【C++ Primer】【学习笔记】【第八章】标准IO库之：文件的输入和输出

迷南。 2022-08-06 11:21 123阅读 0赞

**一、文件流对象的使用**

**1、fstream头文件定义了三种文件IO的类型：**

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    <div style="min-width:2px; background-color:inherit">
      类型 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:463.7999755859375px"> 
    <div style="min-width:2px; background-color:inherit">
      说明 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:93.39999942779542px"> ifstream</td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:463.7999755859375px"> 从istream派生而来，提供读文件的功能；</td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:93.39999942779542px"> 
    <div style="min-width:2px; background-color:inherit">
      ofstream 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:463.7999755859375px"> 
    <div style="min-width:2px; background-color:inherit">
      从ostream派生而来，提供写文件的功能； 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:93.39999942779542px"> 
    <div style="min-width:2px; background-color:inherit">
      fstream 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:463.7999755859375px"> 
    <div style="min-width:2px; background-color:inherit">
      从iostream派生而来，提供读写同一个文件的功能。 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

**2、与iostream类型相比，fstream增加了如下几个操作：**

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   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> fstream增加的操作</td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> 
    <div style="min-width:2px; background-color:inherit">
      说明 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> 
    <div style="min-width:2px; background-color:inherit">
      open( 
     <span style="font-family:微软雅黑; font-size:14.399999618530273px">filename.c_str()</span>); 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> 
    <div style="min-width:2px; background-color:inherit">
      打开文件 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> 
    <div style="min-width:2px; background-color:inherit">
      close(); 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> 
    <div style="min-width:2px; background-color:inherit">
      关闭文件 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> 构造函数(filename.c_str());</td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:279px"> 
    <div style="min-width:2px; background-color:inherit">
      打开文件，构造函数的形参为文件名 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

**3、检查文件打开是否成功**

ifstream infile;

infile.open(filename.c\_str());

if (!infile)

// 文件打开失败的处理

return;

**4、将文件流与新文件重新捆绑**

ifstream infile;

infile.open(filename.c\_str());

infile.close();

infile.open(next\_filename.c\_str());

**5、清除文件流的状态**

重要：若需要重用文件流读写多个文件，必须在打开下一个文件之前使用clear()清除该流的状态。

ifstream infile;

infile.open(filename.c\_str());

// 处理文件filename

infile.close();

infile.clear();

infile.open(next\_filename.c\_str());

// 处理文件next\_filename

**二、文件模式**

**1、文件模式**

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    <div style="min-width:2px; background-color:inherit"> 
     <span style="background-color:inherit">文件模式</span> 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:35px; background-color:inherit; width:366px"> 
    <div style="min-width:2px; background-color:inherit"> 
     <span style="background-color:inherit">说明</span> 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      in 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:366px"> 
    <div style="min-width:2px; background-color:inherit">
      打开文件做读操作 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      out 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:366px"> 
    <div style="min-width:2px; background-color:inherit">
      打开文件做写操作 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      app 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:366px"> 
    <div style="min-width:2px; background-color:inherit"> 
     <span style="font-family:微软雅黑; font-size:14px">打开文件做写操作，并将文件定位在</span>文件尾 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:31px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      ate 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:31px; background-color:inherit; width:366px"> 
    <div style="min-width:2px; background-color:inherit">
      打开文件后立即将文件定位在文件尾 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      trunc 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:366px"> 
    <div style="min-width:2px; background-color:inherit">
      打开文件时情况已存在的文件流 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:26px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      binary 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:26px; background-color:inherit; width:366px"> 
    <div style="min-width:2px; background-color:inherit">
      以二进制模式进行IO操作 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

**2、文件模式的组合**

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    <div style="min-width:2px; background-color:inherit"> 
     <span style="background-color:inherit">文件模式的组合</span> 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:35px; background-color:inherit; width:367px"> 
    <div style="min-width:2px; background-color:inherit"> 
     <span style="background-color:inherit">说明</span> 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:35px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      out 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:35px; background-color:inherit; width:367px"> 
    <div style="min-width:2px; background-color:inherit">
      打开文件做读操作，删除文件中已有的数据 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      out | app 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:367px"> 
    <div style="min-width:2px; background-color:inherit">
      打开文件做写操作，在文件尾写入 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      out | trunc 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:367px"> 
    <div style="min-width:2px; background-color:inherit">
      与out模式相同 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:31px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      in 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:31px; background-color:inherit; width:367px"> 
    <div style="min-width:2px; background-color:inherit"> 
     <span style="font-family:微软雅黑; font-size:14px">打开文件做读操作</span> 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      in | out 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:25px; background-color:inherit; width:367px"> 
    <div style="min-width:2px; background-color:inherit"> 
     <span style="font-family:微软雅黑; font-size:14px">打开文件做读、写操作，并定位于文件开头处</span> 
    </div> </td> 
  </tr> 
  <tr style="background-color:inherit"> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:26px; background-color:inherit; width:110px"> 
    <div style="min-width:2px; background-color:inherit">
      in | out | trunc 
    </div> </td> 
   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:26px; background-color:inherit; width:367px"> 
    <div style="min-width:2px; background-color:inherit">
      打开文件做读、写操作，删除文件中已有的数据 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

**注：以上所有模式的组合均可以加上ate模式，ate模式只会改变文件打开时的初始定位，在第一次读或写之前，将文件定位于文件末尾处。**

**习题8.6：读取文件**

#include <iostream>
    #include <fstream>
    #include <stdexcept>
    #include <string>
    using namespace std;
    
    istream& read_stream(istream& in)
    {
        string str;
    
        while (in >> str, !in.eof())
        {
            if (in.bad())
            {
                throw runtime_error("IO stream corrupted!");
            }
    
            if (in.fail())
            {
                cout << "Read fail, try again." << endl;
                in.clear();
                in.ignore(2, ' ');
                continue;
            }
    
            // str is ok, process it
            cout << str << " ";
        }
    
        cout << endl;
        in.clear();
        return in;
    }
    
    int main()
    {
        string filename;
        cout << "Enter filename: " << endl;
        cin >> filename;
    
        ifstream infile(filename.c_str());
        if (!infile)
        {
            cout << "error: can't open file: " << filename << endl;
            return -1;
        }
    
        read_stream(infile);
        return 0;
    }

[chapter8]$ ./a.out 
    Enter filename: 
    a.cpp 
    int main() { cout << a << endl; } int a() { } int b() { aaa; } int c() { }

**习题8.9：按行读取文件**

#include <iostream>
    #include <fstream>
    #include <stdexcept>
    #include <string>
    #include <vector>
    using namespace std;
    
    void file_to_vector(const string &filename, vector<string> &svec)
    {
        ifstream infile(filename.c_str());
        if (!infile)
        {
            cout << "Error: open file(" << filename << ") fail." << endl;
            return;
        }
    
        string s;
        while (getline(infile, s))
        {
            svec.push_back(s);
        }
    
        infile.close();
       
        if (infile.eof())
        {
            cout << "OK: read to eof." << endl;
        }
    
        if (infile.bad())
        {
            cout << "Error: system fail." << endl;
        }
    
        if (infile.fail())
        {
            cout << "Error: read fail." << endl;
        }
    
        return;
    }
    
    int main()
    {
        string filename;
        cout << "Enter filename: " << endl;
        cin >> filename;
    
        vector<string> svec;
        file_to_vector(filename.c_str(), svec);
    
        cout << "vector's size: " << svec.size() << endl;
        cout << "vector:" << endl;
        for (vector<string>::iterator it = svec.begin(); it != svec.end(); ++it)
        {
            cout << *it << endl;
        }
    
        return 0;
    }

[chapter8]$ ./a.out 
    Enter filename:
    a.cpp
    OK: read to eof.
    Error: read fail.
    vector's size: 20
    vector:
    int main()
    {
        cout << a << endl;
    }
    
    int a()
    {
       
    }
    
    int b()
    {
        aaa;
    }
    
    int c()
    {
       
    }

**习题8.10：按单词读取文件**

#include <iostream>
    #include <fstream>
    #include <stdexcept>
    #include <string>
    #include <vector>
    using namespace std;
    
    void file_to_vector(const string &filename, vector<string> &svec)
    {
        ifstream infile(filename.c_str());
        if (!infile)
        {
            cout << "Error: open file(" << filename << ") fail." << endl;
            return;
        }
    
        string s;
        while (infile >> s)
        {
            svec.push_back(s);
        }
    
        infile.close();
       
        if (infile.eof())
        {
            cout << "OK: read to eof." << endl;
        }
    
        if (infile.bad())
        {
            cout << "Error: system fail." << endl;
        }
    
        if (infile.fail())
        {
            cout << "Error: read fail." << endl;
        }
    
        return;
    }
    
    int main()
    {
        string filename;
        cout << "Enter filename: " << endl;
        cin >> filename;
    
        vector<string> svec;
        file_to_vector(filename.c_str(), svec);
    
        cout << "vector's size: " << svec.size() << endl;
        cout << "vector:" << endl;
        for (vector<string>::iterator it = svec.begin(); it != svec.end(); ++it)
        {
            cout << *it << endl;
        }
    
        return 0;
    }

[chapter8]$ ./a.out
    Enter filename:
    a.cpp
    OK: read to eof.
    Error: read fail.
    vector's size: 22
    vector:
    int
    main()
    {
    cout
    <<
    a
    <<
    endl;
    }
    int
    a()
    {
    }
    int
    b()
    {
    aaa;
    }
    int
    c()
    {
    }