How to seek and read from binary file?

Question

I'm still new to advance file and structures. I'm just having trouble figuring out how to seek and read specific records from a file. and how to display the information of a specific record through it's record number?

    #include <iostream>
    #include <fstream>
    using namespace std;

struct catalog
{
  char title[50];
  char author[50];
  char publisher[30];
  int yearpublish;
  double price;
};

void showRec(catalog);
long byteNum(int);

int main()
{
  catalog book;

  fstream fbook("book.dat",ios::in|ios::binary);

  if (!fbook)
  {
    cout <<"Error opening file";
    return 0;
  }

  //Seek and read record 2 (the third record)


  showRec(book);  // Display record 2

  //Seek and read record 0 (the first record)


  showRec(book); // Display record 0

  fbook.close();
  return 0;
}

 void showRec(catalog record)
 {
    cout << "Title:" << record.title << endl;
    cout << "Author:" << record.author << endl;
    cout << "Publisher name:" << record.publisher << endl;
    cout << "Year publish:" << record.yearpublish << endl;
    cout << "Price:" << record.price << endl << endl;
 }

long byteNum(int recNum)
{
 return sizeof(catalog) * recNum;
 }

Please Help. Thank you.

Answer 1

The C++ istream methods you need for this are seekg and read.

// Seek and read record 2
fbook.seekg(byteNum(2));
fbook.read((char*)&book, sizeof(book));