how to execute a classes object's method simultaneously?

i'm new to multi threading, while i try to develop an application which processes a file i'm stuck at a point. below is the detailed description. please help.

my application reads a directory and loads all the files into class objects. the File class contains the data of the file along with few operational methods. after loading all[limited by constant] the files into memory i want to execute a method void processFile() for each object. i'm thinking to go to multi threading to improve performance so please tell me a way to achieve this.

my class interface:

class D3v_File{
private:
       string* lines;
       string fname;
       int    fileState;
       int    T_lineCnt;
       int    C_lineCnt;
       static int fileCnt;

public:
      D3v_File(const string& f_name);
      string next_line();
      void load_now();
      void process_file();
      ~D3_File();
};

so in my code some where i have a container

vector < D3v_File > File_Obs;

i want to execute the process_file function of objects in the container File_obs parallel.

suggest me the best approach.