C++基礎::檔案流

構造及檔案開啟與否的判斷

• 構造

  const std::string filename;
  std::ofstream ofs(filename);
                      // C++11
  std::ofstream ofs(filename.c_str());
                      // before C++11

• 檔案開啟成功與否的判斷

  std::ifstream ifs(filename);
  if (!ifs)
                      // if (!ifs.good())
  {
      std::cerr << "cannot open the input file \"" << filename << "\"" << std::endl;
      exit(EXIT_FAILURE);
  }

從檔案流中讀資料的方式

std::ifstream ifs(filename);
assert(ifs.good());

• 逐字元

  char c;
  while (ifs.get())
      std::cout << c;

• 逐行

  std::string line;
  while (std::getline(ifs, line, '\n'))
                          // std::getline()的標頭檔案在 <string>
      std::cout << line << std::endl;

• 逐單詞（以空格為分割）讀取

  std::string word;
  while (ifs >> word)
      std::cout << word << std::endl;

臨時建立的檔案流

std::ofstream("./1.txt") << "hello";
std::ofstream("./1.txt", std::ios::app) << " world!" << std::endl;

std::ifstream ifs("./1.txt");
assert(ifs.good());
std::string line;
while (std::getline(ifs, line, '\n'))
    std::cout << line << std::endl;