Re: Very confusing error message when calling a class method from an invariant
On Wednesday, 10 March 2021 at 04:57:19 UTC, Paul Backus wrote:
On Wednesday, 10 March 2021 at 03:39:15 UTC, Meta wrote:
class Human {
static immutable MAX_AGE = 122;
bool alive = true;
int age = 0;
//Error: mutable method onlineapp.Human.checkAge is not
callable using a const object
invariant(checkAge());
[...]
What the hell does this even mean, and where does it come
from? Adding `inout` to `checkAge` actually does cause it to
compile and run too. WTF?
From the language spec [1]:
The invariant is in the form of a const member function.
So, inside the invariant, the object is treated as const, which
means you can't modify it and can only call const methods.
[1] https://dlang.org/spec/class.html#invariants
Now that you mention it, I'm pretty sure I've run into this
before; I must've forgotten about it. I understand the rationale
behind this, but it doesn't really make sense IMO that only
invariants treat the object as const, and not pre/post conditions
as well. Ah well. Thanks for quick answer.
Re: Very confusing error message when calling a class method from an invariant
On Wednesday, 10 March 2021 at 03:39:15 UTC, Meta wrote:
class Human {
static immutable MAX_AGE = 122;
bool alive = true;
int age = 0;
//Error: mutable method onlineapp.Human.checkAge is not
callable using a const object
invariant(checkAge());
[...]
What the hell does this even mean, and where does it come from?
Adding `inout` to `checkAge` actually does cause it to compile
and run too. WTF?
From the language spec [1]:
The invariant is in the form of a const member function.
So, inside the invariant, the object is treated as const, which
means you can't modify it and can only call const methods.
[1] https://dlang.org/spec/class.html#invariants
Very confusing error message when calling a class method from an invariant
class Human {
static immutable MAX_AGE = 122;
bool alive = true;
int age = 0;
//Error: mutable method onlineapp.Human.checkAge is not
callable using a const object
invariant(checkAge());
void growOlder()
in(alive)
out(; checkAge())
{
age++;
if (age > MAX_AGE)
die();
}
void die()
in(alive)
out(; !alive) {
alive = false;
}
bool checkAge() {
return age >= 0 && age <= MAX_AGE || !alive;
}
}
void main() {
Human h = new Human();
h.growOlder();
}
What the hell does this even mean, and where does it come from?
Adding `inout` to `checkAge` actually does cause it to compile
and run too. WTF?
