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::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.