[boost] Virtual function parameter variance

From: Dave (better_cs_now_at_[hidden])
Date: 2004-08-08 02:09:17

Hello all,

Quoting from page 24 of "The Boost Graph Library; User Guide and Reference
Manual":

"It turns out that by the contravariance subtyping rule, the parameter type
in the derived classes member function must be either the same type or a
base class of the type as the parameter in the base class."

Now please consider this code:

#include <iostream>

struct base_1 {};
struct derived_1: base_1 {};

struct base_2
{
   virtual void foo(derived_1 *p) {std::cout << "base_2::foo()\n";}
};

struct derived_2: base_2
{
   virtual void foo(base_1 *p) {std::cout << "derived_2::foo()\n";}
};

int main()
{
   base_2 *ptr = new derived_2;
   ptr->foo(new derived_1);
}

This outputs base_2::foo().  Why?

The quoted passage implies derived_2::foo() should override base_2::foo().
Clearly, it does not.  Furthermore, I cannot find anything in the Standard
that indicates it should.  So, I am clearly misinterpreting the quoted
passage.  Can anybody explain to me what was meant in that passage?

Thank you,
Dave