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Binary file - replace data at specific point

mekkatorqu (416)
I'm making a binary file that has 100 "empty spaces", and then I want to rewrite specific place with info, however it always writes the info at the end of the file, no matter what I try to get position before I call write() it tells me correct position...

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#include <Windows.h>
#include <iostream>
#include <string>
#include <fstream>
#include <vector>

using namespace std;

class hw
{
protected:
	int id;
	string name;
public:
	hw();
	void SetID(int);
	void SetName(string);

	int GetID();
	string GetName();
};

hw::hw()
{
	id = 0;
	name = "NULL";
}

void hw::SetID(int i)
{
	id = i;
}

void hw::SetName(string str)
{
	name = str;
}

int hw::GetID()
{
	return id;
}

string hw::GetName()
{
	return name;
}

int main()
{
	hw HW;
	int i;
	string str;
	int sel;

	bool go = 1;

	fstream file("hardware.dat", ios::in | ios::out | ios::app | ios::binary);
	if (!file.is_open())
	{
		cout << "can't open file ...." << endl;
		system("pause");
		return 0;
	}

	file.seekg(0, file.end);
	streamoff a = file.tellg();
	if (a == 0)
	{
		for (int j = 0; j < 100; j++)
			file.write(reinterpret_cast<char*>(&HW), sizeof(hw));

		file.flush();
	}

	while (go)
	{
		system("cls");
		cout << "1 -> write\n2 -> read\nselection -> ";
		cin >> sel;
		cin.sync();
		file.sync();

		switch (sel)
		{
			case 1:
			{
				cout << "Id: ";
				cin >> i;
				cin.sync();
				cout << "name: ";
				getline(cin, str);

				HW.SetID(i);
				HW.SetName(str);

				streampos pos = HW.GetID()*sizeof(hw);

				file.seekg(pos);

				file.write(reinterpret_cast<char*>(&HW), sizeof(hw));
				file.flush();
				break;
			}
			case 2:
			{
				vector<char> buff(sizeof(hw));

				/*cout << "Id to find: ";
				int id;
				cin >> id;
				cin.sync();

				streampos pos = id*sizeof(hw);
				file.seekg(pos);*/

				file.seekg(0, file.end);
				int size = file.tellg();
				file.seekg(0, file.beg);

				for (unsigned j = 0; j < size/sizeof(hw); j++)
				{
					file.read(&buff[0], sizeof(hw));

					ZeroMemory(&HW, sizeof(hw));
					memcpy(&HW, &buff[0], sizeof(hw));
					buff.clear();

					cout << HW.GetID() << endl << HW.GetName() << endl;
				}
				
				system("pause");
				break;
			}
			default:
			{
				go = 0;
				break;
			}
		}
	}

	file.close();

	system("pause");
	return 0;
}


suggestions? reading whole file works fine
LB (13399)
You are specifying ios::app which forces appending to the end of the file no matter what. Just don't specify that flag.
mekkatorqu (416)
@LB

how am I supposed to not rewrite whole file after opening when not using app?
giblit (3750)
Have you tried ate instead of app?

http://www.cplusplus.com/reference/fstream/fstream/fstream/

though you are using seek anyways so as LB mentioned you could remove the flag.
Last edited on
mekkatorqu (416)
works perfectly with ate, thanks!
LB (13399)
The only time the file is erased when you open it is if you specify the ios::trunc flag. Since you're using seek you don't need ios::ate.
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