Douglas Gregor wrote:
On Tuesday 09 March 2004 05:26 pm, Brian McNamara wrote:
In my opinion, lambda(x) { return f(x) == g(x) }. is best. Whenever I see a placeholder, I expect the whole expression (modulo operator precedence and "constant(foo)" issues) to get lambda-ized, and thus I expect operator== to create a lambda expression. (I dunno if this is the 'right' expectation to have, given the way the libraries work now, but I think it is a reasonable/simple 'ideal' expectation.)
I'd think that this is the right expectation for Lambda, but not for FC++ or Bind. Not that I'm helping any :)
No, I don't think so. Expressions that are valid for both Bind and Lambda should have the same semantics. And in fact, this is not just theory: #include <boost/bind.hpp> namespace boost { namespace _bi { struct equal { template<class V, class W> bool operator()(V const & v, W const & w) const { return v == w; } }; template<class R, class F, class L, class A2> bind_t< bool, equal, list2< bind_t<R, F, L>, typename add_value<A2>::type > > operator== (bind_t<R, F, L> const & f, A2 a2) { typedef list2< bind_t<R, F, L>, typename add_value<A2>::type > list_type; return bind_t<bool, equal, list_type> ( equal(), list_type(f, a2) ); } } // namespace _bi } // namespace boost #include <iostream> int f(int x) { return x + x; } long g(int x) { return 2 * x; } int main() { int x = 4; std::cout << ( boost::bind(f, _1) == 8 )(x) << std::endl; std::cout << ( boost::bind(f, _1) == boost::bind(g, _1) )(x) << std::endl; }
More generally, I think it's not a good idea to try to create "function equality" with operator==. I didn't participate in any of the earlier discussions, but I think that if you want to register/unregister function objects with an event handler, for instance, you should use a separate "handle" object to keep track of "function object identity".
The problem is that there are a whole lot of use cases for equality of function objects, and people really want them. The especially want
delegate<void()> f; f += some_function_object; // connect f -= some_function_object; // disconnect
There is no mention of operator== anywhere in the above, though.