"Matthias Schabel" <boost@schabel-family.org> wrote
For various reasons, it would be really nice to be able to generate a series of unique ordinal values (say integers, but really anything that is easily less_than_comparable) at compile time. A concrete example :
struct system1 : ordinal<0> { }; struct system2 : ordinal<1> { };
etc...
Now, if these tags can be defined in user code in various places, it can be difficult to ensure that there is no duplication of the ordinal values. What I want is something like this:
struct system1 : ordinal<get_next_system_ordinal()> { }; struct system2 : ordinal<get_next_system_ordinal()> { };
where I don't really care what the specific order is, just that the ordinal values are unique... Anyone have a bright idea on how to accomplish this? Preprocessor?
Yes, the preprocessor can be used. VC++ has a __COUNTER__ extension. Also there is BOOST_PP_COUNTER/BOOST_PP_UPDATE_COUNTER()(introduced in 1.34). One problem -- if this used in different headers, the generated numbers depend on the order of includes, which potentially can lead to the ODR violation :-( I once thought that the problem may be solved by compile-time GUID, but for me four numbers instead of one didn't work... but if you want _anything_ less_than_comparable, this might be an option. Regards, Arkadiy