读取和写入 .dat 文件

Question

为什么这个文件总是包含一个额外的Fraction 结构？如果用户在程序刚启动时输入选项 3 来查看所有分数（文件应该为空），则打印出0/0。如果用户选择选项一并输入一个分数1/1，然后选择选项3，则会打印出两个分数1/1 1/1。

注意：要运行它，您需要创建一个名为 "fractions.dat" 的空 .dat 文件

非常感谢您的帮助！

#include <iostream>
#include <fstream>

using namespace std;

struct Fraction
{
    int num, den;
};

int search(fstream& file, Fraction* f);
int menu();
void proccessChoice(int i);
Fraction* readFrac();

Fraction* fracs[100];
int index;
string fileName = "fractions.dat";
fstream file(fileName,  ios::in | ios::out | ios::binary | ios::trunc);

//structure called Fraction with num and den as its only integer members
int main()
{
    if (!file)
    {
        cout << "Error opening file. Program aborting.\n";
        system("pause");
        exit(1);
    }
    int choice;
    do
    {
        choice = menu();
        proccessChoice(choice);
    } while(choice != 4);


    system("pause");
    return 0;
}



int menu()
{
    int c;
    cout << "1.Enter new fraction" << endl;
    cout << "2.Display/update a fraction" << endl;
    cout << "3.Display all fractions" << endl;
    cout << "4.Quit" << endl;
    cin >> c;
    return c;
}

void proccessChoice(int i)
{
    switch(i)
    {
    case 1:
        {
            cout << "Enter a fraction to add: ";
            fracs[index] = readFrac();
            file.seekp(0, ios::end);
            file.write(reinterpret_cast<char*>(fracs[index]), sizeof(Fraction));
            /*cout << fracs[index]->num << "/" << fracs[index]->den ;*/
            index++;
        }
            break;
    case 2:
        {
            cout << "Enter fraction to view/update: ";
            Fraction* fToFind = readFrac();
            int i = search(file, fToFind);
            if (i == -1)
            {
                cout << "Fraction not found." << endl;
            }
            else
            {
                char r;
                cout << fToFind->num << "/" << fToFind->den << " found at index " << i << ". Update? [y/n]";
                cin >> r;
                if(r == 'y' || r == 'Y')
                {
                    cout << "Enter a new fraction: ";
                    Fraction* fToWrite = readFrac();
                    file.seekp(sizeof(Fraction) * (i));
                    file.write(reinterpret_cast<char*>(fToWrite), sizeof(Fraction));
                }
            }
            cout << i << endl;
            cout << "Case 2" << endl;
        }
            break;
    case 3:
        {
            Fraction* cur = new Fraction();
            //int pos = -1;
            if (!file)
            {
                cout << "Error opening file. Program aborting.\n";
                system("pause");
                exit(1);
            }
            file.seekg(0, ios::beg);
            while(!file.eof())
            {
                file.read(reinterpret_cast<char *>(cur), sizeof(Fraction));
                cout << cur->num << "/" << cur->den << endl;
            }
                    cout << "Case 3" << endl;
        }
            break;
    }
}

int search(fstream& file, Fraction* f)
{
    Fraction* cur = new Fraction();
    int pos = -1;
    if (!file)
    {
        cout << "Error opening file. Program aborting.\n";
        system("pause");
        exit(1);
    }
    file.seekg(0, ios::beg);
    while(!file.eof())
    {
        file.read(reinterpret_cast<char *>(cur), sizeof(Fraction));
        if((cur->num == f->num) && (cur->den == f->den))
        {
            cout << "\nFrac found!" << cur->num << "/" << cur->den<<endl;
            pos = file.tellg()/sizeof(Fraction) - 1;
            return pos;
        }
    }
    return pos;
}

Fraction* readFrac()
{
    Fraction* f = new Fraction();
    char slash;
    cin >> f->num >> slash >> f->den;
    return f;
}

尝试的解决方案 1：适用于用户第一次尝试查看所有分数，但在第二次尝试时找不到文件。

case 3:
        {
            Fraction* cur = new Fraction();
            //int pos = -1;
            if (!file)
            {
                cout << "Error opening file. Program aborting.\n";
                system("pause");
                exit(1);
            }
            file.seekg(0, ios::beg);
            while(!file.eof())
            {
                if(file.read(reinterpret_cast<char *>(cur), sizeof(Fraction)).gcount()== sizeof(Fraction))
                {
                    cout << cur->num << "/" << cur->den << endl;
                }
            }
                    cout << "Case 3" << endl;
        }
            break;
    }

Answer 1

因为你还没有到达文件的末尾！

while(!file.eof())

您打开了空文件，但是请注意，读取光标当前位于位置0。只要您没有尝试读取数据file.eof() 就会为假，因为光标尚未到达文件末尾。

您需要检查您是否真的能够提取分数：

if(
   file.read(reinterpret_cast<char *>(cur), sizeof(Fraction)).gcount() 
   == sizeof(Fraction)
){
     // Fraction has been extracted
}
else
     // either eof() or some other error.

请注意，如果您为此提供自定义运算符，这将变得更加简单：

std::istream& operator>>(std:istream& in, Fraction& f){
    return in.read(reinterpret_cast<char *>(&f), sizeof(Fraction));
}

// ....
while(file >> *cur){
   // ... your code.
}