Question

Why this code does not compile:

#include <boost/mpl/vector.hpp>
#include <boost/mpl/for_each.hpp>
#include <iostream>

using namespace std;
using namespace boost;

template <class T>   // specific visitor for type printing
static void print_type(T t)
    {
        std::cout << typeid(T).name() << std::endl;
    }


typedef mpl::vector<int, long, char*> s;
int main ()
{
    mpl::for_each<s>(print_type());
}

I wonder – how to make boost mpl for_each work with free functions from same class?

Answer 1

As stated you need a functor.

The code below includes an additional wrap template that allows the print functor to cope with references.

#include <iostream>
#include <typeinfo>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/for_each.hpp>
#include <boost/mpl/placeholders.hpp>

using namespace std;
using namespace boost;

template <typename T>
struct wrap {};

struct print_type
{
    template< typename T>
    void operator()( wrap<T> ) const
    {
        cout << typeid(T).name() << "\n";
    }
};

typedef mpl::vector<int, long&, char*> s;

int main ()
{
    mpl::for_each<s, wrap<mpl::placeholders::_1> >(print_type());
    return 0;
}

Note: this code based on examples in the book ‘C++ Template Metaprogramming by David Abrahams and Aleksy Gurtovoy