IntrusivePtr.weak

static struct Foo{
    ControlBlock!(int, int) c;
    int i;

    this(int i)pure nothrow @safe @nogc{
        this.i = i;
    }
}

IntrusivePtr!Foo x = IntrusivePtr!Foo.make(123);
assert(x.useCount == 1);

auto wx = x.weak;   //weak pointer
assert(wx.expired == false);
assert(wx.lock.get.i == 123);
assert(wx.useCount == 1);

x = null;
assert(wx.expired == true);
assert(wx.useCount == 0);