c++ - 如何修复 ifstream 语法错误，以及 C++ 中可能丢失数据的转换错误？

Question

我的代码有来自 Source.cpp 文件的代码。我在这个实验室的中间，我创建了一个跟踪相机商店的库存。它几乎完成了，但我遇到错误 C4244(使用 = on like 24 可能丢失数据)和错误 C3867(ios::base_fail/ios::basic_ifstream)。我会在出现错误的地方旁边写上“这里”，因为我无法告诉你是哪一行。这是有问题的代码:

#include<iostream>
#include<string>
#include<fstream>
#include "LinkedList.h"
using namespace std;

int main()
{
LinkedList ll; // Creating object of linked list

struct info record;

char dataToBeRead[100];

ifstream fp; 

fp.open("fileInventory.txt");

if (fp.fail) !!!HERE!!! 
{
    cout << "The file cannot be opened\n";
}
else


{
    while (fp.open, dataToBeRead[100] != NULL) !!!HERE!!!
    {
        char *token = strtok(dataToBeRead, " "); 
        strcpy(record.camera, token); 
        token = strtok(NULL, " ");
        record.price = atof(token);
        token = strtok(NULL, " ");
        record.quantity1 = atoi(token); !!!HERE!!! (possible loss of data?)
        token = strtok(NULL, " ");
        record.quantity2 = atoi(token);
        token = strtok(NULL, " ");
        ll.addEnd(record);
        ll.addReverse(record);

    }
    int ch = 0;
    char camera[50];
    float price;
    int quantity1;
    int quantity2;
    do //Must open the menu
    {
        cout << "1. Display the inventory in alphabetic order:\n";
        cout << "2. Display the inventory in reverse alphabetic order:\n";
        cout << "3. Add an item to the inventory:\n";
        cout << "4. Delete an item from inventory:\n";
        cout << "5. Change any info for an item:\n";
        cout << "6. Exit:\n";
        cin >> ch;
        switch (ch)
        {
        case 1:
            ll.showList1();
            break;
        case 2:
            ll.showList2();
            break;
        case 3:
            cout << "Enter camera:\n";
            cin >> camera;
            cout << "Enter price:\n";
            cin >> price;
            cout << "Enter quantity1:\n";
            cin >> quantity1;
            cout << "Enter quantity2:\n";
            cin >> quantity2;
            strcpy(record.camera, camera);
            record.price = price;
            record.quantity1 = quantity1;
            record.quantity2 = quantity2;
            ll.addEnd(record);
            ll.addReverse(record);
            ll.showList1();
            ll.showList2();
            break;
        case 4:
            cout << "Enter camera name to be deleted:\n";
            cin >> camera;
            ll.removeValue1(camera);
            ll.removeValue2(camera);
            ll.showList1();
            ll.showList2();
            break;
        case 5:
            cout << "Enter the camera name for which you want to edit details:\n";
            cin >> camera;
            cout << "Enter price:\n";
            cin >> price;
            cout << "Enter quantity1:\n";
            cin >> quantity1;
            cout << "Enter quantity2:\n";
            cin >> quantity2;
            ll.editList(camera, price, quantity1, quantity2);
            break;
        case 6:
            break;

        }
    } while (ch != 6); // Menu will run until the ch is not 6


    ofstream myfile;
    myfile.open("newlist.txt"); 

    if (myfile.fail) !!!HERE!!!
    {
        exit(1);
    }

}

return 1;
}

这显然是 fstream 的问题，但我不知道我做错了什么。而且我不确定为什么在那条线上可能会丢失数据。预先感谢您的帮助!

Answer 1

不用说了，这一行:

while (fp.open, dataToBeRead[100] != NULL)

不正确。

此外，要测试文件是否成功打开，您可以简单地这样做:

if (!fp)
{
    // file failed to open
}

但总的来说，最简单的方法是完全不引入 char 数组，而是使用 std::string:

  std::string dataToBeRead;
  while (std::getline(fp, dataToBeRead)) 
  {
   //...
  }

鉴于 dataToBeRead 现在是 std::string，标记化的逻辑需要更改。这只需使用 std::istringstream 即可完成。并使用 operator >>。

std::istringstream strm(dataToBeRead);
strm >> record.camera >> record.price >> record.quantity1 >> record.quantity2;
ll.addEnd(record);
ll.addReverse(record);

因此整个循环是这样的:

  #include <sstream>
  //...
  std::string dataToBeRead;
  while (std::getline(fp, dataToBeRead)) 
  {
     std::istringstream strm(dataToBeRead);
     strm >> record.camera >> record.price >> record.quantity1 >> record.quantity2;
     ll.addEnd(record);
     ll.addReverse(record);
  }

这比使用 C 风格的 strtok 处理要容易和安全得多。

---

您也可以使用 operator >> 直接读取输入，从而跳过整个字符串创建过程:

  while (fp >> record.camera >> record.price >> record.quantity1 >> record.quantity2)
  {
     ll.addEnd(record);
     ll.addReverse(record);
  }

如果数据的解析很简单(在您的情况下，使用空格分隔)，您通常希望使用此方法。如果比这更复杂，则可以使用 istringstream。