1 / 31

IO Streams IOStreams are part of the Standard C++ library Meant to be used in place of C stdio library So why re-invent

IO Streams IOStreams are part of the Standard C++ library Meant to be used in place of C stdio library So why re-invent the standard io library. IO Streams Considerations for IOStreams vs. printf... printf uses format strings

tadhg
Download Presentation

IO Streams IOStreams are part of the Standard C++ library Meant to be used in place of C stdio library So why re-invent

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. IO Streams • IOStreams are part of the Standard C++ library • Meant to be used in place of C stdio library • So why re-invent the standard io library 1 February2006

  2. IO Streams Considerations for IOStreams vs. printf... • printf uses format strings • printf format strings are interpreted at run time not compile time • With printf binding between variables and values occurs at runtime • With IO streams there are no format strings • Cannot overload printf 1 February2006

  3. IOStream Class Hierarchy IOS streambuf* readbuf( ) streambuf ostream const ostream& operator << (type) const ostream& operator << (streambuf*) istream const istream& operator >> (type) const istream& operator >> (streambuf*) ofstream ostrstream ifstream istrstream 1 February2006

  4. Input Streams The extraction Operator >> • Performs the same function as scanf in the C stdio library • Uses white space to delimit its arguments • Type at the keyboard • 234 678 • Execute code • int i, j; • cin >> i >> j; • i contains 234 • j contains 678 1 February2006

  5. Input Streams The extraction Operator >> • Because istream extraction works like scanf….. • It has many of the same problems • Type at the keyboard • my name • Execute code • int i, j; • cin >> i >> j; • values I and j are left unchanged 1 February2006

  6. Input Streams • istream member functions to the rescue…. • get( ) • getline( ) • Allow us to read an entire line or buffer 1 February2006

  7. Input Streams get Member Function Prototypes • class istream • { • public: • ... • istream& get(char* ptr, int len, char delim = `\n'); • istream& get(char& c); • int get(); • istream& getline(char* ptr, int len, char delim = `\n'); • istream& getline(char& c); • int getline(); • istream& read(char* ptr, int n); • }; 1 February2006

  8. Input Streams • Member function get • Leaves delimiter in input stream • Member function getline • Reads and discards delimiter • Member function read • Used to read raw non-delimited bytes 1 February2006

  9. Output Streams The Insertion Operator << • Performs the same function as printf in the C stdio library • Its function members are similar to those in the istream class • class ostream • { • public: • ... • ostream& put(char c); • ostream& write(const char* buffer, int length); • }; 1 February2006

  10. Output Streams The Insertion Operator << • Member Function put • Inserts single character into output stream • Member Function write • Writes specified number of characters ... • From designated buffer into output stream 1 February2006

  11. Error Handling on Input Reading Correct Input Data... • IOstreams designed so they can be tested • if (cin) • if (!cin) • Return true If the last read was successful • Return false If the last read was unsuccessful 1 February2006

  12. Error Handling on Input char buffer[256]; while (cin.getline(myBuff, sizeof(myBuf))) { // process the characters in the buffer. } char buffer[256]; while (cin.get(myBuff, sizeof(myBuf))) { cin.get() // read and discard newline // process the characters in the buffer. } 1 February2006

  13. Error Handling on Input Testing for Errors… • cin.good() • Returns true if no errors • cin.eof() • Returns true if the input stream is at end of file • cin.fail() • Returns true if some input operation has failed • cin.bad() • Returns true if something has gone wrong with streambuf 1 February2006

  14. Error Handling on Input Testing for Errors… • Once an input stream returns an error, it will no longer read any input • To clear an error condition on a stream one must call the clear() error member function • Syntax • cin.clear(); 1 February2006

  15. File IO Streams ifstream and ofstream • To open file… • File stream's constructor is passed the file to open • Prototype • ofstream output( char* myFileName ); 1 February2006

  16. File IO Streams ifstream and ofstream void main( void ) { ofstream output("myFile.dat"); if (!output) { cerr << "Oops problem!!! Can’t open myFile.dat" << endl; } return; } 1 February2006

  17. File IO Streams ifstream and ofstream • Observe the function ignore( ) …. • It’s tossing out the newline character left in input stream by • the get function • Prototype • istream& ignore(int n = 1, int delim = EOF); • n specifies the number of characters to ignore • delim character after which ignore quits. 1 February2006

  18. File IO Streams ifstream and ofstream • Observe the second argument to the ifstream constructor • ios::in • Specifies file to be opened for input • Default is input on an input stream 1 February2006

  19. IO Streams Seeking in IOStreams... • Absolute stream location in a stream called streampos • Can obtain current stream position • tellp( ) member function of ostream • Identifies the current position in the output streambuf • tellg( ) member function of istream • Identifies the current position in the input streambuf • To move within a streambuf • seekp member function of ostream • seekg member function of istream 1 February2006

  20. IO Streams Seeking in IOStreams... • Prototypes • seekp (position); // absolute position • seekp (offset, ios::seekdir); // relative position • seekg (position); // absolute position • seekg (offset, ios::seekdir); // relative position • seekdir • beg - seek from beginning of current file • curr - seek from current position • end - seek backwards from end of current file 1 February2006

  21. Strstreams Formatting Using Streams… • Often when working with display screens necessary to • dynamically create messages • Useful functions in the C stdio library for doing such formatting • functions sprintf and sscanf • C++ also provides similar functionality with strstreams. • Must include <strstrea.h> 1 February2006

  22. Strstreams istrstreams… • Constructors for istrstream • Create an input stream with the string passed to • constructor as input istrstream::istrstream(char* buf); istrstream::istrstream(char* buf, int size); 1 February2006

  23. Strstreams ostrstreams… • Constructors for ostrstream • Create an output stream with the string passed to • constructor as input • ostrstream::ostrstream(char* buf, int size, int = ios::out); • out - characters formatted into starting address of buffer • Write pointer positioned at start of buffer • app - characters appended to end of buffer • Write pointer positioned at null terminator at end of string 1 February2006

  24. Strstreams ostrstreams… • The ostrstream will only allow size characters to be inserted • ostrstream does not automatically insert a null terminator into buffer….Special manipulator ends provided 1 February2006

  25. Strstreams ostrstreams and Storage Allocation • User Specified • char myBuffer[128]; • ostrstream myString(myBuffer, sizeof(myBuffer)); • myString << "Bonjour la Monde" << ends; • Automatic • ostrstream myString; • // myString automatically allocates storage to hold the string. • myString << " Bonjour la Monde” << ends;; 1 February2006

  26. Strstreams Accessing ostrstream Contents... • To access the string an ostrstream contains • Use member function str( ) ….. • ….Returns a pointer to internal buffer holding the string char* cp = A.str(); 1 February2006

  27. Strstreams Accessing ostrstream Contents and Freezing the Stream • Invoking member function str ( )freezes the stream • Action causes ostrstream return pointer to itsinternal • buffer • User invoking str ( ) is now responsible formanaging the • storage 1 February2006

  28. Strstreams Accessing ostrstream Contents and Unfreezing the Stream • Frozen stream can be unfrozen ... returns pointer to ostrstream • Syntax • ostrstreamInstance.rdbuff()->freeze(int arg = 1); • arg – 0 • Unfreeze the stream • arg – 1 (or none) • Freeze the stream 1 February2006

  29. IO Streams Overloading the Insertion Operator • When working with streams a particularly useful operator to • overload is insertion operator • Can specify how to send data to standard out or to a file • Such ability can also be useful for debugging or saving the • state of a program 1 February2006

  30. IO Streams Overloading the Insertion or Extraction Operators • When overloading insertion or extraction operator • Must support constructs of form cout << A << B << C… cin >> J >> L >> M… 1 February2006

  31. IO Streams Overloading the Insertion or Extraction Operators • Syntax • ostream& operator<<(ostream& streamObject, const type& instance); • streamObject - instance of a steam object passed by • reference • instance - instance of a user defined variable • passed by const reference 1 February2006

More Related