Re: [boost] inherited operators

From: Jonathan Wakely (cow_at_[hidden])
Date: 2005-02-17 04:48:18

On Thu, Feb 17, 2005 at 12:48:35AM -0500, Jason Hise wrote:

> Sometimes I like to play around with obscure code, just to see what it 
> does.  The following really surprised me:
> 
> 
> #include <iostream>
> 
> struct A
> {
> public:
>    bool operator == ( const A & rhs )
>    {
>        std::cout << "A\n";
>        return true;
>    }
> };
> 
> struct B
> {
> public:
>    bool operator == ( const B & rhs )
>    {
>        std::cout << "B\n";
>        return false;
>    }
> };
> 
> struct C : public A, public B {  };
> 
> int main (  )
> {
>    C c1, c2;
>    if ( c1 == c2 ) std::cout << "Equal\n";
>    return 0;
> }
> 
> Compiled on MSVC 7.1, this outputs:
> A
> Equal
> 
> I would have expected this code not to compile at all, because the == 
> overload seems ambiguous.  Is this result standards conforming?  If not, 
> what would the standards conforming result be?  Does this fall into the 
> realm of undefined behavior?

No, you're first expectation was right - it's ambiguous, so it's an error.

Comeau's online compiler rejects it, as does every version of GCC I can
lay my hands on.  Seems like a VC++ bug.

GCC says:
ambig.cc: In function 'int main()':
ambig.cc:28: error: request for member 'operator==' is ambiguous
ambig.cc:16: error: candidates are: bool B::operator==(const B&)
ambig.cc:6: error:                 bool A::operator==(const A&)

Comeau says:
"ComeauTest.c", line 28: error: more than one operator "==" matches these operands:
            "C::operator==" (ambiguous by inheritance)
            operand types are: C == C
     if ( c1 == c2 ) std::cout << "Equal\n";

jon

-- 
"For a successful technology, reality must take precedence over public
 relations, for Nature cannot be fooled."
	- Richard P. Feynman