[cpp] Null Check Interfaces

[Aidan63]

Adds a null check to the interface object before we start casting, dereferencing, and doing anything which will cause an access violation.

interface IFoo {
    function foo():Void;
}

function main() {
    final o : IFoo = null;

    try {
        o.foo();
    catch (exn:Any) {
        trace(exn);
    }

    o.foo();
}

With the current behaviour null interface access cannot be caught and you will not get stack traces, adding the null check means it can now be caught and you will get stack traces. The used NullReference function is also used by the hxcpp debugger, so null interfaces will no longer crash the debugger.

[rainyt]

Very nice, i will try it.

[rainyt]

Is there any way for me to directly download the pre constructed Mac executable file? I encountered difficulties constructing haxe from the code source.

[rainyt]

OK, I found it, thank you

[rainyt]

I cannot directly obtain version 4.3.5, it seems that I can only construct from the source.

[Simn]

Is there a meaningful test we could add for this?

[kLabz]

Is there a meaningful test we could add for this?

Yeah, snippet in PR description segfaults without this fix, and exception can now be caught

[Aidan63]

I've added a test to make sure accessing a null interface does throw.

Also updated the generated code a bit so the null check is guarded behind the HXCPP_CHECK_POINTER define. For reference this is the null checking code when accessing class objects.

      #ifdef HXCPP_CHECK_POINTER
      if (!mPtr) NullReference("Object", true);
      // The handler might have fixed up the null value
      if (!mPtr) NullReference("Object", false);
      #ifdef HXCPP_GC_CHECK_POINTER
         GCCheckPointer(mPtr);
      #endif
      #endif
      return mPtr;

I have not done the double NullReference call as that boolean parameter "fixup" isn't used so I'm guessing its a hold over from a old implementation.