• Carlos Santander B. (24/24) Dec 10 2003 I was expecting this to work:
• Antti =?iso-8859-1?Q?Syk=E4ri?= (37/60) Dec 10 2003 I noticed the same thing in C++ -- namely that protected access

"Carlos Santander B." <carlos8294 msn.com> writes:

I was expecting this to work:

----------------a.d----------------
class A { protected int a; }
----------------b.d----------------
import std.c.stdio;
import a;

class B : A {
 void print(A a) {
  printf("%d\n",a.a);  //here
 }
}
----------------c.d----------------
import b;

void main() {
 (new B).print(new A);
}
-----------------------------------

But instead I get "b.d(6): class A member a is not accessible". I'd call it
a bug, but it might be feature. If it is a feature, why is that?

-------------------------
Carlos Santander


---

Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.550 / Virus Database: 342 - Release Date: 2003-12-09

Antti =?iso-8859-1?Q?Syk=E4ri?= <jsykari gamma.hut.fi> writes:

In article <br8cad$2a47$1 digitaldaemon.com>, Carlos Santander B. wrote:
 I was expecting this to work:
 
 ----------------a.d----------------
 class A { protected int a; }
 ----------------b.d----------------
 import std.c.stdio;
 import a;
 
 class B : A {
  void print(A a) {
   printf("%d\n",a.a);  //here
  }
 }
 ----------------c.d----------------
 import b;
 
 void main() {
  (new B).print(new A);
 }
 -----------------------------------
 
 But instead I get "b.d(6): class A member a is not accessible". I'd call it
 a bug, but it might be feature. If it is a feature, why is that?

I noticed the same thing in C++ -- namely that protected access
restrictions can be bypassed only when accessing from the deriving
object itself, not outside it.

Which is kind of inconsistent because private access restrictions are
controlled on class level, instead of the object level. But on the other
hand, it's kind of logical: that the A object in question might not be a
B, or even another type derived from A. Or it could be another type that
does not expect that _anyone_ can access its member a except itself:

class A { protected int a; }

class B : A
{
    void betray(A a)
    {
        a.a = 5;
    }
}

class C : A
{
    void gullible(B traitor)
    {
        a = 1;
        // In good faith we give our destiny into its hands
        traitor.betray(this);
        assert(a == 1); // crash! who changed my A?
    }
}

int main()
{
    B b = new B;
    C c = new C;

    c.gullible(b);
}

(Warning - The code is untested. DMD doesn't work with my Linux box
because it appears to be compiled with an older libc/gcc and I don't
have any older libraries for compatibility)

-Antti