1. 1 Chap4. Fundamental File Structure Concepts

2. 2 Chapter Objectives  Introduce file structure concepts dealing with Stream files Reading and writing fields and records Field and record boundaries Fixed-length and variable-length fields and records Packing and unpacking records and buffers  Present an object-oriented approach to file structures Methods of encapsulating object value and behavior in classes Classes for buffer manipulation Class hierarchy for buffer and file objects and operations Inheritance and virtual functions Template classes

3. 3 Contents 4.1 Field and Record Organization 4.2 Using Classes to Manipulate Buffers 4.3 Using Inheritance for Record Buffer Classes 4.4 Managing Fixed-Length, Fixed-Field Buffers 4.5 An Object-Oriented Class for Record Files

4. A Stream file  File structure ==> Persistency ==> Programs outlive the data in files  Simple representation: a file organized as a stream of bytes  Simple, but Reverse Humpty-Dumpty problem In case of putting all information as a byte of stream, there is no way to get it apart Solution : Use field structure

5. The Need of Field Concept Consider the function “write Person as a stream of bytes”! Ostream & operator << (ostream & outputFile, Person & p) { // insert (write) fields into stream outputFile << p.LastName << p.FirstName << p.Address << p.City << p.State << p.ZipCode; return outputFile; } (input) Mary Ames 123 Maple Stillwater, Ok 74074 Alan Mason 90 Eastgate Ada, Ok 74820 (output) AmesMary123 MapleStillwaterOK74075MasonAlan90 Eastgate…..

6. 6 Field Organization  Field: The smallest logically meaningful unit of information in a file (not physical)  Field structures (4 methods) ÀFix the length of fields ÁBegin each field with a length indicator ÂSeparate the fields with delimiters ÃUse a “Keyword = value” expression Continued

7. 7 Four methods for organizing files Ames John 123 Maple Stillwater OK74075377-1808 Mason Alan 90 Eastgate Ada OK74820 (a) Field lengths fixed. Place blanks in the spaces where the phone number would go. Ames|John|123 Maple|Stillwater|OK|74075|377-1808| Mason|Alan|90 Eastgate|Ada|OK|74820|| (b) Delimiters are used to indicate the end of a field. Place the delimiter for the "empty"field immediately after the delimiter for the previous field. Ames|...|Stillwater|OK|74075|377-1808|#Mason|... 90Eastgate|Ada|OK|74820|#... (c) Place the field for business phone at the end of the record. If the end-of-record mark is encountered, assume that the field is missing. SURNAME=Ames|FIRSTNAME=John|STREET=123 Maple|...|ZIP=74075|PHONE=377-1808|#... (d) Use a keyword to identify each field each field. If the ketword is missing, the corresponding field is assumed to missing.

8. RW files with Field Concept  Extraction operator for delimited fields into a Person object istream & operator >> (istream & stream, Person & p) { // read delimited fields from file char delim; stream.getline(p.LastName, 30, delim); if (strlen(p.LastName) == 0) return stream; stream.getline(p.FirstName,30,delim); stream.getline(p.Address,30,delim); ….. return stream; } ** By 부록 D.5 과 D.7 Last Name ‘Ames’ First Name ‘Mary’ Address ‘123 Maple’ ………. Last Name ‘Mason’ First Name ‘Alan’ ……...

9. 9 Record Organization  Record: a set of fields that belong together  Record organization(5 methods) ÀMake records a predictable number of bytes (Fixed-length records) Fig4.5. (a)(b) ÁMake records a predictable number of fields Fig4.5. (c) ÂBegin each record with a length indicator Fig4.6. (a) ÃUse an index to keep track of addresses Fig4.6. (b) ÄPlace a delimiter at the end of each record Fig4.6. (c)

10. 10 The method for organizing records (1)  Three ways of making the lengths of records constant and predictable Fixed-length record w/ fixed-length fields Fixed-length record w/ variable-length fields Six fields per record Fig. 4.5

11. 11 The method for organizing records (2)  Record structure for variable record with a length indicator using a index file with delimiter(#) Fig. 4.6 40 36

12. Write a var-length delimited buffer to a file (from memory to disk) Const int MaxBUfferSize = 200; int WritePerson(ostream & stream, Person & p) { char buffer [MaxBufferSize]; strcpy(buffer, p.LastName); strcat(buffer, “|”); strcat(buffer, p.FistName); strcat(buffer, “|”); ….. strcat(buffer,p.Zipcode); strcat(buffer, “|”); short length=strlen(buffer); stream.write (&length, sizeof(length)); stream.write(&buffer, length) } Figure 4.7 (pp 129)

13. Reading Variable Records  Records preceded by lengths (variable length records) 40 Ames|Mary|123 Maple|Stillwater|OK|74075| 36 Mason|Alan|90 Eastgate|Ada|OK|74820 int ReadVariablePerson (istream & stream, Person & p) { // read a variable sized record from stream and store it in p short length; stream. read (&length, sizeof(lenth)); char * buffer = new char[length + 1]; // create a buffer space stream. read (buffer, length); buffer [ length] = 0; // treminate buffer with null istrstream strbuff (buffer); // create a string stream strbuff >> p; // use the istream extraction operator return 1; }

14. 14 Read-file using File Dump  File-dump gives us the ability to look inside a file at the actual bytes that are stored  Octal Dump: od -xc filename  e.g. The number 40, stored as ASCII characters and as a short integer (a) 40 stored as ASCII chars: (b) 40 stored as a 2-byte integer: Decimal value of number 40 Hex value stored in bytes ASCII character form '4''0' '\0'"(" 34 30 00 28

15. Using Classes to Manipulate Buffers  Examples of three C++ classes to encapsulate operation of buffer object Function : Pack, Unpack, Read, Write –Output: pack into a buffer & write a buffer to a file –Input: read into a buffer from a file & unpack a buffer –‘pack and unpack’ deals with only one field DelimTextBuffer class for delimited fields LengthTextBuffer class for length-based fields FixedTextBuffer class for fixed-length fields  Appendix E : Full implementation

16. Buffer Class for Deliminated Text Fields(1)  Variable-length buffer  Fields are represented as delimited text Class DelimTextBuffer { public: DelimTextBuffer (char Delim = ‘|’, int maxBtytes = 1000); int Read(istream & file); int Write (ostream & file) const; int Pack(const char * str, int size = -1); int Unpack(char * str); private: char Delim;// delimiter character char * Buffer; // character array to hold field values int BufferSize;// current size of packed fields int MaxBytes;// maximum # of characters in the buffer int NextByte;// packing/unpacking position in buffer };

17. Buffer Class for Deliminated Text Fields(2) int DelimTextBuffer :: Pack (const char * str, int size) // set the value of the next field of the buffer; // if size = -1 (default) use strlen(str) as Delim of field { short len; // length of string to be packed if (size >= 0) len = size; else len = strlen (str); if (len > strlen(str)) // str is too short! return FALSE; int start = NextByte; // first character to be packed NextByte += len + 1; if (NextByte > MaxBytes) return FALSE; memcpy (&Buffer[start], str, len); Buffer [start+len] = Delim; // add delimeter BufferSize = NextByte; return TRUE; } Pack() method copies the characters of its argument to the buffer and then adds the delimiter characters.

18. Buffer Class for Deliminated Text Fields(3) int DelimTextBuffer::Unpack(char *str) // extract the value of the next field of the buffer { int len = -1;// length of packed string int start = NextByte; // first character to be unpacked for(int i = start; i < BufferSize; i++) if(Buffer[i] == Delim) {len = i-start; break;} if(len == -1) return FALSE;// delimiter not found NextByte += len + 1; if(NextByte > BufferSize) return FALSE; strncpy (str, &Buffer[start], len); str[len] = 0;// zero termination for string return TRUE; } Unpack() is extracking one field from a record in a buffer.

19. Buffer Class for Deliminated Text Fields(4)  Read method of DelimTextBuffer Clears the current buffer contents Extracts the record size Read the proper number of bytes into buffer Set the buffer size int DelimTextBuffer::Read(istream & stream) { Clear(); stream.read((char *)&BufferSize, sizeof(BufferSize)); if (Stream.fail()) return FALSE; if (BufferSize > MaxBytes) return FALSE; // buffer overflow stream.read(Buffer, BufferSize); return stream.good(); }

20. Extending Class Person with Buffer Operations class Person{public: char lastname[11]; char firstname[11]; … char zipcode[10]; // method … int Pack(DelimTextBuffer &buf) const; // buffer operation Pack... } int Person::Pack(DelimTextBuffer &buf) const { // pack the fields into a DelimTextBuffer int result; result = buf.Pack(lastname); result = result && buf.Pack(firstname); … return result = result && buf.Pack(zipcode); } * pack deals with only one field!

21. Buffer Classes for Length-Based Fields  Almost same as the deliminated field class (compare with the previous page)  Change in the implementations of the Pack and Unpack class LengthTextBuffer { public: LengthTextBuffer(int maxBytes = 1000); int Read(istream & file); int Write(ostream & file) const; int Pack(const char * field, int size = -1); int Unpack(char * field); private: char * Buffer;// character array to hold field values int BufferSize; // size of packed fields int MaxBytyes; // maximum # of characters in the buffer int NextByte;// packing/unpacking position in buffer };

22. Buffer Classes for Fixed-length Fields Class FixedTextBuffer { public: FixedTextBuffer (int maxBytes = 1000); int AddField (int fieldSize); int Read(isteram * file); int Write(ostream *file) const; int Pack(const char * field); int Unpack (char * field); private: // character array to hold field values char * Buffer; // size of packed fields int BufferSize; // Max # of chars in the buffer int MaxBytes; // packing/unpacking position in buffer int NextByte; // array of field sizes int * FieldSizes; } int Person::InitBuffer (FixedTextBuffer &buffer) { buffer.Init(6, 61); // 6 필드, 61 바이트 buffer.AddField (10); buffer.AddField (15); buffer.AddField (2); buffer.AddField (9); return 1; }

23. Inheritance in the C++ Stream Classes class istream: virtual public ios { … class ostream: virtual public ios { … class iostream: virtual istream, public ostream { … class ifstream: public fstreambase, public istream { … class ostream: public fstreambase, public ostream {… class fstream: public fstreambase, public iostream { … Operations that work on base class objects also work on derived class objects

24. Class Hierarchy for Record Buffer Objects(1) Appendix F : full implementation  Inheritance allows multiple classes share members and methods

25. Class Hierarchy for Record Buffer Objects(2) class IOBuffer { public: IOBuffer (int maxBytes = 1000); // a MAX of maxByte virtual int Read (istream &) = 0; // read a buffer virtual int Write (ostream &) = 0; // write a buffer virtual int Pack (const void * field, int size = -1) = 0; virtual int Unpack (void * field, int maxbytes = -1) = 0; protected: char * Bufffer; // character array to hold field values int BufferSize; // sum of the sizes of packed fields int MaxBytes; // MAX # of characters in the buffer };

26. Class Hierarchy for Record Buffer Objects(3) Class VariableLengthBuffer: public IOBuffer { public: VariableLengthBuffer (int MaxBytes = 1000); int Read (istream &); int Write (ostream &) const; int SizeOfBuffer () const; // return current size of buffer }; class DelimFieldBuffer: public VariableLengthBuffer { public: DelimFieldBuffer (char Delim = -1, int maxBytes = 1000); int Pack (const void *, int size = -1); int Unpack (void *field, int maxBytes = -1); protected: char Delim; };

27. Managing Fixed-Length, Fixed-Field Buffers class FixedFieldBuffer: public FixedLengthBuffer { public: FixedFieldBuffer (int maxFields, int RecordSzie = 1000); FixedFieldBuffer (int maxFields, int *fieldSize); int AddField (int fieldSize); // define the next field int Pack(const void * field, int size = -1); int Unpack(void * field, int maxBytes = -1); int NumberOfFields () const; // return # of defined fields protected: int * FieldSzie; // array to hold field sizes int MaxFields; // MAX # of fields int NumFields; // actual # of defined fields };

28. Object-Oriented Class for Record Files  So far, we defined buffer classes  Now, we encapsulate all of our file operations! class BufferFile // file with buffers { public: BufferFile (IOBuffer &); // create with a buffer int Open(char * fname, int MODE); // open an existing file int Create (char * fname, int MODE); // create a new file int Close(); int Rewind();// reset to the first data record // Input and Output operations int Read(int recaddr = -1); int Write(int recaddr = -1); int Append(); // write the current buffer at the end of file protected: IOBuffer & Buffer; // reference to the file’s buffer fstream File;// the C++ stream of the file }; Usage: DelimFieldBuffer buffer; BufferFile file(buffer); file.open(myfile); file.Read(); buffer.Unpack(myobject);