MFC的CFile类以追加模式写文件

CFile类的Open函数有CFile::modeNoTruncate模式，该模式是非截断的意思，再配合CFile::modeCreate，意味着如果文件不存在，则创建，如果文件存在，则不会将文件内容清空。当对文件进行追加写入时，有一个小细节就是打开文件模式中虽然添加了这两种OpenFlag，但是文件指针并未移动到末尾，因此追加出现异常。正确的做法是在打开成功后，将文件指针移动至末尾，如下所示：

    CFile file;
    if (file.Open(_T("d:\\a.txt"), CFile::modeCreate | CFile::modeWrite | CFile::typeBinary | CFile::modeNoTruncate))
    {
        file.SeekToEnd();
    }

    TCHAR* buf = _T("abcd123456\n");
    file.Write(buf, _tcslen(buf)*sizeof(TCHAR));
    file.Close();

有些时候，文件可能有BOM头，此时就需要特殊处理下，在SeekToEnd后，获取文件指针位置，判断是否是0，是0则写BOM，否则跳过，如下所示：

    CFile file;
    if (file.Open(_T("d:\\a.txt"), CFile::modeCreate | CFile::modeWrite | CFile::typeBinary | CFile::modeNoTruncate))
    {
        file.SeekToEnd();
        if (file.GetPosition() == 0)
        {
            BYTE bom[] = { 0xFF, 0xFE };
            file.Write(bom, _countof(bom));
        }
    }

    TCHAR* buf = _T("abcd123456\n");
    file.Write(buf, _tcslen(buf)*sizeof(TCHAR));
    file.Close();

参考MSDN：

• CFile::modeNoTruncate   Combine this value with modeCreate. If the file being created already exists, it is not truncated to 0 length. Furthermore, if the file being created already exists, the underlying file pointer will point to the beginning of the file. This flag guarantees the file to open, either as a newly created file or as an existing file. This might be useful, for example, when opening a settings file that may or may not exist already. This option applies to CStdioFile as well.