@safe ref counted pointer

[Stanislav Blinov]

On Wednesday, 12 January 2022 at 21:17:13 UTC, vit wrote:
> Hello,
> I want implement @safe ref counted pointer (similar to 
> std::shared_ptr).
> Problem is that access to managed data of ref counted pointer 
> is inherently unsafe because ref counted pointer can be 
> released (managed data is destroyed) and scope 
> reference/pointer can be still on the stack:
> Example:
>
> ```d
>
> import std.typecons;
>
> scope rc = RefCounted!int(5);
>
> (scope ref int data){
>     rc = RefCounted!int(42);
>
>     data = -1; ///dangling reference!
> }(rc);
>
> ```
>
> It look like i must choose from 2 options:
>
> 1) Method which can release managed data or move ownership of 
> managed data are @safe but methods accessing managed data must 
> be @system.
>
> 2) Method which can release managed data or move ownership of 
> managed data are  @system but methods accessing managed data 
> can be @safe

3) Leave `get()` `@system` and simply make a `@safe` caller:

```d
struct SharedPtr(T)
{
     // ...
     auto apply(Dg)(scope Dg dg)
     if (is(typeof(dg(typeof(this).init.get))))
     {
         auto tmp = this; // borrow another reference
         return dg(ref () @trusted { return get(); } ());
     }
     // ...
}

void main() @safe
{
     scope rc = SharedPtr!int(5);
     rc.apply((scope ref data) {
         rc = SharedPtr!int(42);
         data = -1; // ok
     });
}

```