• berni44 (38/38) Nov 11 2019 While debugging phobos I came across some stuff I don't
• Adam D. Ruppe (11/12) Nov 11 2019 The .ptr method is a pointer to the data associated with the

berni44 <dlang d-ecke.de> writes:

While debugging phobos I came across some stuff I don't 
understand. A small example:

void foo(void* p)
{
     Object o = cast(Object) p;
     ClassInfo oc = typeid(o);
}

class Bar
{
     void some_func(int i) {}

     void do_something(void delegate(int) d)
     {
         // is it possible to check here, if d.ptr can be passed 
to foo?
         foo(d.ptr);
     }
}

void main()
{
     auto b = new Bar();

     b.do_something(&b.some_func); // OK
     b.do_something(i => b.some_func(i)); // crashes
}

While the first call to do_something succeeds, the second one 
crashes. That's probably, because a closure is not an 'Object' 
and the cast in foo should never happen (a normal delegate seems 
to be an Object somehow, though). Is there any chance to notice 
this in "do_something" and emergency-exit there (e.g. with some 
assert call)?

Background of the question: Issue 9603 [1] reports some crashes 
when using std.signals. I meanwhile found out, that the crash 
happens inside "_d_toObject", which is a function in the runtime 
[2]. The documentation tells, that this function crashes if the 
pointer does not point to a class, an interface or is the null 
pointer. It would be nice to avoid the crash by some check before 
the call to _d_toObject.

[1] https://issues.dlang.org/show_bug.cgi?id=9603
[2] https://dlang.org/library/rt/cast_/_d_to_object.html

Adam D. Ruppe <destructionator gmail.com> writes:

On Monday, 11 November 2019 at 13:10:44 UTC, berni44 wrote:
 (a normal delegate seems to be an Object somehow, though).

The .ptr method is a pointer to the data associated with the 
delegate. It might be an object (class or struct) or it might be 
a memory block (for local variables  on the stack or a closure).

There is no way to tell the difference between these in D, even 
at runtime. The .ptr member drops all source information.

The code should probably be rewritten to avoid inspecting the 
.ptr member entirely. Maybe have it wrap the function internally 
- right at the point when it receives the delegate or write a 
whole new function that takes object and method separately - or 
something.