C++ How to Program 14.6

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Sylvia Casey
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1 C++ How to Program Random-Access Files pg.611-pg.612 -Unlike sequential files, R.A. files are instant-access applications. Any transaction-processing system. Requiring rapid access to specific data. -C++ does not impose structure on a file. Application must create them!!! -There are a variety of techniques that can be used. -The easiest way might be to require all records in a file to be same fixed length. -Same sized fixed length records allow programs to quickly calculate the exact location of any record relative to the beginning of the file. -This facilitates immediate access to records even in large files. fig.14.8 pg.612 C++ view of a random access file. data can be inserted, without destroying other data. data can be stored without needing to rewrite the entire file. offset O 1... Each block represents a fixed amount of byte space, in this case they are 100 bytes (fig.14.8 pg.612)

2 14.7 Creating a Random-Access File pg Writing Bytes with ostream Member Function write -Typically you could use outfile << number; -This would write a four-byte integer data type that could be a single digit number, or as many as 11 (10 digits plus a sign) each potential digit would consume a single byte of storage. OSTREAM WRITE pg.613 -outfile.write(reinterpret_cast< const char* > (&number), sizeof( number )) -const char * points to one byte. -From that location &number the function write outputs the number of bytes specified by its second argument sizeof(number) number being an integer of type size_t. -reinterpret_cast operator this is necessary because of compilation issues, specifically dealing with passing an int* to a const char*. Compiler would view this as inconsistent. -This operator is performed at compile time, and does not change the value of the object. Rather a request is issued by the operator to reinterpret the operand as the target type.

3 14.7 Creating a Random-Access File pg String Objects do not have fixed lengths, rather the use of dynamically allocated memory to accommodate the variants in length. -The record information must maintain the fixed length entegrity. -First and Last name for instance, would need to initialized as fixed length char arrays. -In conjunction with the string Library the length of the string can be evaluated and truncate if necessary. This task could be performed using the.size(), and.copy() functionality. (pg.615 fig lines 38-41) Opening a File for Output in Binary Mode pg.617 fig line 11 -ofstream outcredit( Credit.dat, ios::out ios::binary ); -ofstream object = outcredit -File being opened is Credit.dat -ios::out ios::binary = the second argument for ofstream object for constructor. -Indicates that the data being being written is in binary mode, a requirement for fixed length records.

4 14.8 Writing Data Randomly to Random-Access File (pg ) -From last slide the example Main (pg.617) essentially sets up 100 blank records. -Now we are ready to write to exact locations in the credit.dat file. -To perform this task we will need to take advantage of the fstream functions write & seekp. -seekp sets a put file-position pointer to a specific position in.dat file. -write function then outputs the data. -(fig pg.617) On line 16 a fstream object is created. The file is opened for input and output in binary mode. ios::in ios::out ios::binary Positioning the File Position Pointer -outcredit.seekp((client.getaccountnumber() -1) * sizeof(clientdata); <- -1 because start at byte position 0.

5 14.9 Reading from a Random-Access File Sequentially (pg ) -istream function read inputs specified number of bytes from the current position in the stream into an object. In the case of the books example the object is the client. -Example on pg.619 fig lines reads the number of bytes specified by the sizeof(clientdata), from the file associated with the ifstream object incredit then finally storing the data in the client record Case Study: Transaction-Processing Program ( fig pg.621) -pg.624 line 119 updaterecord function.

6 14.11 Object Serialization. -Serialized objects are represented by a series of bytes that includes the object's data as well as information about the objects type and the types of objects that are stored within it. -This is necessary when objects of different types are stored as bytes. Otherwise it is hard for C++ to recreate the data object, and the object types within it. -The open source Boost C++ Libraries provide serialization object support. Text, Binary, and extensible markup language formats (XML).

7 Quick Recap Questions 1.) What are some things to consider when using reinterpret_cast? Hint: Error/Portability Tips pg ) What member function does write belong to? 3.) What is an example of space time trade off? How does it relate to reading from Random-Access files sequentially. 4.) Does C++ support Object Serialization?

by Pearson Education, Inc. All Rights Reserved. 2

by Pearson Education, Inc. All Rights Reserved. 2 Data that is formatted and written to a sequential file as shown in Section 17.4 cannot be modified without the risk of destroying other data in the file. For example, if the name White needs to be changed