I think a better example is the "actual time". The actual time, is a constantly changing value, which is read-only.

That is no better example.

Application code, whether Perl or C/C++ or any other language, does not have direct access to the hardware timers that underlie real-time clocks.

Therefore, it makes no sense for an application class or object to have a "actual time" attribute. They may have an actual time method, but that will of necessity make a system call to obtain the value.

Even if the class we were discussing was a kernel system class, an "actual time" method would still not return a direct read of a read-only attribute. Clocks are volatile hardware counters, and they do not count in program usable real-time units directly. They count in something related to bus clocks, which vary from processor to processor, and even with the current energy saving or turbo boost states.

So, any "actual time" method is going to involve some conversion from hardware clock units to real-worlds units. In other words, it is going to be a derived value.