Sorry trivial questions I'm sure.

Why doesn't the lexical_cast work below?

I think it meets the concept description.

Source is OutputStreamable, meaning that an operator<< is defined that takes a std::ostream or std::wostream object on the left hand side and an instance of the argument type on the right.
Target is InputStreamable, meaning that an operator>> is defined that takes a std::istream or std::wistream object on the left hand side and an instance of the result type on the right.
Target is CopyConstructible [20.1.3].
Target is DefaultConstructible, meaning that it is possible to default-initialize an object of that type [8.5, 20.1.4].

#include <vector>
#include <iostream>
#include <algorithm>

#include <boost/bind.hpp>
#include <boost/lexical_cast.hpp>
#include <boost/filesystem.hpp>
#include <sstream>

using namespace std;
using namespace boost;
using namespace boost::filesystem;

template<class T>
ostream&
operator<< (ostream& os, const vector<T>& v)
{
copy(v.begin(), v.end(), ostream_iterator<T>(os, ";"));
return os;
}

template<class T>
string
v2s(const vector<T>& v)
{
stringstream ss;
copy(v.begin(), v.end(), ostream_iterator<T>(ss, ";"));
return ss.str();
}

int
main(int argc,
char* argv[])
{
vector<string> v;
v.push_back("hello");
v.push_back("world");

cout << v << endl;

cout << v2s(v) << endl;

cout << lexical_cast<string>(v) << endl; // XXX doesn't compile...XXX
return 0;
}

Any help greatly appreciated.