Understanding RefCounted

[Steven Schveighoffer]

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