Question

我使用fstream对象

创建了一个读/写字节到二进制文件中

#include <iostream>
#include <fstream>

using namespace std;

#define COL_WIDTH 20

int writeBinaryFile(char *desPath);
int readBinaryFile(char *desPath);

int age;
char name[COL_WIDTH];
int recsize = sizeof(int)+sizeof(name);
int n;

int main(){

    char desPath[MAX_PATH+1];
    char input[2];

    while(true){
        cout  << "Main Menu:\n1 Write Binary File\n2 Read Binary File\n3 EXIT" << endl;
        cin.getline(input, 2);          


        if(atoi(input)== 1){
            cout << "Enter destination path:" << endl;
            cin.getline(desPath, MAX_PATH+1);
            for(;;){
                int output = writeBinaryFile(desPath);
                if(output == 1) break;
                else if(output == 2) { *input = '4'; break;}
            }
        }
        else if(atoi(input) == 2){
            cout << "Enter destination path:" << endl;
            cin.getline(desPath, MAX_PATH+1);
            for(;;){
                int output = readBinaryFile(desPath);
                if(output == 1) break;
                else if(output == 2){ *input = '4'; break;}
            }
        }

        else if(atoi(input) == 3)
            break;
        else cout << "You entered wrong input, enter only 1 or 2." << endl;

        if(atoi(input) == 4) continue;
        else break;

    }

    system("pause");
    return 0;   
}

int writeBinaryFile(char *desPath){
    char option[2];
    fstream wBin(desPath, ios::binary | ios::out);
    if(!wBin){
        cout << desPath << " is not good path."<< endl;
        return 1;
    }
    cout << "Enter name: " << endl;
    cin.getline(name,COL_WIDTH);
    cout << "Enter age: " << endl;
    cin >> age;

    cout << "Enter record number: " << endl;
    cin >> n;

    wBin.seekp(n*recsize); 
    wBin.write(name, sizeof(name));
    wBin.write((char*)&age, sizeof(age));
    wBin.close();

    cout << "Enter record again? Press:" << endl
         << "Enter to continue" << endl
         << "Q to quit" << endl
         << "M to go back to main menu" << endl;

    cin.ignore(256,'\n');
    cin.getline(option,2);

    if(option[0] == 'q' ||option[0] == 'Q') return 1;
    else if(option[0] == 'm' ||option[0] == 'M') return 2;

    return 0;
}


int readBinaryFile(char *desPath){       
    char option[2];
    fstream rBin(desPath, ios:: binary | ios:: in);
    if(! rBin){
        cout << "File not found!" << endl;
        return 1;
    }
    cout << "What record number?" <<endl;
    cin >> n;
    rBin.seekp((n*recsize));
    rBin.read(name,sizeof(name));
    rBin.read((char*)&age, sizeof(int));


    cout << name <<endl;
    cout << age << endl;
    rBin.close();
    cout << "Print more? Press enter. Press Q to QUIT or M to go back." << endl;
    cin.clear();
    cin.sync();
    cin.getline(option, 2);

    if(option[0] == 'q'|| option[0] == 'Q'){rBin.close(); return 1;}
    else if(option[0] == 'm' || option[0] == 'M'){rBin.close(); return 2;}

    return 0;
}

我首先创建了一个二进制文件，其名称，年龄，记录编号（字节位置为0）然后在第二个循环上输入名称，年龄，记录编号2。当我试图读取字节pos（0）的第一对（char *，int）但它返回错误的结果但是当我试图读取pos（1）中的最后一对时它返回正确的结果。

我也试过输入3次，但只有最后一个char *和int是正确的。

为什么在第一个字节（名称（20字节），年龄（4字节））中得到错误结果但是在最后一个字节中得到正确的结果？

Answer 1

在每次调用writeBinaryFile时，重新打开输出，销毁文件的先前内容。

您需要使用ios::binary | ios::ate | ios::out | ios::in。

但是，如果该文件不存在，该位掩码将不创建该文件。您可以使用以下代码序列解决该问题：

int writeBinaryFile(char *desPath) {
  ...
  // Create the file only if it doesn't exist
  fstream(desPath, ios::binary|ios::out|ios::app);

  // Now it exists -- open it
  fstream wBin(desPath, ios::binary|ios::out|ios::in);
  ...
}

感谢@JoeFish获得答案的有用部分。

使用fstream c ++读取字节

1 个答案: