C++ input and output streams

input and output stream

• Description: IO objects have no copy or assignment operations, and there are many flag bits that can determine the current state of IO

console input and output

• mainly cinwithcout
• The standard library will be cinassociated coutwith cin, and the cout buffer will also be flushed when using cin

file input and output

file mode

• Classification
  
  • in: open for reading
  • out: open for writing
  • app: Before each write operation, locate the end of the file
  • ate: After opening the file, immediately locate the end of the file
  • trunc: truncate file
  • binary: run io in binary mode
• illustrate
  
  • Only set out mode on ofstreamor fstreamobject
  • Only if the in mode is set on the ifstreamor fstreamobject
  • The trunc mode can only be set when the out mode is also set.
  • Only using out mode will overwrite the content of the original file
• Note: Use the open function to open the input and output stream, and use close to close it. Of course, for the declared variable, it can be closed because the object is destroyed, but it is recommended to close it manually as much as possible to prevent subsequent misoperations

• code

  void test()
  {
      vector<string> words;
      string s;
      char ch[1024];
      ifstream infile("text.txt", ios::in);
      ofstream ofile("sentence.txt", ios::out);
      if (ofile)
      {
          // 按行读入
          while (infile.getline(ch, 1024))
          {
              cout << ch << endl;
              ofile << ch << endl;
          }
          infile.close();
          ofile.close();
      }
  
      infile.open("text.txt", ios::in);
      ofile.open("words.txt", ios::out);
      if (ofile)
      {
          // 按单词读入(遇到空格、换行、制表符等都会视为一个单词的结束)
          while (infile >> s)
          {
              cout << s << endl;
              ofile << s << endl;
          }
          infile.close();
          ofile.close();
      }
  }
  

string stream

Introduction

• Three classes are defined in the sstream header file to support memory IO, istringstreamread data from ostringstreamstring, write data to string, and stringstreamsupport the above two operations

• code

  void test()
  {
      vector<string> words;
      string s;
      char ch[1024];
      ifstream infile("contact.txt", ios::in);
  
      unordered_map<string, vector<int>> infos;
  
      while (getline(infile, s))
      {
          string key;
          vector<int> vec;
          int tmp = 0;
          istringstream is(s);
          is >> key; // 构造一个包含字符串的读入缓冲区，然后可以从这个缓冲区中读取所需要的信息
          while (is >> tmp)
          {
              vec.push_back(tmp);
  
          }
          infos[key] = vec;
          cout << s << endl;
      }
      cout << "=====infos======" << endl;
      ostringstream os;
      for (auto it = infos.begin(); it != infos.end(); it++)
      {
          os << "key is : " << it->first << ", value is : ";
          for (auto & num : it->second)
              os << num << " ";
          cout << os.str() << endl;
          os.str("");
          //os.clear(); //这句话只是清空缓冲区的标志位，并没有清空缓冲区的内容
      }
  
  }
  

• Content in contact

  AA 1 4 7
  BB 2 5 8
  CC 3 6 9