Re: [Boost-users] comparing function objects

Subject: Re: [Boost-users] comparing function objects
From: Noah Roberts (roberts.noah_at_[hidden])
Date: 2008-09-24 18:43:10

Noah Roberts wrote:
> "Function object wrappers can be compared via ==  or != against any 
> function object that can be stored within the wrapper. If the function 
> object wrapper contains a function object of that type, it will be 
> compared against the given function object (which must be either be 
> EqualityComparable or have an overloaded boost::function_equal)."
> 
> But how do I compare two boost::functions?  This code doesn't work:
> 
> #include <boost/function.hpp>
> #include <boost/bind.hpp>
> 
> double f(double)
> {
>   return 42.0;
> }
> 
> int main()
> {
>   boost::function<double(double)> f1 = boost::bind(&f, _1);
>   boost::function<double(double)> f2 = boost::bind(&f, _1);
> 
>   //if (f1 == f2)
>   if (boost::function_equal<boost::function<double(double)>, 
> boost::function<double(double)> >(f1,f2))
>    ;
> }
> 
> NOTE: tried to specifically instantiate the function_equal template as 
> attempt3.  Previous attempt didn't do that...it also did not work.

Oh, and the errors from mingw:

g++ -Ie:\boost\include -O0 -g3 -Wall -c -fmessage-length=0 -omain.o 
..\main.cpp
e:/boost/include/boost/function_equal.hpp: In function `bool 
boost::function_equal_impl(const F&, const G&, long int) [with F = 
boost::function<double ()(double)>, G = boost::function<double 
()(double)>]':
e:/boost/include/boost/function_equal.hpp:24:   instantiated from `bool 
boost::function_equal(const F&, const G&) [with F = 
boost::function<double ()(double)>, G = boost::function<double ()(double)>]'
..\main.cpp:15:   instantiated from here
e:/boost/include/boost/function_equal.hpp:17: error: ambiguous overload 
for 'operator==' in 'f == g'
e:/boost/include/boost/function_equal.hpp:17: note: candidates are: 
operator==(void (boost::function1<double, double>::dummy::*)(), void 
(boost::function1<double, double>::dummy::*)()) <built-in>
e:/boost/include/boost/function/function_base.hpp:739: note: 
      typename boost::enable_if_c< boost::type_traits::ice_not< 
boost::is_integral<T>::value>::value, bool>::type 
boost::operator==(const boost::function_base&, Functor) [with Functor = 
boost::function<double ()(double)>]
e:/boost/include/boost/function/function_base.hpp:748: note: 
      typename boost::enable_if_c< boost::type_traits::ice_not< 
boost::is_integral<T>::value>::value, bool>::type 
boost::operator==(Functor, const boost::function_base&) [with Functor = 
boost::function<double ()(double)>]
e:/boost/include/boost/function/function_template.hpp:826: note: 
          void boost::operator==(const boost::function1<R, T0>&, const 
boost::function1<R, T0>&) [with R = double, T0 = double]
Build error occurred, build is stopped
Time consumed: 781  ms.