C++ :: Ifstream Selectively Read In Information?

If I have a data file.dat like this, for example:

...
other information
...
physics:
4,5,6,7
...

line:
(4,2), (2,4)

line:
(1,2), (2,3)

...
...

I want to design a class and corresponding code so that every time when it reads "line:" for the file.dat , it will push_back a new line into the line_t vector, and each time when it encounter physics it will put the values to physics, How can I implement this?

every data in file.dat is useful, they need to be read into different class type. How can I implement this?

class line_t {
public:
vector<point_t> P(2, point_t());

[Code].....