On 5/12/21 1:16 PM, JG wrote:
On Wednesday, 12 May 2021 at 13:38:10 UTC, Steven Schveighoffer wrote:
On 5/12/21 3:28 AM, JG wrote:
Reading the documentation on RefCounted I get the impression that the
following can lead to memory errors. Could someone explain exactly
how that could happen? I suppose that problem would be the call
something to do with front?
```
private struct RefCountedRangeReturnType(R)
{
import std.typecons : RefCounted;
private RefCounted!R r;
auto empty() { return r.refCountedPayload.empty; }
auto front() { return r.refCountedPayload.front; }
void popFront() { r.refCountedPayload.popFront; }
auto save() { return
typeof(this)(RefCounted!R(r.refCountedPayload.save)); }
}
auto refCountedRange(R)(R r)
{
import std.typecons : RefCounted;
return RefCountedRangeReturnType!R(RefCounted!R(r));
}
```
You don't need to access refCountedPayload. RefCounted is supposed to
be like a transparent reference type, and should forward all calls to
the referenced item.
I don't see how you will get memory errors from your code. Maybe you
can elaborate why you think that is?
To be honest I can't see the problem. But the following from the
documentation made me wonder if I was doing something that could lead to
memory problems:
"RefCounted is unsafe and should be used with care. No references to the
payload should be escaped outside the RefCounted object."
In particular I wondered if in some special case holding a reference to
front might cause a problem, but perhaps that is incorrect.
Ah, ok. So reference counting provides a single thing you can point at
and pass around without worrying about memory cleanup. But only as long
as you refer to it strictly through a RefCounted struct. If you keep a
pointer to something in the payload that isn't wrapped in a RefCounted
struct (and specifically the original RefCounted struct), then it's
possible the RefCounted struct will free the memory while you still hold
a reference.
You are returning from front by value, so there shouldn't be a problem
with lifetime issues. However, there are possible exceptions, but they
would be really rare.
As an example of something that would be a bad idea:
```d
struct S
{
int x;
}
int *bad;
{
auto rc = S(1).refCounted;
bad = &rc.x; // escape a reference to the payload
} // here, the scope closes and rc is freed
*bad = 5; // leaving a dangling pointer that can be used
```
-Steve
