I found a strange error when I try the vec3 sample from proto, my code is like this:

struct Vec3SubscriptCtx
: proto::callable_context< Vec3SubscriptCtx const >
{
    typedef int result_type;

    Vec3SubscriptCtx(int i)
      : i_(i)
    {}

    // Index array terminals with our subscript. Everything
    // else will be handled by the default evaluation context.
    int operator ()(proto::tag::terminal, int const (&arr)[3]) const
    {
        return arr[this->i_];
    }

    int i_;
};
struct Vec3
: proto::extends<proto::terminal<int[3]>::type, Vec3>
{
   explicit Vec3(int i=0, int j=0, int k=0)
    {
        (*this)[0] = i;
        (*this)[1] = j;
        (*this)[2] = k;
    }

    int &operator [](int i)
    {
        return proto::value(*this)[i];
    }

    int const &operator [](int i) const
    {
        return proto::value(*this)[i];
    }

    // Here we define a operator = for Vec3 terminals that
    // takes a Vec3 expression.
    template< typename Expr >
    Vec3 &operator =(Expr const & expr)
    {
        typedef Vec3SubscriptCtx const CVec3SubscriptCtx;
        (*this)[0] = proto::eval(proto::as_expr(expr), CVec3SubscriptCtx(0));
        (*this)[1] = proto::eval(proto::as_expr(expr), CVec3SubscriptCtx(1));
        (*this)[2] = proto::eval(proto::as_expr(expr), CVec3SubscriptCtx(2));
        return *this;
    }

    template< typename Expr >
    Vec3 (Expr const & expr)
    {
        typedef Vec3SubscriptCtx const CVec3SubscriptCtx;
        (*this)[0] = proto::eval(proto::as_expr(expr), CVec3SubscriptCtx(0));
        (*this)[1] = proto::eval(proto::as_expr(expr), CVec3SubscriptCtx(1));
        (*this)[2] = proto::eval(proto::as_expr(expr), CVec3SubscriptCtx(2));
    }

    void print() const
    {
        std::cout << '{' << (*this)[0]
                  << ", " << (*this)[1]
                  << ", " << (*this)[2]
                  << '}' << std::endl;
    }
};
int main(int argc, const char* argv[])
{
    Vec3 a(1, 2, 3);
    Vec3 b(3, 2, 1);
    Vec3 c;
    c = b;
    c.print();

    return 0;
}

I expected that when c is printed it will display 3, 2, 1 ( the value of b ) but instead it print 0,0,0 like b is never assigned to c. I try this using vs 2010 with boost 1.44 in windows 7 64.