C++ Crash Kurs

Dr. Dennis Pfisterer

Institut für Telematik, Universität zu Lübeck

http://www.itm.uni-luebeck.de/people/pfisterer

Strings and iostreams

• Strings are a major component of any C program– Used a constants on the stack, e.g.: “Hello World”

• C Strings– Strings are represented as arrays of characters– A char* pointer points to the first character of a string

Introduction to strings

– A char* pointer points to the first character of a string– End of a string is indicated by the character ‘\0’– Libc functions for string manipulation (strlen, strcat, …)

• C Strings are also– a major source of errors– time-consuming to use

� C++ provides a superior, common string class

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C++ strings• C++ strings hide the internal data representation from the user

– Automatic memory management, string termination, …– “Escaping” to c-strings possible

• Avoid the most common errors in C– Overwriting allocated char array bounds

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– Overwriting allocated char array bounds– Accessing arrays through uninitialized pointers– Using pointers pointing to already free‘d memory regions

• The C++ standard defines a behavior, not an implementation– Different implementations but same behavior– E.g., class wrappers for C-strings, reference counting, copy-on-write, …

• Defined in the namespace std as string– #include <string>

C++ strings: Construction and Initialization• Strings are constructed like normal classes

• Strings may be initialized– as an empty string

– using a character constant

– using (part of) a c-string

– using (part of) another string

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– using (part of) another string

• Example– #include <string>

using namespace std;int main(int argc, char** argv) {string empty_string;string hello_world(“Hello world");

string copy_of_hello_world(hello_world);string substring(hello_world, 0, 5); // will be hello

}

C++ strings: Find, insert & replace characters

• Find the position of a substring– int s.find(string needle, int start_pos = 0);– Returns 0-based index of the occurence– If needle is not found, string::npos is returned

• Insert characters/strings into existing strings

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• Insert characters/strings into existing strings– s.insert(int start_pos, string insert_string);

• Replace characters/string in existing strings– s.replace(int start_pos, int replace_length, string replacement);

• Many overloaded versions available in std:string– find, find_first_of, find_last_of, find_first_not_of, find_last_not_of, rfind, …

C++ strings: Find, insert & replace characters

• Usage of find_first_of and find_first_not_of– const string numbers = "0123456789";

string s = "I was born in 1976";

int start = s.find_first_of(numbers);

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int start = s.find_first_of(numbers);

if( start != string::npos ) {int end = s.find_first_not_of(numbers, start);cout << “First number: " << s.substr(start, end - start) << endl;

}elsecout << “No number found in string” << endl;

C++ strings: Comparison• C++ provides several methods for comparing strings

– Basic operators ==, !=,>, <, >=, <=

– Direct comparison with char* possible

• Also different overloaded compare(…) methods available– Compare two complete strings.

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– Compare two complete strings.– Compare part of either string to a complete string– Compare (different) subsets of two strings

• Example– string s1("One"), s2("Two");char* tmp = “One”;

– if( s1 == s2 ) … ;– if( s1 != s2 ) … ;– if( s1 == tmp ) … ;– if( s1.compare(1, 7, s2, 22, 7) == 0 ) … ;

C++ strings: Revert to C-style strings• strings can be used with functions taking “traditional” char* arguments– Method string::c_str() provides a pointer to a const char* array– It is not allowed to use the string in a non-const way

• Example

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• Example– string s1 = “Test”;const char* tmp = s1.c_str();

const char* tmp = s2.c_str();printf("This is a standard c-string: %s\n", tmp);

if( tmp == s2 )cout << "Yes, tmp and s2 are the same" << endl;

See Demo: strings.cpp

iostreamsiostreams

Why another I/O library• Libc offers a variety of functions to deal with I/O

– Basic I/O: open, read, write, close, creat, …– Console: printf, scanf, putc, getc, puts, gets, …– File: FILE*, fopen, fclose, fwrite, fseek, …

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• Different functions for (nearly) the same problem– Many hard-to-guess parameters which should have default values

– No integrated support for i18n (internationalization)– No common interface to these similar problems

� C++ iostreams offer a common interface and superior functionality

iostreams: Fundamentals• The fundamental abstractions are streams

– A stream transports and formats characters of a fixed width – Implemented as objects– Divided into input (istream) and output (ostream) streams– Combination of input and output is called iostream– #include <iostream> provides the basic functionality

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• Different implementations using the same interface are available such as for– Console: cout, cin– Files: ifstream, ofstream, and fstream– Strings: istringstream, ostringstream, and stringstream

• Two (overloaded) operators provide a common interface– operator<< is called inserter– operator>> is called extractor

iostreams: Extractors• Extractors parse information from a stream according to the destination type

– Overloaded operators available for each basic data type

• Predefined extractor object available for console interaction– Defined in #include <iostream>, called std::cin

• Example

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• Example– int i;

cin >> i;

– float f;cin >> f;

– char c;cin >> c;

– string s;cin >> s;

iostreams: Inserters• Inserters write information to a stream according to the data type

– Overloaded operators available for each basic data type

• Predefined inserter object available for console interaction– Defined in #include <iostream>, called std::cout

• Example– cout << "i = ";

cout << i;

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– cout << "i = "; cout << i;cout << "\n";

– cout << "f = ";cout << f;cout << "\n";

– cout << "c = ";cout << c;cout << "\n";

– cout << "buf = ";cout << buf;cout << "\n";

iostreams: Convenient use of inserters/extractors

• Standard operator<< methods returns an ostream&– Signature: ostream& operator<<(ostream& o, <type> t);

• Operator calls may be cascaded– Instead of:

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– Instead of:• cout << "i = "; cout << i;cout << "\n";

– Cascaded operator call:• cout << "i = " << i << endl;

• Resolves to– operator<<( operator<<( operator<<(cout, “i = ") , i) , endl );

iostreams: Line based I/O• iostream offers three options for line-based I/O

– Member function get( ) and getline( ) operating on char*– Global function getline() defined in #include <string>

• get and getline may be dangerous

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• get and getline may be dangerous– Use getline and string when possible

• Example– string s;getline(cin, s);

– cout << “You have entered: “ << s << endl;

iostreams: Formatting output• iostreams provides a number of methods to control formatting

• Example– width(int w) causes leading blanks to be added to any value fewer than 10 characters wide for the next output operation

– double values[] = { 1.23, 34.56, 678.9, 1234.56, 123456789.123 }; for( int i = 0; i < 4; i++ )

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– double values[] = { 1.23, 34.56, 678.9, 1234.56, 123456789.123 }; for( int i = 0; i < 4; i++ ) {

cout.width(10); cout << values[i] << endl;

}

• Output– ______1.23 _____35.36 _____653.7 ___4358.24123456789.123

iostreams: Formatting output• Example

– fill(int char) uses another character than blank for padding

– double values[] = { 1.23, 34.56, 678.9, 1234.56, 123456789.123 }; for( int i = 0; i < 4; i++ ) { cout.width(10);

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cout.width(10); cout.fill(‘*’);cout << values[i] << endl;

}

• Output– ******1.23 *****35.36 *****653.7 ***4358.24123456789.123

iostreams: Formatting output• Many other formatting options available using so-called flags

• Flags are controlled using set/unset methods– fmtflags ios::setf(fmtflags ored_flag);– fmtflags ios::unsetf(fmtflags clear_flag);

• Some flags are grouped to a field

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• Some flags are grouped to a field– ios::dec, ios:hex and ios::oct belong to the group ios::basefield

– fmtflags ios::setf(fmtflags bits, fmtflags field); is used to override the current flags of a field with a new value

• Example (switch from decimal to hexadecimal output)– Variant 1: First, unset decimal flag, then set hex flag

• cout.unsetf(ios::dec); cout.setf(ios::hex);

– Variant 2: Override all flags of field ios::basefield• cout.setf(ios::hex, ios::basefield);

iostreams: Formatting output• Example: Decimal and Hex output

– #include <iostream>using namespace ios;using namespace std;

– int main(){int i = 12;

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int i = 12;

cout.setf(hex, basefield);cout << "Hex: " << i;

cout.setf(dec, basefield);cout << ", decimal: " << i << endl;

}

• Output– Hex: c, decimal: 12

Demo: basic.cpp

iostreams: Formatting output using manipulators

• Using methods to change a streams’ status complicates its use– iostreams provides manipulators that modify a stream “inline”

– #include <iomanip>

• Example using member functions

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• Example using member functions– cout.setf(ios::hex, ios::basefield);cout << "Hex: " << i;cout.setf(ios::dec, ios::basefield);cout << ", decimal: " << i << endl;

• Example using manipulators– cout << "Hex: " << ios::hex << i << ", decimal: " << ios::dec<< i << endl;

iostreams: Manipulators w/o argumentsManipulator Effect

dec Same as ostream::setf(ios::dec, ios::basefield)

oct Same as ostream::setf(ios::oct, ios::basefield)

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hex Same as ostream::setf(ios::hex, ios::basefield)

flush Flushes buffered (if any) data to the outut

endl Ends the current line (i.e., ‘\n‘) and calls flush

ws Reads as many whitespace characters (such as ‘ ‘, ‘\n‘, ‘\t‘, …) from the

stream until a non-whitespace character is found

iostreams: Manipulators w/o arguments (1/2)

Manipulator Effect

showbase

noshowbaseShow numeric base (dec, oct, or hex) when printing basic data types

showpos

noshowposShow plus sign (+) for positive values

uppercase

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uppercase

nouppercaseUse uppercase characters A-F for hexadecimal values and display E for scientific values

showpoint

noshowpointShow decimal point and trailing zeros for floating-point values

skipws

noskipwsSkip white space on input

left

right

internal

left-align, pad on right

Right-align, pad on left

Fill between leading sign or base indicator and value

scientific

fixed

Display preference for floating-point output (scientific notation vs. fixed-point decimal).

iostreams: Manipulators with arguments (2/2)

Manipulator Effect

setiosflags(fmtflags n)Equivalent to ostream::setf(n). Remains until the next change (e.g., ios::setf())

resetiosflags(fmtflags n)Clears only the format flags specified by n.

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resetiosflags(fmtflags n)Clears only the format flags specified by n. Also in effect until the next change

setbase(base n)Changes base to n, where n is 10, 8, or 16 Prefer to use dec, oct, and hex

setfill(char n) Sets the fill character to n, such as ios::fill( )

setprecision(int n) Sets the precision to n, such as ios::precision( )

setw(int n) Sets the field width to n, such as ios::width( )

Demo: manipulators.cpp

iostreams: Implementing manipulators• Custom manipulators help to simplify repeating tasks on streams

• Definition of the endl manipulator– ostream& endl(ostream&);

• Example of a custom manipulator

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• Example of a custom manipulator– ostream& ecust_cpp(ostream& os) {return os << “The following student participated in the C++ course:“;

}

– string names[] = { “John Doe”, “Stephen Miller”, “Jessica Smith” };for(int i = 0; i < 3; ++i)cout << ecust_cpp << names[i] << endl;

iostreams: Implementing custom inserters/extractors

• Custom inserter/extractors “flatten” objects to/from streams– Useful for persistence

• Create an operator<< and operator>> for your data type– First parameter is non-const stream reference

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– First parameter is non-const stream reference

– Second parameter is (const) reference to your object

– Method returns the supplied stream reference for cascading

• Signatures– ostream& operator<<(ostream& o, const YourType& t);

– istream& operator>>(istream& o, YourType& t);

iostreams: Implementing custom inserters• Example

– class Point { public: Point(int x, int y) : x_(x), y_(y) {}int x_; int y_; …

};

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– ostream& operator<<(ostream& o, const Point& p){return o << “(“ << p.x_ << “,“ << p.y_ << “)“;

}

– Point p(1,1);cout << “Point p = “ << p << endl;

• Output: – (1,1)

iostreams: Implementing custom extractors• Extractors are slightly more difficult since the input data may have an unexpected format– Extractors signal failure by setting flags on the istream– E.g., ios::failbit, ios:: eofbit, ios:: badbit, …– ios::goodbit signals that the stream is good

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• Interact with flags using member functions– good(), bad(), eof(), fail()– setstate(ios::failbit); sets the fail flag– clear(); resets fail flags

• A stream can also be used in Boolean expressions– while( instream )

do_something(); //Run until !good()

iostreams: Implementing custom extractors

28Demo: customop.cpp

iostreams: File I/O• #include <fstream> provides file I/O functionality

– ifstream for file input– ofstream for file output– Same interface as console I/O

• Different open modes available

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• Different open modes available– ios::app, append to an existing file– ios::ate, seek to the end of a file (input or output)– ios::trunc, truncate an existing file– ios::binary, use binary, not text mode

• Example– ifstream in(“file.txt”);

– ofstream out(“outfile.txt”, ios::app | ios::binary);

iostreams: File I/O• Use like any other stream

• Examples– ofstream out(“outfile.txt”, ios::app | ios::binary);

out << “This is the file’s content” << endl;

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out << “This is the file’s content” << endl;

out.close();

– ifstream in(“infile.txt”);

int i; in >> i;

in.close();

Demo: file-io.cpp

iostreams: String I/O• C++ strings may be used as source and target of stream operations

– std::istringstream and std::ostringstream provided by <sstream>

• Examples– #include <sstream>#include <string>#include <iostream>

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#include <string>#include <iostream>using namespace std;

– istringstream ci("47 1.414 This is a test"); int i; float f; string s;ci >> i >> f >> s;

– ostringstream s;s << “This is a test ” << hex << 10 << dec << 10 << endl;string result = s.str();

iostreams: Read a file into a string

• Example

– ifstream f(“infile.txt“); ostringstream os;string s;

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– os << f.rdbuf(); //rdbuf() returns a streambuffer object s = os.str();