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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

 

程序示例

#include 
   
    #include 
    
     #include 
     
      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;}
     
    
   

 

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

使用循环的方式打开一组文件

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 
   
    #include 
    
     #include 
     
      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;}
     
    
   

那两文件我自己创建的，然后里面随便输入一些字符

文件模式

在oepn函数中添加参数

使用位或运算符'|'来混合使用 比如打开追加 打开截断

代表性的组合

追加

#include 
   
    #include 
    
     #include 
     
      #include 
      
       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

二进制文件

二进制文件的读写统一用read()和writer()

程序示例

#include 
   
    #include 
    
     #include 
     
      #include 
      
       // 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

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

原型

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？

举例

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 
   
    #include 
    
     #include 
     
      #include 
      
       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) <
       
        << ": "       << 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

使用临时文件

嗯。。想得很周全，但是没啥软用 先留着吧

文件I/O完结

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