Updating information in a binary text file using struct (C++) - c++

Question

I'm writing a program to store inventory data in a binary text file. One of the functions is to search for the name of an item (that was already entered) within the text file, and if found, read the information to a structure, update it, and send the structure back to the file.
The trouble is using seekp/seekg to overwrite the correct data.
I.E. The structure holds the item name, total stock, the Wholesale price, and the Retail price. But my current code will only overwrite the item name when sent back to the text file.
I have a feeling this is due to either using seek/read/write incorrectly.
In the following code I only use one struct to temporarily hold data when inputting/outputting to the file. (I hate to dump code like this but I'm not sure what else to do at this point.) Below is the function to modify a specific item that has already been entered:
The structure is
struct inventory
{
string itemname;
int stock;
double costWS,
costRE;
};
but i've only created one named "holder", to use when adding items to the text file or searching/modifying items.
void ModifyRecord()
{
fstream update ("Records.txt" , ios::in|ios::ate|ios::binary);
bool found = false;
long int whereAmI;
string tomodify;
cout << "Enter the name of the item to modify." << endl;
cin >> tomodify;
while ( !update.eof() || !found )
{
//holder is the structure containing inventory data
//this will read only one structure at a time, to send to holder
update.read(reinterpret_cast<char *> (&holder), sizeof(holder));
cout << "Reading new struct." << endl;
whereAmI = update.tellg();//save input position
//check this structure for the name entered
if (holder.itemname.find(tomodify) == 0)
{
found = true;//exit while loop
update.seekg( -( sizeof(holder) ), ios::cur);
//Update data
cout << "String found. \nYou have selected to modify: "
<< holder.itemname << endl;
cout << "Enter new name for the item: " << endl;
cin >> holder.itemname;
do
{
cout << "Enter amount stocked." << endl;
cin >> holder.stock;
}while(holder.stock < 0);
do
{
cout << "Enter wholesale cost." << endl;
cin >> holder.costWS;
}while(holder.costWS < 0);
do
{
cout << "Enter retail cost." << endl;
cin >> holder.costRE;
}while(holder.costRE < 0);
//send back to file
update.write(reinterpret_cast<char *> (&holder), sizeof(holder));
whereAmI = update.tellp();//save current output position
update.close();
break;
}
}
if ( !found )
cout << "Keyword not found." << endl;
}
So how do I overwrite the correct data using seekg/p? Am I using read/write correctly?

Answer 1

First: "binary text file" sounds like "cold hot water". You should decide: do you want to work with binary or text file.
Second: Your struct inventory contains a field of type string, that's why it cannot be used like a POD, but you're trying to do that:
update.seekg( -( sizeof(holder) ), ios::cur);
^^^^^^^^^^^^^^
and
update.write(reinterpret_cast<char *> (&holder), sizeof(holder));
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^
Third: If you'll change your struct like this:
struct inventory {
enum { MAX_ITEM_LENGTH = 100 };
char itemname[ MAX_ITEM_LENGTH + 1 ]; // +1 for trailing '\0'
int stock;
double costWS,
costRE;
};
then the algorithm would be like this:
fstream update ("Records.txt" , ios::in|ios::ate|ios::binary);
bool found = false;
string tomodify;
cout << "Enter the name of the item to modify." << endl;
cin >> tomodify;
while ( !update.eof() )
{
update.read( reinterpret_cast<char *> (&holder), sizeof(holder) );
if ( tomodify == holder.itemname )
{
update.seekg( -( sizeof(holder) ), ios::cur);
cout << "String found. \nYou have selected to modify: "
<< holder.itemname << endl;
cout << "Enter new name for the item: " << endl;
string newItemName;
cin >> newItemName;
strncpy( holder.itemname, newItemName.c_str(), inventory::MAX_ITEM_LENGTH );
do
{
cout << "Enter amount stocked." << endl;
cin >> holder.stock;
} while(holder.stock < 0);
do
{
cout << "Enter wholesale cost." << endl;
cin >> holder.costWS;
} while(holder.costWS < 0);
do
{
cout << "Enter retail cost." << endl;
cin >> holder.costRE;
} while(holder.costRE < 0);
//send back to file
update.write(reinterpret_cast<char *> (&holder), sizeof(holder));
found = true;
break;
}
}
if ( !found )
cout << "Keyword not found." << endl;
}