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1 Chapter 17 File Processing 杨明

2 OBJECTIVES To create, read, write and update files. Sequential file processing. Random-access file processing. To use high-performance unformatted I/O operations. The differences between formatted-data and raw-data file processing. To build a transaction-processing program using random-access file processing. 2

3 17.1 Introduction 17.2 The Data Hierarchy 17.3 Files and Streams 17.4 Creating a Sequential File Topics 17.5 Reading Data from a Sequential File 3

4 17.1 Introduction 程序中的变量 数组 常量等 临时存储 data persistence: File( 文件 ) permanent retention of large amounts of data secondary storage devices sequential files and random-access files 4

5 17.1 Introduction 17.2 The Data Hierarchy 17.3 Files and Streams 17.4 Creating a Sequential File Topics 17.5 Reading Data from a Sequential File 5

6 17.2 The Data Hierarchy 6

7 17.1 Introduction 17.2 The Data Hierarchy 17.3 Files and Streams 17.4 Creating a Sequential File Topics 17.5 Reading Data from a Sequential File 7

8 17.3 Files and Streams C++ views each file as a sequence of bytes and imposes no structure on a file. Each file ends either with an end-of-file marker or at a specific byte number recorded in a systemmaintained, administrative data structure. ( 文件结束标志或指定数量字节数 ) 所谓 stream 流就是一个字节序列 : 当进行输入操作时, 字节从设备 ( 键盘 磁盘等 ) 流向内存 ; 当进行输出操作时, 字节从内存流向外部设备 ( 屏幕 磁盘等 ). 8

9 17.3 Files and Streams Stream I/O template hierarchy public 继承 9

10 1. // char TYPEDEFS, support char I/O 17.3 Files and Streams 2. typedef basic_istream< char, char_traits<char> > istream; 3. typedef basic_ostream< char, char_traits<char> > ostream; 4. typedef basic_iostream< char, char_traits<char> > iostream; 5. typedef basic_ifstream< char, char_traits<char> > ifstream; 6. typedef basic_ofstream< char, char_traits<char> > ofstream; 7. typedef basic_fstream< char, char_traits<char> > fstream; 1. // wchar_t TYPEDEFS 2. typedef basic_ios< wchar_t, char_traits<wchar_t> > wios; 3. typedef basic_istream< wchar_t, char_traits<wchar_t> > wistream; 4. typedef basic_ostream< wchar_t, char_traits<wchar_t> > wostream; 5. typedef basic_iostream< wchar_t, char_traits<wchar_t> > wiostream; 6. typedef basic_ifstream< wchar_t, char_traits<wchar_t> > wifstream; 7. typedef basic_ofstream< wchar_t, char_traits<wchar_t> > wofstream; 8. typedef basic_fstream< wchar_t, char_traits<wchar_t> > wfstream; 10

11 17.3 Files and Streams ios cin istream ostream cout, cerr ifstream iostream ofstream fstream 11

12 17.3 Files and Streams 键盘 屏幕 cin cout 内存 内存 文件 文件 ifstream 的对象 oftream 的对象 内存 内存 主要差异 : 文件操作时需定义 ifstream / ofstream 对象, 以指定所具体操作的文件和操作相关的参数 12

13 头文件 #include <iostream> #include <fstream> <fstream> 包括三种类模板的定义 basic_ifstream ( for file input ) basic_ofstream ( for file output ) 17.3 Files and Streams basic_fstream ( for file input and output ) 提供了处理 char 字符流的类模板特化定义 ifstream: 从文件中读入字符 ( 读文件 ) ofstream: 向文件输出字符 ( 写文件 ) fstream: 支持文件中字符的输入和输出 13

14 17.1 Introduction 17.2 The Data Hierarchy 17.3 Files and Streams 17.4 Creating a Sequential File Topics 17.5 Reading Data from a Sequential File 14

15 17.4 Creating a Sequential File 创建 ofstream 对象 (1) 创建流类对象的同时打开文件 ofstream( const char* filename, int mode) filename: 路径 + 文件名 ( 含后缀 ) "c:\\clients.dat" "clients.dat" // 当前路径 mode: using std::ios; ios::out ofstream 的缺省模式 1 2 若文件存在, 则打开并丢弃现有数据若文件不存在, 则创建 ios::app 向文件末尾添加数据 15

16 17.4 Creating a Sequential File 创建 ofstream 对象 (2) 先创建对象, 后打开文件 缺省构造函数 + open 成员函数 open 与前述构造函数的参数相同 ofstream outclientfile; outclientfile.open( clients.dat, ios::out); 16

17 17.4 Creating a Sequential File 文件的写操作 ( 与 cout 相似 ) outclientfile << account << << name << << balance << endl; 为何写数据的时候需要空格分隔? 17

18 17.4 Creating a Sequential File P 当流引用作为 condition 使用, 会自动隐含调用 void* 重载运算符, 以将其转换成指针 根据上一次流操作是否成功, 得到 : null 指针 : 操作失败, 则 0, 即 False Non-null 指针 : 操作成功, 则非 0, 即 True 一种常见的流读取失败是读到了 EOF 标记, 此时 condition 即为 False (ctrl+z) 18

19 17.4 Creating a Sequential File 文件的关闭 ofstream 对象析构时会自动关闭文件 建议当文件不再需要使用时, 显式调用 close 成员函数关闭 ofstream outclientfile; outclientfile.open( a.dat, ios::out); outclientfile.close(); outclientfile.open( b.dat, ios::out); outclientfile.close(); 19

20 17.1 Introduction 17.2 The Data Hierarchy 17.3 Files and Streams 17.4 Creating a Sequential File Topics 17.5 Reading Data from a Sequential File 20

21 17.5 Reading Data from a Sequential File 创建 ifstream 对象 (1) 创建流类对象的同时打开文件 ifstream inclientfile( "clients.dat", ios::in ); ios::in 缺省模式, 仅能从文件读取数据 ( 最小权限原则 ) (2) 创建对象, 后打开文件 ifstream inclientfile; inclientfile.open("clients.dat", ios::in ); 21

22 17.5 Reading Data from a Sequential File 文件的读操作 ( 与 cin 相似 ) inclientfile >> account >> name >> balance; 为何写文件的时候需要空格分隔? 读文件时, 需要空白字符分隔数据! 22

23 17.5 Reading Data from a Sequential File 已完成功能 : 打开并顺序读取文件内容, 直到文件结束 文件位置指针 : 指向下一个将要读 (get 指针 ) 或写 (put 指针 ) 的字节位置 问题 : 如何重新定位文件位置指针? Backward Forward 23

24 17.5 Reading Data from a Sequential File istream 成员函数 seekg( streamoff, seek_dir ) tellg() // 返回当前 get 指针位置 ostream 成员函数 seekp( streamoff, seek_dir ) tellp() // 返回当前 put 指针位置 24

25 17.5 Reading Data from a Sequential File 文件位置指针的偏移量和 Seek Direction ios::beg the default Positioning relative to the beginning ios::cur Positioning relative to the current position ios::end Positioning relative to the end 25

26 17.5 Reading Data from a Sequential File position to the nth byte of fileobject (default ios::beg) fileobject.seekg( n ); position n bytes forward in fileobject fileobject.seekg( n, ios::cur ); position n bytes back from end of fileobject fileobject.seekg( n, ios::end ); position at end of fileobject fileobject.seekg( 0, ios::end ); assigns the "get" file-position pointer value to variable location of type long: location = fileobject.tellg(); 26

27 17.5 Reading Data from a Sequential File 信用卡账户管理 Zero balance: 没有消费, 没有存款 1 balance == 0 Credit balance: 有存款 2 balance < 0 Debit balance: 有欠款 3 balance > 0 inclientfile.clear(); // reset eof for next input inclientfile.seekg( 0 ); // reposition to beginning of file 27

28 Summary 三种文件流 文件输入流 (ifstream) 文件输出流 (ofstream) 文件输入 / 输出流 (fstream) 文件处理步骤 定义文件流对象 打开文件 : open 读写文件 关闭文件 : close 28

29 Summary 文件读写 顺序文件操作 : 从文件的开始处依次顺序读写文件内容, 不能随机读写文件内容. 读 : 文件流类的 get getline read 成员函数以及抽取符 >> 写 : put write 函数以及插入符 << 29

30 Homework 实验必选题目 ( 交实验报告 ): 实验任选题目 ( 不交实验报告 ): 作业题目 (Homework): 30