C++ Primer Plus 学习笔记第十七章 I/O

墨蓝 2021-12-23 02:53 296阅读 0赞

fstream

写入： ofstream对象  << 或者write()

读取： ifstream对象 >> 或者get()

使用open函数关联文件或者直接用ofstream类型的构造函数来关联文件 没有该文件则会创建该文件

ofstream是ostream的继承类 所以可以使用ostream的所有函数

默认打开方式是截断 也就是清空文件内容

![20190626165454730.png][]

程序示例

#include <iostream>
    #include <fstream>
    #include <string>
    
    int main()
    {
      using namespace std;
      string filename;
    
      cout << "Enter name for new file: ";
      cin >> filename;
    // c_str是将字符串转换为C风格的字符串
      ofstream fout(filename.c_str());
    
      fout << "For your eyes only!\n";
      cout << "Enter your secret number: ";
      float secret;
      cin >> secret;
      fout << "Your secret number is " << secret << endl;
      fout.close();
    
      ifstream fin(filename.c_str());
      cout << "Here are the contents of " << filename << ":\n";
      char ch;
      while (fin.get(ch))
        cout << ch;
      cout << "Done\n";
      fin.close();
    
      return 0;
    }

![20190626170845594.png][]

流状态检查和is\_open()

文件流状态：

fail() 判断是否试图打开一个不存在的文件 （failbit位）

也可以用is\_open()来判断 一样的

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70][]

使用循环的方式打开一组文件

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 1][]

命令行处理技术

在main()中添加参数

int main(int argc, char \*argv\[\])

argc 参数个数 包括命令名本身

argv指向命令参数的字符数组指针 包括命令名本身

#include <iostream>
    #include <fstream>
    #include <cstdlib>
    int main(int argc, char* argv[])
    {
      using namespace std;
      if (argc == 1)
      {
    // [0] 表示的是程序名
        cerr << "Usage: " << argv[0] << " filename[s]\n";
        exit(EXIT_FAILURE);
      }
    
      ifstream fin;
      long count;
      long total = 0;
      char ch;
    
      for (int file = 1; file < argc; file++)
      {
        fin.open(argv[file]);
        if (!fin.is_open())
        {
          cerr << "Could not open " << argv[file] << endl;
          fin.clear();
          continue;
        }
        count = 0;
        while (fin.get(ch))
          count++;
        cout << count << " characters in " << argv[file] << endl;
        total += count;
        fin.clear();
        fin.close();
      }
    
      cout << total << " characters in all fileds:\n";
    
      return 0;
    }

![20190626173132243.png][]

那两文件我自己创建的，然后里面随便输入一些字符

![20190626173235223.png][]

文件模式

在oepn函数中添加参数

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 2][]

使用位或运算符'|'来混合使用 比如打开追加 打开截断

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 3][]

代表性的组合

追加

#include <iostream>
    #include <fstream>
    #include <string>
    #include <cstdlib>
    
    const char* file = "guests.txt";
    
    int main()
    {
      using namespace std;
      char ch;
    // 读文件
      ifstream fin;
      fin.open(file);
    
      if (fin.is_open())
      {
        cout << "Here are the current contents of the "
             << file << " file:\n";
        while(fin.get(ch))
          cout << ch;
        fin.close();
      }
    // 直接用构造函数关联文件 模式采用追加 取文件
      ofstream fout(file, ios::out | ios::app);
      if (!fout.is_open())
      {
        cerr << "Can't open " << file << " file for output.\n";
        exit(EXIT_FAILURE);
      }
    
      cout << "Enter guest names (enter a blank line to quit):\n";
      string name;
      while (getline(cin, name) && name.size() > 0)
      {
        fout << name << endl;
      }
      fout.close();
    // 读文件
      fin.clear();
      fin.open(file);
      if (fin.is_open())
      {
        cout << "Here are the new contents of the "
             << file << " file:\n";
        while (fin.get(ch))
          cout << ch;
        fin.close();
      }
      cout << "Done.\n";
      return 0;
    }

运行结果 运行两次 第一次自动创建文件，第二次追加文件

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 4][]

二进制文件

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 5][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 6][]

二进制文件的读写统一用read()和writer()

![20190626181020351.png][]

程序示例

#include <iostream>
    #include <fstream>
    #include <iomanip>
    #include <cstdlib>
    // 用来将输入流中的换行符去掉
    inline void eatline() { while (std::cin.get() != '\n') continue;}
    struct planet
    {
      char name[20];
      double population;
      double g;
    };
    
    const char* file = "planets.dat";
    
    int main()
    {
      using namespace std;
      planet p1;
      cout << fixed << right;
      ifstream fin;
      fin.open(file, ios_base::in | ios_base::binary);
      if (fin.is_open())
      {
        cout << "Here are the current contents of the "
             << file << " file:\n";
    // 强转成char指针类型是因为read函数必须要该类型，并且保持类型一致
        while(fin.read((char* ) &p1, sizeof p1))
        {
          cout << setw(20) << p1.name << ": "
               << setprecision(0) << setw(12) << p1.population
               << setprecision(2) << setw(6) << p1.g << endl;
        }
        fin.close();
      }
    // 追加写，二进制
      ofstream fout(file, ios_base::out | ios_base::app | ios_base::binary);
      if (!fout.is_open())
      {
        cerr << "Can't open " << file << " file for output:\n";
        exit(EXIT_FAILURE);
      }
    
      cout << "Enter planet name (enter a blank line to quit):\n";
      cin.get(p1.name, 20);
      while(p1.name[0] != '\0')
      {
        eatline();
        cout << "Enter planetary population: ";
        cin >> p1.population;
        cout << "Enter planet's acceleration of gravity: ";
        cin >> p1.g;
        eatline();
        fout.write((char* ) &p1, sizeof p1);
        cout << "Enter planet name (enter a blank line "
             << "to quit): \n";
        cin.get(p1.name, 20);
      }
    
      fout.close();
    
      fin.clear();
      fin.open(file, ios_base::in | ios_base::binary);
      if(fin.is_open())
      {
        cout << "Here are the new contents of the "
             << file << " file:\n";
        while (fin.read((char*) &p1, sizeof p1))
        {
          cout << setw(20) << p1.name << ": "
               << setprecision(0) << setw(12) << p1.population
               << setprecision(2) << setw(6) << p1.g << endl;
        }
        fin.close();
      }
      cout << "Done.\n";
      return 0;
    }

程序示例

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 7][]

结构体中不能用string类型 因为string返回的是指向字符串的地址。哪个保存到文件中是没有卵用的

随机存取

利用输入指针和输出指针在文件中随意跳转进行得写（其实是在缓冲区中）

seekg()输入指针

seekp()输出指针

![20190628114546171.png][]

原型

![20190628115602730.png][]

ios\_base::seekdir是ios\_base类中定义的另一种整形， 有3个值 ios\_base::beg 文件开始的位置 ios\_base::cur 当前位置的偏移量

ios\_base::end文件尾

streamoff:相对于ios\_base所指向的位置的特定距离（单位是字节）

streampos 指的是距离文件头的位置

他在说什么呢？

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 8][]

举例

![20190628131323844.png][]

输入流检查文件当前位置用tellg()

输出流检查用tellp()

返回的是streampos类型的值

#include <iostream>
    #include <fstream>
    #include <iomanip>
    #include <cstdlib>
    
    const int LIM = 20;
    struct planet
    {
      char name[LIM];
      double population;
      double g;
    };
    
    const char* file = "planets.dat";
    inline void eatline() {while (std::cin.get() != '\n') continue;}
    
    int main()
    {
      using namespace std;
      planet pl;
      cout << fixed;
    
      fstream finout;
    // 读写字节格式
      finout.open(file, ios_base::in | ios_base::out | ios_base::binary);
      int ct = 0;
      if (finout.is_open())
      {
    // 输出流（读取）重置到文件开头
        finout.seekg(0);
        cout << "Here are the current contents of the "
             << file << " file:\n";
        while (finout.read((char*) &pl, sizeof pl))
        {
          cout << ct++ << ": " << setw(LIM) << pl.name << ": "
               << setprecision(0) << setw(12) << pl.population
               << setprecision(2) << setw(12) << pl.g << endl;
        }
    // 文件结尾就清输出流缓存
        if (finout.eof())
          finout.clear();
        else{
          cerr << "Error in reading " << file << ".\n";
          exit(EXIT_FAILURE);
        }
      }
      else
      {
        cerr << file << " could not be opened -- bye.\n";
        exit(EXIT_FAILURE);
      }
      cout << "Enter the record number you wish to change: ";
      long rec;
      cin >> rec;
    //自定义函数
      eatline();
      if (rec < 0 || rec >= ct)
      {
        cerr << "Invalid record number -- bye\n";
        exit(EXIT_FAILURE);
      }
    // stream类中自定义的int类型
    // 跳转到文件内容的相应位置，以一个planet为长度
      streampos place = rec * sizeof pl;
      finout.seekg(place);
    // 出现超尾，报异常
      if (finout.fail())
      {
        cerr << "Error on attempted seek\n";
        exit(EXIT_FAILURE);
      }
    
      finout.read((char*) &pl, sizeof pl);
      cout << "Your selection:\n";
      cout << rec << ": " << setw(LIM) <<pl.name << ": "
           << setprecision(0) << setw(12) << pl.population
           << setprecision(2) << setw(6) << pl.g << endl;
      if(finout.eof())
        finout.clear();
    
      cout << "Enter planet name: ";
      cin.get(pl.name, LIM);
      eatline();
      cout << "Enter planetary population: ";
      cin >> pl.population;
      cout << "Enter planet's acceleration of gravity: ";
      cin >> pl.g;
    //输入流指针定位至客户要求的指定步长
      finout.seekp(place);
     // flush表示手动刷新，一刷新数据就写入到文件中
      finout.write((char *) &pl, sizeof pl) << flush;
      if(finout.fail())
      {
        cerr << "Error on attempted write\n";
        exit(EXIT_FAILURE);
      }
    
      ct = 0;
    // 重新定位输出流至文件流开头
      finout.seekg(0);
      cout << "Here are the new contents of the " << file
           << " file:\n";
      while(finout.read((char *) &pl, sizeof pl))
      {
        cout << ct++ << ": " << setw(LIM) << pl.name << ": "
             << setprecision(0) << setw(12) << pl.population
             << setprecision(2) << setw(6) << pl.g << endl;
      }
      finout.close();
      cout << "Done.\n";
      return 0;
    }

运行结果

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 9][]

使用临时文件

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 10][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 11][]

嗯。。想得很周全，但是没啥软用 先留着吧

文件I/O完结

[20190626165454730.png]: /images/20211223/87df4ba7c4514ff0adb724896251ccd0.png
[20190626170845594.png]: /images/20211223/316999cdc0b845b68bdfc804cd3f86f6.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70]: /images/20211223/fa1ee9248cc04185964568764796c033.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 1]: /images/20211223/2378d16133eb490994ab268b0dc352f6.png
[20190626173132243.png]: /images/20211223/3007a139ca5744cc86629f578739f17e.png
[20190626173235223.png]: /images/20211223/da18f62b22ac4bde8c83e32c313f4c61.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 2]: /images/20211223/a40c060862394ea386af4ad8988152fd.png
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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 4]: /images/20211223/55de64e40c694231b5e2f1f457a5fe89.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 5]: /images/20211223/02b4f28042ba42afa2922ee7ff7c8150.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 6]: /images/20211223/9d6e463c81554b2ca82f3d05f000acbc.png
[20190626181020351.png]: /images/20211223/aacee538a31c490e841c5ad9dc3be7e7.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 7]: https://img-blog.csdnimg.cn/20190628110809734.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI=,size_16,color_FFFFFF,t_70
[20190628114546171.png]: https://img-blog.csdnimg.cn/20190628114546171.png
[20190628115602730.png]: https://img-blog.csdnimg.cn/20190628115602730.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 8]: https://img-blog.csdnimg.cn/20190628115646754.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI=,size_16,color_FFFFFF,t_70
[20190628131323844.png]: https://img-blog.csdnimg.cn/20190628131323844.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 9]: https://img-blog.csdnimg.cn/20190702095047916.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI=,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 10]: https://img-blog.csdnimg.cn/20190702100106803.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI=,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70 11]: https://img-blog.csdnimg.cn/20190702100113481.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI=,size_16,color_FFFFFF,t_70