Template functor error in g++

Question

I've got some code where I would like to build a vector of elements using the mapped values in a map. The code below works fine in Visual Studio (and seems legit as far as I can tell), but g++ disagrees.

template<class PAIR>
typename PAIR::second_type foo(const PAIR& arg)
{
    return (arg.second);
}

class A
{
private:
    typedef std::map<int, std::wstring> map_t;
    map_t m_map;

public:
    void bar()
    {
        // Attempt to pulled the mapped type from the map into the vector
        std::vector<std::wstring>vect(m_map.size());
        std::transform(m_map.begin(), m_map.end(), vect.begin(),
            &foo<map_t::value_type>);  // <-- error here, see below, also

        // other attempts that all failed:
        // - std::transform(..., boost::bind(foo<map_t::value_type>, _1));
        // - std::transform(..., boost::bind(&map_t::value_type::second, _1));
        // - also tried casting foo to a specific function type
        // - also tried "template<class T> T itself(T arg) { return T; }" applied to all the above functor candidates, a la "std::transform(..., itself(<<functor>>));"
    }
};

Unfortunately, I don't have the exact error text with me (something about not being able to figure out which overloaded function to use) at the moment or the specific version of g++ (the latest being distributed with Ubuntu), but I'll update this post when I get that.

In the meantime, can anyone explain why g++ can't resolve the type of the functor being provided?

Answer 1

The following compiles on my machine:

#include <map>
#include <vector>
#include <algorithm>
#include <string>

template<class PAIR>
typename PAIR::second_type foo(const PAIR& arg)
{
    return (arg.second);
}

class A
{
private:
    typedef std::map<int, std::wstring> map_t;
    map_t m_map;

public:
    void bar()
    {
        std::vector<std::wstring>vect(m_map.size());
        std::transform(m_map.begin(), m_map.end(), vect.begin(),
            &foo<map_t::value_type>);  
    }
};

command line:

g++ -c overt.cpp

version:

$ g++ --version
i686-apple-darwin9-g++-4.0.1 (GCC) 4.0.1 (Apple Inc. build 5490)
Copyright (C) 2005 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.