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