istream::read problem

[hentaiw]

// Use random-access I/O to read specific inventory records 
// from a data file. This program reads the file InvDat.dat, 
// which is created by the example program in the recipe: 
//  
//   Write Unformatted Binary Data to a File 
 
#include <iostream> 
#include <fstream> 
#include <cstdlib> 
 
using namespace std; 
 
// A simple inventory structure. 
struct inventory { 
  char item[20]; 
  int quantity; 
  double cost; 
}; 
 
int main(int argc, char *argv[]) 
{ 
  inventory entry; 
  long record_num; 
 
  if(argc != 2) { 
    cout << "Usage: ShowRecord <record-num>\n"; 
    return 1; 
  } 
 
  // Convert the string representation of the entry 
  // number into a long value. 
  record_num = atol(argv[1]); 
 
  // Confirm that the record number is greater than or 
  // equal to zero. 
  if(record_num < 0) { 
    cout << "Record numbers must be greater than or equal to 0.\n"; 
    return 1; 
  } 
 
  // Open the file for binary input. 
  ifstream fInvDB("InvDat.dat", ios::in | ios::binary); 
 
  // Confirm that the file opened without error. 
  if(!fInvDB) { 
    cout << "Cannot open file.\n"; 
    return 1; 
  } 
 
  // Read and display the entry specified on the command line. 

 // First, seek to the desired record. 
 fInvDB.seekg(sizeof(inventory) * record_num, ios::beg); 
 // Next, read the record. 
 fInvDB.read((char *) &entry, sizeof(inventory)); 
 // Close the file. 
 fInvDB.close(); 
 // Confirm that there were no file errors. 
 if(!fInvDB.good()) { 
   cout << "A file error occurred.\n"; 
   return 1; 
 } 
 // Display the inventory for the specified entry. 
 cout << entry.item << endl; 
 cout << "Quantity on hand: " << entry.quantity; 
 cout << "\nCost: " << entry.cost << endl; 
 return 0; 
}

After going through data to set the pointer in the front of the requested record

fInvDB.seekg(sizeof(inventory) * record_num, ios::beg);

And start reading :

fInvDB.read((char *) &entry, sizeof(inventory));

How does it knows that &entry is referencing to an inventory (a structure) ? Because of the lines :

  // Display the inventory for the specified entry. 
  cout << entry.item << endl; 
  cout << "Quantity on hand: " << entry.quantity; 
  cout << "\nCost: " << entry.cost << endl; 

shows that the program knows entry is a structure,class of some sort.

[Cubbi]

It knows that entry is an inventory because of the line 22, where you told the compiler that entry is an inventory.