation troubles (and maybe also because Walter is a bit disappointed about DbC). Example: a class member function performs a certain computation and changes some attributes. In the postcondition you want to test that such changes are correct. To do this well it's useful to know what the original state of those attributes was. This is what the "old" feature allows you to do. Similar things are possible with free functions too. So preconditions in a function allow you to assert that general rules about inputs are fulfilled, postconditions allow you to assert that general rules about its outputs are fulfilled, and the "old" (pre-state) feature allows you to assert that general rules about its input-output pairs are fulfilled. So it's not a small thing :-) It was discussed three or more times: http://www.digitalmars.com/d/archives/digitalmars/D/why_no_old_operator_in_function_postconditions_as_in_Eiffel_54654.html http://www.digitalmars.com/d/archives/digitalmars/D/Communicating_between_in_and_out_contracts_98252.html >I definitely need a "mental jump" to think in terms of DbC; nearly never use >it, simply don't think at it.< DbC looks like a very simple thing, but you need some time, thinking, and reading about what it is and what its purposes are, to learn to use it :-) Bye, bearophile
