Did you send this to the OP ("Dave" <better_cs_now@yahoo.com>)? Jeremy Graham Siek <jsiek@osl.iu.edu> writes:

Hello Dave,

Sorry about the confusion. I should have distinguished between how this works in C++, and what is theoretically possible. The contravariance rule I spoke of describes what makes sense theoretically. However, C++ is more conservative in this case and restricts parameter types of overloads to be the same type. This is a similar issue to covariant return types. Early on, C++ did not allow them, even though it made sense theoretically, then later on covariant return types were allowed.

Best Regards, Jeremy

On 8/8/04 2:09 AM, "Dave" <better_cs_now@yahoo.com> wrote:

...

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

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_______________________________________________ Jeremy Siek <jsiek@osl.iu.edu> http://www.osl.iu.edu/~jsiek Ph.D. Candidate, Indiana University Bloomington C++ Booster (http://www.boost.org) _______________________________________________

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-- Dave Abrahams Boost Consulting http://www.boost-consulting.com

Hello Dave,

Sorry about the confusion. I should have distinguished between how this works in C++, and what is theoretically possible. The contravariance rule I spoke of describes what makes sense theoretically. However, C++ is more conservative in this case and restricts parameter types of overloads to be the same type. This is a similar issue to covariant return types. Early on, C++ did not allow them, even though it made sense theoretically, then later on covariant return types were allowed.

Best Regards, Jeremy

On 8/8/04 2:09 AM, "Dave" <better_cs_now@yahoo.com> wrote:

...

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

OK, I appreciate the clarification! Great lib and book BTW! "Jeremy Graham Siek" <jsiek@osl.iu.edu> wrote in message news:BD3BAC4B.302C%jsiek@osl.iu.edu... 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

_______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost

_______________________________________________ Jeremy Siek <jsiek@osl.iu.edu> http://www.osl.iu.edu/~jsiek Ph.D. Candidate, Indiana University Bloomington C++ Booster (http://www.boost.org) _______________________________________________

_______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost