我的显示功能在C++控制台中不显示文件内容，第二次进入显示选项（在菜单中是2）。

Question

#include <iostream>
#include <string>
#include <fstream>
using namespace std;

//code to register, display and delete a constituency for election using file handling

fstream constituencies("constituencies.txt", ios::out | ios::in);
fstream temp("temp.txt", ios::out | ios::in);

/* declaring files for constituencies registration and when to delete, a temp file to copy the previous data into it and later rename it as constituencies*/

void registerConstituency() // function to register constituency
{
    // Proccess for constituency
    string consti;
    cout << "\n\nEnter unique constituency\n";
    getline(cin, consti);

    string get_file;
    static bool flag = false;
    /* flag to detect whether the constituency to be registered is already registered or not */

    // searching file if entry by user already exists or not through while loop

    while (getline(constituencies, get_file)) {
        if (get_file == consti) {
            flag = true;
            break;
        }
    }

    if (flag == true) {
        cout << "\nConstituency already exists, try another one\n";
    }
    else {
        ofstream constituencies("constituencies.txt", ios::app);
        constituencies << consti << endl;
        cout << "Your Constituency has been registered\n";
        constituencies.close();
    }
}

void displayConstituency()
{
    string get_file;
    while (getline(constituencies, get_file))
        cout << get_file << endl;
}

void updateOrDelete()
{
    string deleteline;
    string line;

    ofstream temp("temp.txt", ios::app);
    cout << "Which constituency do you want to remove? \n";
    cin >> deleteline;

    while (getline(constituencies, line)) {
        if (line != deleteline) {
            temp << line << endl;
        }
    }

    remove("constituencies.txt");
    rename("temp.txt", "constituencies.txt");
}

void menu()
{
    cout << "Select an option:" << endl
         << endl;
    cout << "1) Register a national assembly constituency (e.g. NA-1)\n";
    cout << "2) List all constituencies\n";
    cout << "3) Update/Delete Constituencies \n";

    int option;
    cout << "option: ";
    cin >> option;

    switch (option) {
    case 1:
        registerConstituency();
        break;

    case 2:
        displayConstituency();
        break;

    case 3:
        updateOrDelete();
        break;

    default: // Invalid option
        cout << "INVALID OPTION!!" << endl;
        break;
    }
}

int main()
{
    for (;;) {
        menu();

        int exit;
        cout << "\nENTER ANY NUMBER TO EXIT\n";
        cin >> exit;
        system("CLS");
    }
}

Answer 1

首先，你需要添加 fflush(stdin) 之前 getline(cin, consti) 在第一个定义的函数中，因为它是跳过要求输入的。

现在，我们来解决主要问题。

问题是，第二次执行时，文件内容没有任何显示。

原因是。 即使不读，它也会转到文件的结尾处 while(getline()) 语句。

要解决这个问题，请跳到第二个函数 void displayConstituency():

void displayConstituency()
{
    string file = "";

    while (getline(constituencies, file))
        cout << file << endl;
        // goes end of file and never jumps to beginning for next execution
}

与其这样，不如这样做。

void displayConstituency()
{
    string file = "";

    constituencies.clear(); // this
    constituencies.seekg(0, ios::beg); // this

    while (getline(constituencies, file))
        cout << file << endl;
}

它清除EOF，第二行再找文件的开头，供将来使用。