C++ fstream

fstream

• <fstream> 라이브러리

• 기본 옵션은 ASCII 포맷으로 덮어쓰기이다.

  • 옵션에 대한 플래그는 다음과 같다.

    xiosbase

    static constexpr _Openmode in         = static_cast<_Openmode>(0x01);
    static constexpr _Openmode out        = static_cast<_Openmode>(0x02);
    static constexpr _Openmode ate        = static_cast<_Openmode>(0x04);
    static constexpr _Openmode app        = static_cast<_Openmode>(0x08);
    static constexpr _Openmode trunc      = static_cast<_Openmode>(0x10);
    static constexpr _Openmode _Nocreate  = static_cast<_Openmode>(0x40);
    static constexpr _Openmode _Noreplace = static_cast<_Openmode>(0x80);
    static constexpr _Openmode binary     = static_cast<_Openmode>(0x20);

• 옵션으로 이어쓰기, binary 포맷으로 열기가 가능하다.

• 클래스의 소멸자에서 파일을 닫아주기 때문에 굳이 fstream.close()를 할 필요가 없다. (RAII)

  • 하지만 가독성 등의 이유로 명시하는게 낫다고 생각한다.

• 비주얼 스튜디오에서 Ctrl + K를 누르고 R을 누르면(컨트롤 없이) 작업 중인 폴더가 익스플로러로 열린다.

---

예제

• ASCII 포맷으로 저장 후 불러오는 예제

  #include <iostream>
  #include <string>
  #include <fstream>
  
  int         main()
  {
      using namespace std;
  
      string filename = "my_first_file.txt";
  
      {
          ofstream ofs(filename);
  
          if (!ofs)
          {
              cerr << "Fail to open file\n";
              exit(1);
          }
  
          ofs << "Line 1\n";
          ofs << "Line 2\n";
  
          ofs.close();
      }
  
      {
          ifstream ifs(filename);
  
          if (!ifs)
          {
              cerr << "Fail to open file\n";
              exit(1);
          }
  
          while (ifs)
          {
              string str;
              getline(ifs, str);
              cout << str << endl;
          }
  
          ifs.close();
      }
  }
  
  /* stdout stderr
  Line 1
  Line 2
  */

  ---

• ios::app 사용

  #include <iostream>
  #include <string>
  #include <fstream>
  
  int         main()
  {
      using namespace std;
  
      string filename = "my_first_file.txt";
  
      {
          ofstream ofs(filename, ios::app);
  
          if (!ofs)
          {
              cerr << "Fail to open file\n";
              exit(1);
          }
  
          ofs << "Line 1\n";
          ofs << "Line 2\n";
  
          ofs.close();
      }
  
      {
          ifstream ifs(filename);
  
          if (!ifs)
          {
              cerr << "Fail to open file\n";
              exit(1);
          }
  
          while (ifs)
          {
              string str;
              getline(ifs, str);
              cout << str << endl;
          }
  
          ifs.close();
      }
  }
  
  /* stdout stderr (3번 반복 실행 시)
  Line 1
  Line 2
  Line 1
  Line 2
  Line 1
  Line 2
  */

  ---

• binary 포맷 예제

  #include <iostream>
  #include <string>
  #include <fstream>
  
  int         main()
  {
      using namespace std;
  
      string filename = "my_first_file.txt";
  
      {
          ofstream ofs(filename, ios::binary);
  
          if (!ofs)
          {
              cerr << "Fail to open file\n";
              exit(1);
          }
  
          const unsigned num_data = 10;
          ofs.write((char*)&num_data, sizeof(num_data));
          for (int i = 0; i < num_data; ++i)
              ofs.write((char*)&i, sizeof(i));
  
          ofs.close();
      }
  
      {
          ifstream ifs(filename, ios::binary);
  
          if (!ifs)
          {
              cerr << "Fail to open file\n";
              exit(1);
          }
  
          unsigned num_data = 0;
          ifs.read((char*)&num_data, sizeof(num_data));
          cout << "Number of data : " << num_data << endl;
          for (unsigned i = 0; i < num_data; ++i)
          {
              int num;
              ifs.read((char*)&num, sizeof(num));
              cout << num << endl;
          }
  
          ifs.close();
      }
  }
  
  /* stdout stderr
  Number of data : 10
  0
  1
  2
  3
  4
  5
  6
  7
  8
  9
  */

  ---

• stringstream 사용

  #include <iostream>
  #include <string>
  #include <fstream>
  #include <sstream>
  
  int         main()
  {
      using namespace std;
  
      string filename = "my_first_file.txt";
  
      {
          ofstream ofs(filename, ios::binary);
  
          if (!ofs)
          {
              cerr << "Fail to open file\n";
              exit(1);
          }
  
          stringstream ss;
          ss << "Line 1\n";
          ss << "Line 2\n";
          string str = ss.str();
  
          unsigned str_length = str.size();
          ofs.write((char*)&str_length, sizeof(str_length));
          ofs.write(str.c_str(), str_length);
  
          ofs.close();
      }
  
      {
          ifstream ifs(filename, ios::binary);
  
          if (!ifs)
          {
              cerr << "Fail to open file\n";
              exit(1);
          }
  
          unsigned str_length = 0;
          ifs.read((char*)&str_length, sizeof(str_length));
          cout << "Size of string: " << str_length << endl;
          string str;
          str.resize(str_length);
          ifs.read(&str[0], str_length);
          cout << str << endl;
  
          ifs.close();
      }
  }
  
  /* stdout stderr
  Size of string: 14
  Line 1
  Line 2
  */