After i edit the code, it become like this:
#include <iostream>
#include <fstream>
#include <stdio.h>

using namespace std;

int main(int argc, char *argv[])

    int x ;
    char y[1000] ;
    char str[10];
    char op [5] ;

    cout << "Wait for input: ";
    // get input, if the input is not "open", wait for another input

    //Creates an instance of ofstream, and opens example.txt
    ofstream a_file ( "G:\\ccc\\example.txt" );
    // Outputs to example.txt through a_file
    cout<<"type some numbers :";
    cin>> x ;
    a_file<<"number :" <<x ;
    cout<<"type some alphabet :";
    cin>> y ;
    a_file<<"alphabet: "<<y ;
     while (true) {
        cin >> op;
        if(strcmp(op, "open")== 0){
                      fopen("g:\\ccc\\example.txt","r"); break;
        else cout << endl << "Hint: open" << endl << "type again:" ;
    // Close the file stream explicitly
    //Opens for reading the file
    ifstream b_file ( "g:\\ccc\\example.txt" );
    //Reads one string from the file
    b_file>> str;
    //Should output 'this'
    cout<< str <<"\n";
    FILE *fp;
    fp=fopen("g:\\ccc\\example.txt", "r");
I am sure that the example.txt with this location:G:\ccc is exists.
But weird why no file open?