開啟檔案

對所有 3 個檔案流（ifstream，ofstream 和 fstream）以相同的方式開啟檔案。

你可以直接在建構函式中開啟檔案：

std::ifstream ifs("foo.txt");  // ifstream: Opens file "foo.txt" for reading only.

std::ofstream ofs("foo.txt");  // ofstream: Opens file "foo.txt" for writing only.

std::fstream iofs("foo.txt");  // fstream:  Opens file "foo.txt" for reading and writing.

或者，你可以使用檔案流的成員函式 open()：

std::ifstream ifs;
ifs.open("bar.txt");           // ifstream: Opens file "bar.txt" for reading only.

std::ofstream ofs;
ofs.open("bar.txt");           // ofstream: Opens file "bar.txt" for writing only.

std::fstream iofs;
iofs.open("bar.txt");          // fstream:  Opens file "bar.txt" for reading and writing.

你應該始終檢查檔案是否已成功開啟（即使在寫入時）。失敗可能包括：檔案不存在，檔案沒有正確的訪問許可權，檔案已被使用，發生磁碟錯誤，驅動器斷開連線…檢查可以按如下方式進行：

// Try to read the file 'foo.txt'.
std::ifstream ifs("fooo.txt");  // Note the typo; the file can't be opened.

// Check if the file has been opened successfully.
if (!ifs.is_open()) {
    // The file hasn't been opened; take appropriate actions here.
    throw CustomException(ifs, "File could not be opened");
}

當檔案路徑包含反斜槓時（例如，在 Windows 系統上），你應該正確地轉義它們：

// Open the file 'c:\folder\foo.txt' on Windows.
std::ifstream ifs("c:\\folder\\foo.txt"); // using escaped backslashes

Version >= C++ 11

或使用原始文字：

// Open the file 'c:\folder\foo.txt' on Windows.
std::ifstream ifs(R"(c:\folder\foo.txt)"); // using raw literal

或者使用正斜槓代替：

// Open the file 'c:\folder\foo.txt' on Windows.
std::ifstream ifs("c:/folder/foo.txt");

Version >= C++ 11

如果要在 Windows 上的路徑中開啟包含非 ASCII 字元的檔案，則可以使用非標準的寬字元路徑引數：

// Open the file 'пример\foo.txt' on Windows.
std::ifstream ifs(LR"(пример\foo.txt)"); // using wide characters with raw literal