IO Streams IOStreams are part of the Standard C++ library

Exploring the C++ Stream LibraryExploring the C++ Stream Library

Copyright 2008 Oxford Consulting, Ltd 1 October 2008

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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



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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



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IOStream Class HierarchyIOS

streambuf* readbuf( )IOS

streambuf* readbuf( )

ostreamconst ostream& operator << (type)

const ostream& operator << (streambuf*)

ostreamconst ostream& operator << (type)

const ostream& operator << (streambuf*)

istrstreamistrstreamifstreamifstreamofstreamofstream ostrstreamostrstream

istreamconst istream& operator >> (type)

const istream& operator >> (streambuf*)

istreamconst istream& operator >> (type)

const istream& operator >> (streambuf*)

streambufstreambuf



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Input StreamsThe 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


234 678

Execute code

int i, j;

cin >> i >> j;

i contains 234

j contains 678



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Input Streams

The extraction Operator >>

Because istream extraction works like scanf….. It has many of the same problems


my name

Execute code

int i, j;

cin >> i >> j;

values I and j are left unchanged


my name

Execute code

int i, j;

cin >> i >> j;

values I and j are left unchanged



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Input Streams

istream member functions to the rescue….

get( )

getline( )

Allow us to read an entire line or buffer



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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);

};

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);

};



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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



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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);

};

class ostream{

public:...ostream& put(char c);ostream& write(const char* buffer, int length);

};



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Output Streams

The Insertion Operator <<

Member Function putInserts single character into output stream

Member Function writeWrites specified number of characters ...From designated buffer into output stream



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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



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char buffer[256];

while (cin.getline(myBuff, sizeof(myBuf))) { // process the characters in the buffer.}

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.}

char buffer[256];

while (cin.get(myBuff, sizeof(myBuf))) { cin.get() // read and discard newline // process the characters in the buffer.}




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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



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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();



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File IO Streams

ifstream and ofstreamTo open file…

File stream's constructor is passed the file to open

Prototypeofstream output( char* myFileName );



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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;}

void main( void ){ ofstream output("myFile.dat"); if (!output) { cerr << "Oops problem!!! Can’t open myFile.dat" << endl; } return;}



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File IO Streams

ifstream and ofstreamObserve the function ignore( ) ….

It’s tossing out the newline character left in input stream by the get function

Prototypeistream& ignore(int n = 1, int delim = EOF); n specifies the number of characters

to ignore delim character after which ignore quits.



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File IO Streams

ifstream and ofstream

Observe the second argument to the ifstream constructorios::in

Specifies file to be opened for input

Default is input on an input stream



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IO StreamsSeeking 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



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IO Streams

Seeking in IOStreams...Prototypes

seekp (position); // absolute positionseekp (offset, ios::seekdir); // relative position

seekg (position); // absolute positionseekg (offset, ios::seekdir); // relative position

seekdir beg - seek from beginning of current filecurr - seek from current positionend - seek backwards from end of current file

Prototypesseekp (position); // absolute positionseekp (offset, ios::seekdir); // relative position

seekg (position); // absolute positionseekg (offset, ios::seekdir); // relative position

seekdir beg - seek from beginning of current filecurr - seek from current positionend - seek backwards from end of current file



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StrstreamsFormatting 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>



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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);

istrstream::istrstream(char* buf);istrstream::istrstream(char* buf, int size);



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Strstreamsostrstreams…

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 bufferapp - characters appended to end of buffer

Write pointer positioned at null terminator at end of string

ostrstream::ostrstream(char* buf, int size, int = ios::out);out - characters formatted into starting address of buffer

Write pointer positioned at start of bufferapp - characters appended to end of buffer

Write pointer positioned at null terminator at end of string



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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



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Strstreams

ostrstreams and Storage Allocation

User Specified

char myBuffer[128];ostrstream myString(myBuffer, sizeof(myBuffer));myString << "Bonjour la Monde" << ends;

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;;

Automatic

ostrstream myString;// myString automatically allocates storage to hold the string.myString << " Bonjour la Monde” << ends;;



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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();char* cp = A.str();



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Strstreams

Accessing ostrstream Contents and Freezing the Stream

Invoking member function str ( ) freezes the stream

Action causes ostrstream return pointer to its internal buffer

User invoking str ( ) is now responsible for managing the storage



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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 streamarg – 1 (or none) Freeze the stream



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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



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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…

cout << A << B << C…cin >> J >> L >> M…



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IO Streams

Overloading the Insertion or Extraction Operators

Syntaxostream& 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

Syntaxostream& 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

IO Streams IOStreams are part of the Standard C++ library

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