How can I call the data type from the function of a class

everyone. I defined a class as well as member function. And now i would like to call the data type(x.dat) imported from outside. how could I do that? Thank you very much in advance.

Sorry, I have no codes. But it would be sth like this:

class abs{
private:
...
public:
...
void function(data){     //here i would like to use the external data  x.dat
...
}
}