On Monday, 19 November 2018 at 02:39:32 UTC, Stanislav Blinov
wrote:
You're skimming the examples ;)
I'm not saying your examples don't work, I'm trying to understand
the minimum requirements. You said:
"That's [constructors needing to be pure is] only for types with
indirections (pointers), since `pure` guarantees that you do not
mutate any global state."
My example was only to show that:
- a const constructor is insufficient for creating an immutable
struct S
- an immutable or pure constructor is sufficient for creating an
immutable struct S
You also showed that an inout constructor is sufficient too.
I don't understand why not any constructor would work. After all:
"value types are all copyable between mutable/const/immutable. So
even if `return x + y` would yield a `const T`, you can still
instantiate a T from it."
Recall that member functions (including constructors) are just
functions in disguise:
...
what that boils down to, conceptually, is:
I get what inout does know. But continuing on the constructor
lowering, what strikes me is that:
S __ctor(ref const S this, int x) {
this.x = x; // allowed, while `this` is a const(S)!
// return this; automatically inserted
}
Knowing that a constructor may mutate the const(this) or
immutable(this) members, why can't a mutable(this) constructor be
called on an immutable(this), *unless* it is pure (which seems
irrelevant to me)?
