Walter Bright wrote:
Steven Schveighoffer wrote:
A pure function also cannot modify any data via its parameters. In
other words, its parameters must be transitively const.
Yes, a strongly pure function must have those traits.
No, that would be a weakly pure one.
You're operating with a different definition. There are actually three
levels. For extra clarity, let's split strongly-pure into two:
immutably pure == all parameters are immutable
const pure == all parameters are const
weakly pure == no access to globals
The interestingly thing is that an immutably pure function can safely
call a weakly-pure function. The only things the weakly pure function
will be able to change, are variables which are local to the immutably
pure functions.
The *only* difference between weakly pure and immutably pure is the
guarantees which are made about the parameters. And this is already
covered by the const modifiers in the function signature. So that
concept doesn't actually need to be part of pure.
Note that this is possible only because immutable and const are
transitive. It wouldn't be possible with head-const, or logical const.
Transitive const gives us an absolutely huge win.
