需要有关通过fstream保存变量的帮助，我是否需要使用向量？

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, eg 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";
            }
        }
    }
}