#include <vector>
#include <iostream>
#include <ext/functional>
#include <boost/variant.hpp>

using namespace std;
using namespace __gnu_cxx;      // for SGI extensions compose1 and select1st
using namespace boost;

// define dummy visitor class
class dummy
    : public boost::static_visitor<>
{
public:
    template<class T>
    void operator()(const T &x) const
    {
        cout << x << endl;
    }
};

typedef variant<int,double> TMixed;
typedef pair<TMixed,const char*> TPair;

int main()
{
    TPair p=make_pair(0, "zero");

    vector<TPair> v;
    v.push_back( make_pair(1, "int") );
    v.push_back( make_pair(3.4, "double") );

    dummy _dummy;

    for_each( v.begin(), v.end(),
              compose1(apply_visitor(_dummy), select1st<TPair>() ) );  // yields a const error

    // the const error comes from here
    compose1(apply_visitor(_dummy), select1st<TPair>() )(p);

    // but this works
    apply_visitor(_dummy)( select1st<TPair>()(p) );

    // again in detail
    const select1st<TPair> g=select1st<TPair>();
    const apply_visitor_delayed_t<dummy> f(_dummy);     // remove the const and
    f( g(p) );                                          // this line works!
}