Re: future of the C++

Walter Bright <>
Wed, 7 Jul 2010 14:16:00 CST
Joshua Maurice wrote:

On this question, could someone define "pure" exactly in context
please? The possible counterexample I immediately thought of union
find. A find operation on a forest of rank trees in union find is
"logically const" in that it doesn't change observable state, but it
does greatly mutate internal state to get better performance on
subsequent lookups. operator== on some data structures may do
operations like this to optimize future operations. However, this
reminds me a lot of the mutable keyword as "an exception" to const
correctness. Perhaps a similar concept is needed for pure functions.

Pure functions would not allow "logical const". While logical constness hides
the mutation from the programmer, the mutation is still very much there (as you
said) and the consequences are the function cannot be treated as pure by the
compiler. For example, the usual synchronization issues arise when doing
concurrent programming with such impure functions.

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