Overloaded [] operator + template

Hi All,

Can anybody explain how to fix the [] operator in the code below, Visual C++
gives the following error message:

error C2678: binary '[' : no operator found which takes a left-hand operand
of type 'const bool_vector<len>'

I tried a number of code permutations but ended up with more errors (I am a
beginner :-),

Thanks,
Hans

#include <iostream>
using namespace std;

template<int len> class bool_vector;
template<int len> ostream &operator<< (ostream &os, const bool_vector<len>
&v);

template<int len> class bool_vector {
private:
    bool *v;
    int i;
public:
    int sz;
    bool_vector() {
        v=new bool[sz=len];
        for (i=0; i<sz; i++) v[i]=0;
    };
    ~bool_vector() {
        delete [] v;
    };

    friend ostream& operator << <>(ostream& os, const bool_vector<len>&); //
required?

    void write(const string& initstr) {
        for (i=0; i<len; i++) v[i]=initstr[i]-'0';
    }

    bool operator[] (const int& x){
        return v[x];
    }
};

template<int len> ostream& operator << (ostream& os, const
bool_vector<len>& v) {
    for (int i=0;i<v.sz;i++) os << v[i]; // **** Error C2678 ****
    return os;
}

int main()
{
    bool_vector<4> a;
    a.write("1011");
    cout << "Using overloaded ostream " << a << endl;
}