C++ Streams © Bruce M. Reynolds & Cliff Green, 2002 1 C++ Programming Certificate University of Washington Cliff Green

© Bruce M. Reynolds & Cliff Green, 2002

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C++ Streams

C++ Programming CertificateUniversity of Washington

Cliff Green


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C++ Streams

Input and Output

• C++ uses streams for input and output

• Like C, these are not defined within the language, but in the library

• What is a stream?– Think of a stream as a sequence of characters

• To use streams, we must add “#include <iostream>”• All standard library components are in the std

namespace, so appropriate directives or qualifications must be used– using std::cout;


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C++ Streams

Why use streams?

• printf potentially wastes run-time memory– entire printf library is loaded to output a single char

• printf is theoretically slower than C++ streams– the format string is parsed at run-time instead of

compile-time– the C library does benefit from more years of

performance tuning, however• printf allows no compile-time type checking

– no protection against arguments not matching the format specifier

• printf is hard to extend to new types – %c, %d, %s, %f


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C++ Streams

Types of streams

• Streams for:

– Input: istream, ifstream, istringstream

– Output: ostream, ofstream, ostringstream


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C++ Streams

Stream operators

• ostream– output is called “insertion”– << is the “insertion” operator– Value is inserted into the output stream

• istream– input is called “extraction”– >> is the “extraction” operator– Value is extracted from the input stream


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C++ Streams

Well-known streams

• Global stream objects are defined in <iostream>

– cin istream object connected to standard input

– cout ostream object connected to standard output

– cerr ostream object connected to standard error

– clog ostream object connected to standard error providing buffered output


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C++ Streams

Examples

#include <iostream>using std::cout; using std::cin; using std::endl;

cout << “Hello, World” << endl;// endl is equivalent to “\n”, plus a buffer flush

int nValue = 123;cout << “Value is” << nValue << endl;

cin >> nValue;cout << “The new value is” << nValue << endl;


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C++ Streams

Type-safety

• Streams can accept

– Any built-in data type

– Any complex expression that evaluates to a built-in data type

– Any type that can be implicitly converted to a built-in data type

– Any user-defined type for which the << or >> operators have been defined


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C++ Streams

C++ classes preview (or review)

• Streams are instances of C++ classes.• Class instances (generally called objects) are like

structures in that they have fields (generally called members).

• Members can be data or functions (methods).• We use the “.” syntax to access members

infile.get(ch); // get a characteroutfile.close(); // close the streamoutfile.eof(); // end of stream?cout.precision(); // get the current precision


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C++ Streams

Formatting

• precision() // sets the number of decimal places

• hex // change the base to hex• dec // change the base to dec• oct // change the base to oct

#include <iomanip>using std::cout; using std::endl;cout.precision( 4 )cout << std::hex << 100 << endl; // prints “64”cout << std::dec << 100 << endl; // prints “100”cout << std::oct << 100 << endl; // prints “144”


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C++ Streams

Formatting

• left, right justification• fixed, scientific format of floats• width, precisionformat of numbers• fill sets/gets fill character• endl adds a CR and flushes• flush writes out the buffer• ws eats white space

• plus lots, lots more...


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C++ Streams

Output

put( char c )Output a single character

cout.put( ‘a’ ); // prints “a”

write ( char const * s, int n )

Output n characters from string schar const * str = “My name is”;cout.write( str, 6 ); // prints “my nam”


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C++ Streams

Input

get ( char c ) // inputs a single characterchar c;cin.get( c ); // reads one character

getline ( char * s, int n )Input up to n characters from standard input until “\n”

getline ( istream&, string& )Inputs characters from istream until “\n” (safer and more convenient than getline into a character array)

getline (cin, str);


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C++ Streams

Streams are sophisticated objects!

ios

istream ostream

iostreamifstream ofstream

fstream


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C++ Streams

Stream error states

• cin.eof() – true if end-of-file has been encountered

• cin.fail() – true when a format error has occurred – characters have not been lost– it is normally possible to recover from such a failure

• cin.bad() – true when an error occurs that results in loss of data– serious failures are normally not recoverable

• cin.good() – true if bad(), fail() and eof() are all false


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C++ Streams

File processing

• Must include <iostream>• Must also include <fstream>

• File open and creation modes• ios::app append only• ios::ate open and seek to the end• ios::in input• ios::out output• ios::trunc if file exists, overwrite it• ios::binary open file in binary mode


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C++ Streams

File processing

• Output file stream example– ofstream outFile( “filename”, ios::trunc );

• Input file stream example– ifstream inFile( “filename”, ios::in );


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C++ Streams

File processing

ios overloaded operator member functions

• operator! – returns true if fail() or bad() would return true– example: if ( !outFile ) ...

• operator void*– converts the stream to a pointer which is NULL if fail()

or bad() would return true– example: while ( inFile >> name ) ...


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C++ Streams

In-core formatting

• istringtream for input stream processing

#include <sstream> // old hdr is strstream

#include <string>

using std::istringstream; using std::string;

istringstream s(“1.414 47 This is a test”);

int i; float f;

string buff;

s >> f >> i >> buff; // buff == “This”


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C++ Streams

In-core formatting

• ostringstream for output stream processing• the str() method of ostringstream returns a string object

#include <sstream>

#include <string>

using std::ostringstream; using std::string;

int i(20), j(30);

ostringstream os;

os << “i = “ << i << “, j = “ << j;

string s (os.str());


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C++ Streams

Manipulators

• Manipulators perform formatting tasks

• Requires the inclusion of <iomanip>

• Can take zero or one argument

• Examples– setw– setprecision– dec, oct, hex, setbase


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C++ Streams

Manipulators

• Manipulators are functions

• Manipulators may be user-defined

– ostream& bell( ostream& os ) { return os << ‘\a’; }

– ostream& tab( ostream& os ) { return os << ‘\t’; }


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C++ Streams

istream functions

• get( char ) gets one character– returns istream– returns EOF on end-of-file

• get() gets one character– returns the character– returns EOF on end-of-file

• get( char* buff, int limit, char delimiter)– terminates when the delimiter is read– reads up to limit-1 characters– does not read the delimiter


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C++ Streams

istream functions

• getline() – operates like the three-argument get using CR– does not store the delimiter– alternate version is non-member fct, takes istream as

first arg, string as second, delimiter as third

• ignore skips the specified number of characters

• putback places the previous character obtained by get back on the stream

• peek returns the next character, but does not remove it from the stream


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C++ Streams

istream functions

• failbit is set if the fewer than the specified number of characters is read

• gcount returns the number of characters input by the previous read


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C++ Streams

istream functions

• operator>>( char *)• operator>>( string )

– inputs the next word (delimited by white space)– automatically skips white space– setw sets the maximum string size


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C++ Streams

Other useful manipulators and functions

• showpoint – output a floating-point number with a decimal point– number of significant digits specified by the precision

• uppercase– forces X or E for hex or floating point– forces all characters of hex to be uppercase

• boolalpha– forces boolean values to be output as “true”, “false”


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C++ Streams

Other useful manipulators

• tie– synchronizes input and output to ensure that outputs

occur before subsequent inputs


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C++ Streams

Hints for using iostreams

• Seeking within streams– ios::begin– ios::end– ios::cur

• Don’t forget operator precedence– Precedence of << is relatively low– Most expressions are OK without parentheses

• cout << a * b + c << endl;– Other operators have lower precedence!

• cout << ( a == b ? “equal” : “not equal” );


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C++ Streams

<< and >> as non-member operators

• The stream insertion operator<< and stream extraction operator>> are overloaded in compiler libraries to handle data items of– built-in types– strings– pointer values

• To use these operators with user-defined types, each class must overload the operators

• Cannot be a member function of the class, since the stream (not the class object) is the left-hand argument to the operator.


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C++ Streams

<< and >> as non-member operators, traditional technique

class MyClass {

friend ostream &operator<<(

ostream&, MyClass const& );

};

ostream& operator<<( // implementation

ostream &os, MyClass const& rhs) {

...

}


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C++ Streams

<< and >> as non-member operators, alternative technique (no friend functions)

class MyClass {public:

ostream& streamOut (ostream&) const;};ostream& operator<<(ostream &os,

MyClass const& rhs) {return rhs.streamOut(os);

}

ostream& MyClass::streamOut(ostream& os) const {return os << “ … “;

}


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C++ Streams

<< and >> as non-member operators

• Serialization: define ostream<< and istream>> overloading for all your classes.– Allows the class to write itself to any persistent

storage (file) and recover its contents back from the file.

• Debugging hint: Define ostream<< for all your classes!– Allows the developer to see the internal structure of

the object with one line of code.

Documents

C++ Streams © Bruce M. Reynolds & Cliff Green, 2002 1 C++ Programming Certificate University of Washington Cliff Green