递归算法(Recursive Algorithm)

 您必须使用反向参数顺序将运算符*声明为非成员函数（外部类），并从中调用另一个 
 
template<typename T>
Vector3d<T> operator*(T& s, const Vector3d<T> &v)
{
    return Vector3d(v.v[0] * s, v.v[1] * s, v.v[2] * s);
} 
template<typename T>
Vector3d<T> operator*(const Vector3d<T> &v, T& s)
{
    return s * v; //call the other overload
} 
 
 并且不要忘记指定模板参数： 
 
Vector3d<T>
        ^^^
 
 还有一个问题......为什么要采用T&不是const T&或T ？ 在当前形式中，您将阻止rvalues传递。 例如，这不会编译： 
 
Vector3d<int> v;
v*3; //3 isn't an lvalue, cannot bind to a nonconst reference

You must declare your operator* as a nonmember function (outside class) with inverse argument order and call the other one from it
 
template<typename T>
Vector3d<T> operator*(T& s, const Vector3d<T> &v)
{
    return Vector3d(v.v[0] * s, v.v[1] * s, v.v[2] * s);
} 
template<typename T>
Vector3d<T> operator*(const Vector3d<T> &v, T& s)
{
    return s * v; //call the other overload
} 
 
And don't forget specifying the template parameters:
 
Vector3d<T>
        ^^^
 
One more issue... Why take T& istead of const T& or just T? In current form you're preventing rvalues to be passed. For example, this wouldn't compile:
 
Vector3d<int> v;
v*3; //3 isn't an lvalue, cannot bind to a nonconst reference