Reading and Writing to Binary file

Question

#include <iostream>
#include <fstream>
#include <cstring>




struct Data {
  char *Hold1;
  char *Hold2;
};



int main() {
  
//Variables
  int choice , loop = 0, RecAmount = 0;
  std::string DepName, output1;
  double inp2, output2;

////File Check
   std::fstream inventory;
    inventory.open("inventory.dat", std::ios::out|std::ios::in|std::ios::binary );  
    if (inventory.fail()){
      inventory.open("inventory.dat", std::ios::out|std::ios::in | std::ios::binary|           
      std::ios::trunc);
      std::cout << "Error opening file....";
        return 0; }

  while (loop < 1) {
    std::cout << "\nRead/write testing\n";
    std::cout << "1. Input Data\n";
    std::cout << "2. Read Data Output\n";
    std::cin >> choice;

////Input 2 strings
    if (choice == 1) {
      std::cout << "Testing input\n";

      struct Data test1;
      test1.Hold1 = "First test";
      test1.Hold2 = 29;

      
     
      }

    if (choice == 2) {
      std::cout << "Testing output\n";
      
      //// Add way to choose which data is read and displayed
      inventory.read ((char *)& inp2, sizeof(Data));

      

      std::cout << "Character input = " << output1 << std::endl;
    }
  }
}

I don't know how to access or use Binary files. I need to:

• Write numbers and character arrays to a binary file (named "inventory.dat" in this)
• Be able to seek and read back chosen information (say I input 3 sets of data and want to only display the 2nd set)
• Overwrite any chosen data (find the second set of data and change it)

I've tried looking up info before and couldn't find anything I could understand or use so any help would be greatly appreciated.

Answer 1

Here's an example using a char array. This is not quite complete but should get you started in the right direction. If definitely needs more error checking.

const int BUFFER_SIZE = 256;

// A FIXED SIZE data structure
struct data {
    char string1[BUFFER_SIZE];
    char string2[BUFFER_SIZE];
    int i1;
    double d1;
};

if (choice == 1) {
    data test;
    std::cin >> test.string1;
    std::cin >> test.string2;
    std::cin >> test.i1;
    std::cin >> test.d1;
    // Append to end of file
    inventory.seekg(0, std::ios_base::end);
    inventory.write(reinterpret_cast<const char *>(&test), sizeof(data));
}
else {
    int recordNumber;
    std::cout << "Enter record number to read: ";
    std::cin >> recordNumber;
    data test;
    // Jump to record.
    // Ex: user enters "5". To get the file pointer to point to record #5
    // seek from the beginning of the file to 4 * sizeof(a single record)
    if (inventory.seekg((recordNumber - 1) * sizeof(test))) {
        inventory.read(reinterpret_cast<char *>(&test), sizeof(test));
        std::cout << test.string1 << ','
                  << test.string2 << ','
                  << test.i1 << ','
                  << test.d1;

    }
}