• Yuxuan Shui (27/50) Jan 05 2017 I was trying to read DIP1000, and I've made about half way

Yuxuan Shui <yshuiv7 gmail.com> writes:

I was trying to read DIP1000, and I've made about half way 
through it. I found it confusing/hard to follow, and sometimes 
inconsistent. So I want to ask for some clarification. Help would 
be appreciated.


 A garbage collected language is inherently memory safe.

I think in order for this sentence to make sense, we need to 
define what is a "garbage collected language". Does any language 
that supports GC count? Because there's Boehm GC for C. Does D 
count? Because dmd has this compiler switch: -boundscheck=off, 
which clearly makes the language not memory safe.

 [...]


 We also define lifetime for each value, which is the extent 
 during which a value can be safely used.

OK.

 * For an unrestricted pointer, [...] lifetime is dictated by 
 the lifetime of the data to which the pointer points to.

Why? The pointer itself can be used safely in its lexical scope 
just like a value type. It's *pointer which has the lifetime of 
the data. Maybe 'using' a pointer means dereferencing it?

 [...]

 expression	lifetime	notes
 *e		∞		Lifetime is not transitive

Huh? I guess this is true if *e is a value type, since then it 
would be copied. But what about:

int a;
int *b = &a;
int **c = &b;

*c = b so lifetime(*c) = lifetime(b), right? And then as per the 
example given

 if (...) {
    int x;
    p = &x; // lifetime(p) is now equal to lifetime(x)
 }

lifetime(b) = lifetime(a). So lifetime(*c) = lifetime(a) != ∞.

Maybe that example is poorly written? Because:


 [...]
     scope int* a = &global_var; // OK per rule 1, 
 lifetime(&global_var) > lifetime(a)
     a = &global_var;       // OK per rule 1, 
 lifetime(&global_var) > lifetime(a)

As per the previous example. lifetime(a) will become 
lifetime(&global_var) after first assignment. Would it be better 
if we say lifetime(&global_var) > reachability(a)?

BTW, reachability() is defined at the beginning of this DIP, but 
never used afterwards. Why bother defining it?