使用c++ 11 std::enable_if来启用成员函数，如果向量是特定的长度

I am writing a simple vector class and I would like to have some member functions that are only available in vectors of certain lengths (cross product for a 3 element vector for example). I stumbled across std::enable_if and it looks like it may be able to do what I want, but I don't seem to be able to get it working correctly.I am writing a simple vector class and I would