C++ boost function overloaded template

Question

I cannot figure out why this segment gives unresolved overloaded function error (gcc version 4.3.4 (Debian 4.3.4-6)):

#include <algorithm>
#include <boost/function.hpp>

// this does not work
int main1()
{
    typedef boost::function<const int&(const int&, const int&)> max;
    max m(&std::max<int>);
}

// this does not work
int main2() {
    typedef boost::function2<const int&, const int&, const int&> max;
    max m(static_cast<max>(&std::max<int>));
}

can you help me, thanks

test.cpp: In function âint main()â:
test.cpp:7: error: no matching function for call to âboost::function2<const int&, const int&, const int&>::function2(<unresolved overloaded function type>)â
/usr/include/boost/function/function_template.hpp:747: note: candidates are: boost::function2<R, T1, T2>::function2(const boost::function2<R, T1, T2>&) [with R = const int&, T0 = const int&\
, T1 = const int&]
/usr/include/boost/function/function_template.hpp:739: note:                 boost::function2<R, T1, T2>::function2(boost::function2<R, T1, T2>::clear_type*) [with R = const int&, T0 = cons\
t int&, T1 = const int&]
/usr/include/boost/function/function_template.hpp:707: note:                 boost::function2<R, T1, T2>::function2() [with R = const int&, T0 = const int&, T1 = const int&]

max/min is defined as

  template<typename _Tp>
    inline const _Tp&
    max(const _Tp& __a, const _Tp& __b)
    {
      // concept requirements
      __glibcxx_function_requires(_LessThanComparableConcept<_Tp>)
      //return  __a < __b ? __b : __a;
      if (__a < __b)
        return __b;
      return __a;
    }

I have tried all sorts of template explicit instantiation but nothing seems to work. Same problem appears with g++ 4.1 but not with ICC

this works

#include <algorithm>
#include <boost/function.hpp>

namespace std_ {
    template<typename _Tp>
    inline const _Tp&
    max(const _Tp& __a, const _Tp& __b)
    {
        // concept requirements
        //return  __a < __b ? __b : __a;
        if (__a < __b)
            return __b;
        return __a;
    }
}

int main()
{
    typedef const int &T;
    typedef boost::function<T(T,T)> min_;
    //typedef const int&(*min_)(const int&, const int&);
    min_ m(::std_::max<int>);
}

and this

#include <algorithm>
#include <boost/function.hpp>

int main()
{
    //typedef const int &T;
    //typedef boost::function<T(T,T)> min_;
    typedef const int&(*min_)(const int&, const int&);
    min_ m(::std::max<int>);
}

Answer 1

To critique the code, there's no reason to static_cast that. Consider all the cast is going to do is use the constructor of boost::function2 to make a new boost::function2, then it will be copy-constructed into m. Just construct directly into m:

#include <algorithm>
#include <boost/function.hpp>

int main()
{
    typedef boost::function2<const int&, const int&, const int&> max;
    max m(&std::max<int>);
}

Lastly, the preferred syntax of boost::function is:

#include <algorithm>
#include <boost/function.hpp>

int main()
{
    typedef boost::function<const int&(const int&, const int&)> max;
    max m(&std::max<int>);
}

The n-ary specific classes are for older compiler support.

Answer 2

It seems to be a problem with the definition of the std::max template function with releases of gcc < 4.4

With gcc-4.4.0 and msvc Express9 it works.

The following works also for gcc-3.4 and gcc-4.3

int main1()
{
    int res = std::max(1,2);
    typedef boost::function<const int&(const int&, const int&)> max;
    max m(static_cast<const int&(*)(const int&, const int&)>(std::max<int>));

    return 0
}