How can I define a struct in (*.cpp) instead (*.h) in a class

How can I define a struct in (*.cpp) instead (*.h) in a class

I want to define a struct out of the head file.
Below is what I declare in the (*.h)
class MyVertex
{
public:
struct Vertex{};
};
How can I define the struct Vertex more detailedly in the (*.cpp)? I've
tried several times with several ways but failed.