Need help regarding saving variables via fstream,

Question

I doing this project, where I want to save some variables for a device; Devicename, ID and type.

bool Enhedsliste::newDevice(string deviceName, string type)
{
fstream myFile;
string line;
char idDest[4];

myFile.open("Devices.txt", ios::app | ios::in | ios::out); //Opretter/Åbner fil, app = startlinje er den nederste linje, in out = input output
if (myFile.is_open())
{
    while (getline(myFile, line)) //Går filen igennem for hver linje
    {
        if (line == deviceName)
        {
            cout << deviceName << "-device already exists." << endl;
            return false;
        }
    }
}
else
{
    cout << "Uable to open file." << endl;
    return false;
}
myFile.close();

myFile.open("Devices.txt", ios::app | ios::in | ios::out); //Opretter/Åbner fil, app = startlinje er den nederste linje, in out = input output
if (myFile.is_open())
{
    if (type == "Lampe")
        type_ = 1;
    else if (type == "Roegalarm")
        type_ = 2;
    else if (type == "Tyverialarm")
        type_ = 3;
    else
    {
        cout << "Type does not exists." << endl;
        return false;
    }

    deviceName_ = deviceName;
    myFile << deviceName_ << endl;

    id_++;
    sprintf_s(idDest, "%03d", id_);
    myFile << idDest << endl;

    myFile << type_ << endl;

    myFile.close();

    return true;
}
else
{
    cout << "Uable to open file." << endl;
    return false;
}
}

Now I also want to make a deleteDevice, where I can take deviceName as a parameter and it will find that line and delete ID and type, but I have no idea on how to do this.

Do I need to rewrite my addDevice to vector? And how do I do this?

Thanks in advance and sorry for bad code, explanation etc. I'm new to this.

Answer 1

To delete a line from your current text file, you will have to read in all lines, e.g. storing the data in a std::map, remove the relevant item, and write it all back out again. An alternative is to use a database or binary fixed size record file, but reading everything into memory is common. By the way, I would remove the ios::app append mode for opening the file for reading. It translates to the equivalent of append mode for fopen, but the C99 standard is unclear on what it means for reading, if anything.

Answer 2

To delete a single device you might read the file and write to a temporary file. After transferring/filtering the data, rename and delete files:

#include <cstdio>
#include <fstream>
#include <iostream>

int main() {
    std::string remove_device = "Remove";
    std::ifstream in("Devices.txt");
    if( ! in) std::cerr << "Missing File\n";
    else {
        std::ofstream out("Devices.tmp");
        if( ! out) std::cerr << "Unable to create file\n";
        else {
            std::string device;
            std::string id;
            std::string type;
            while(out && std::getline(in, device) && std::getline(in, id) && std::getline(in, type)) {
                if(device != remove_device) {
                    out << device << '\n' << id << '\n' << type << '\n';
                }
            }
            if( ! in.eof() || ! out) std::cerr << "Update failure\n";
            else {
                in.close();
                out.close();
                if( ! (std::rename("Devices.txt", "Devices.old") == 0
                && std::rename("Devices.tmp", "Devices.txt") == 0
                && std::remove("Devices.old") == 0))
                    std::cerr << "Unable to rename/remove file`\n";
            }
        }
    }
}