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You need to use fstream for input / output files. The syntax is simple EX:

#include <fstream>
// ...
std::ofstream flux ; // to open a file in ouput mode
flux.open("myfile.whatever") ; 

Or

#include <fstream>
// ...
std::ifstream flux ; // open a file in input mode
flux.open("myfile.whatever") ;

Other actions are possible on your file: append, binary, trunc, etc. You would use the same syntax as above instead we put std::ios::(flags), for instance:

• ios::out for output operation
• ios::in for input operation
• ios::binary for binary (raw byte) IO operation, instead of character-based
• ios::app for to start to write at the end of the file
• ios::trunc for if the file already exist replace delete the old content and replace by new
• ios::ate - position the file pointer "at the end" for input/output

Ex:

#include <fstream>
// ...
std::ifstream flux ;
flux.open("myfile.whatever" , ios::binary) ;

Here is a more complete but simple example.

#include <iostream>
#include <fstream>

using namespace std ;

int input ;
int New_Apple ;
int Apple_Instock ;
int Eat_Apple ;
int Apple ;

int  main()
{
  bool shouldQuit = false;
  New_Apple = 0 ;
  Apple_Instock = 0 ;
  Eat_Apple = 0 ;
  
  while( !shouldQuit )
  {
    cout << "------------------------------------- /n";
    cout << "1) add some apple " << endl ;
    cout << "2) check apple in stock " << endl ;
    cout << "3) eat some apple " << endl ;
    cout << "4) quit " << endl ;
    cout << "------------------------------------- /n";
    cin >> input ;
    
    switch (input)
    {
      case 1 :
      {
        system("cls") ;
      
        cout << "------------------------------------ /n";
        cout << " how much apple do you want to add /n";
        cout << "------------------------------------ /n";      
        cin >> New_Apple ;
        
        ifstream apple_checker ;
        apple_checker.open("apple.apl") ;
        apple_checker >> Apple_Instock ;
        apple_checker.close() ; 
        
        Apple = New_Apple + Apple_Instock ;
        
        ofstream apple_adder ;
        apple_adder.open("apple.apl") ;
        apple_adder << Apple ;
        apple_adder.close() ;
        
        cout << "------------------------------------ /n";
        cout << New_Apple << " Apple has been added ! /n";
        cout << "------------------------------------ /n";
        break;
      }
    
      case 2 :  
      {
        system("cls") ;
         
        ifstream apple_checker ;
        apple_checker.open("apple.apl") ;
        apple_checker >> Apple_Instock ;
        apple_checker.close() ;
        
        cout << "------------------------------------ /n";
        cout << " there is " << Apple_Instock ;
        cout << "apple in stock /n" ;
        cout << "------------------------------------ /n";
        break;
      }
       
      case 3 :
      {
        system("cls") ;
         
        cout << "------------------------------------ /n";
        cout << "How many apple do you want to eat /n" ;
        cout << "------------------------------------ /n";
        cin >> Eat_Apple ;
        
        ifstream apple_checker ;
        apple_checker.open("apple.apl") ;
        apple_checker >> Apple_Instock ;
        apple_checker.close() ;
        
        Apple = Apple_Instock - Eat_Apple ; 
        
        ofstream apple_eater ;
        apple_eater.open("apple.apl") ;
        apple_eater << Apple ;
        apple_eater.close() ;
        
        cout << "----------------------------------- /n";
        cout << Eat_Apple ;
        cout << " Apple has been eated! /n";
        cout << "----------------------------------- /n";
        cout << Apple << " Apple left in stock /n";
        cout << "----------------------------------- /n";
        break;
      }
      
      case 4 :
      {
        shouldQuit = true;
        break;
      }
      
      default :
      {
        system("cls") ;
         
        cout << "------------------------------------ /n";
        cout << " invalide choice ! /n";
        cout << "------------------------------------ /n"; 
        break;
      }
    }
  }
  return 0;
}