On Thursday, 26 February 2015 at 20:46:07 UTC, deadalnix wrote:
Consider :
void foo(T** a) {
T** b = a; // OK
T* = ...;
*b = c; // Legal because of your transitive clause,
// but not safe as a can have an
// arbitrary large lifetime.
}
This example's incomplete, but I can guess you meant something
like this:
void foo(T** a) {
T** b = a; // OK
T d;
T* c = &d;
*b = c; // Legal because of your transitive clause,
// but not safe as a can have an
// arbitrary large lifetime.
}
This show that anything you reach through an indirection can
have from the same lifetime as the indirection up to an
infinite lifetime (and anything in between). When using it as
an lvalue, you should consider the largest possible lifetime,
when using it as an rvalue, you should consider the smallest
(this is the only way to be safe).
I'm starting to see what you mean. I guess it's only applicable
to variables with double (or more) indirections (e.g. T**, T***,
etc.), since only they can lose information with transitive
scopes. Looks like we need a new rule: variables assigning to one
of their double indirections cannot acquire a scope-depth greater
than (or lifetime less than) their current one. Does that fix the
problem?
