Re: The interface's 'in' contract passes if it makes a virtual function call

[Steven Schveighoffer via Digitalmars-d-learn]

On 11/4/14 3:26 PM, Steven Schveighoffer wrote:
On 11/4/14 3:01 PM, Ali Çehreli wrote:
Perhaps I am expecting too much from the current 'in' contract design
and implementation. ;)
Still, the virtual function call in the following interface's 'in'
contract should be dispatched to the implementaion in the derived class,
right?

It seems like mere presence of that virtual function call causes the
'in' contract of the interface succeed and the derived's 'in' contract
never gets called.

import std.stdio;

void main()
{
     /* EXPECTATION: The following call should execute both the
      * base's and the derived's in contracts 50% of the time
      * because the base's contract fails randomly. */
     (new C()).foo();
}

interface I
{
     void foo()
     in {
         writeln("I.foo.in");

         /* This check succeeds without calling virtualCheck! */
         assert(virtualCheck());
     }

     bool virtualCheck();
}

class C : I
{
     void foo()
     in {
         writeln("C.foo.in");
     }
     body
     {}

     bool virtualCheck()
     {
         writeln("C.virtualCheck");

         /* Fail randomly 50% of the time */
         import std.random;
         import std.conv;
         return uniform(0, 2).to!bool;
     }
}

The output has no mention of C.virtualCheck nor C.foo.in:

I.foo.in
           <-- Where is C.virtualCheck?
           <-- Where is C.foo.in?

Ali

This looks like a dmd bug. My theory is that the call to virtualCheck is
going to the WRONG vtbl address. I have seen stuff like this before. It
likely is calling something like toString. You would have to debug to
figure it out.

So what I think happens is it calls the wrong virtual function, which
returns non-zero always, and obviously doesn't print anything, and then
continues on. I added a writeln("after virtual check") to the in
contract of I.foo, and it writes that too.

Yep. I debugged it. It's calling toHash instead.

Proof (the weird casting thing is because I wanted to call writeln from 
toHash, but toHash is nothrow and writeln is not) :

import std.stdio;

void main()
{
    /* EXPECTATION: The following call should execute both the
     *      * base's and the derived's in contracts 50% of the time
     *           * because the base's contract fails randomly. */
    (new C()).foo();
}

interface I
{
    void foo()
        in {
            writeln("I.foo.in");

            /* This check succeeds without calling virtualCheck! */
            assert(this.virtualCheck());
            writeln("after virtual check");
        }

    bool virtualCheck();
}

void printToHash() { writeln("in toHash");}

class C : I
{
    void foo()
        in {
            writeln("C.foo.in");
        }
    body
    {}

    bool virtualCheck()
    {
        writeln("C.virtualCheck");

        /* Fail randomly 50% of the time */
        import std.random;
        import std.conv;
        return uniform(0, 2).to!bool;
    }
    override size_t toHash() @trusted
    {
        auto f = cast(void function() nothrow)&printToHash;
        f();
        return 1;
    }
}


output:

I.foo.in
in toHash
after virtual check

Please report to bugzilla.

-Steve