Generate error the C++ way

Transcription

1 Reference informa9on Lecture 3 Stream I/O Consult reference for complete informa9on! UNIX man- pages (available on exam): man topic man istream man ostream ios, basic_string, stringstream, ctype, numeric_limits Online (not available on exam): File separa9on Related (cohesive) func9ons can be gathered in one file to form a package. A package can be compiled separately, and do not need recompila9on unless you change a package source file. Public declara9ons are place in a header file *.h Defini9ons are placed in a implementa9on file *.cc Header and implementa9on files should have the same name, except for the extension Header file must have a preprocessor guard to protect from mul9ple inclusion #ifndef _FILE_NAME_H_ #define _FILE_NAME_H_ // public declarations #endif Genera9ng errors Your goal should be to no-ce the program user in a clear and understandable way and then recover. We will cover the recover part later in the course. C ways Message in program or system log (good) Message to standard error (poor) Message to standard output (bad) Exit program (bad) Return error code from func9on (bad) C++ ways Throw anonymous error (bad) Throw specific error (good) Generate error the C++ way #include <exception> string msg = error message throw invalid_argument(msg); throw logic_error( bad bool ); throw domain_error( bad luck ); #include <iostream> throw ios::failure( bad file ); Stream concept An ordered stream of bytes One source generates bytes One des9na9on consumes bytes Not possible to break the given order Des9na9on can not receive again! Blind: can not see future! 1

2 Think of speech! A sequence of words. Have a source: mouth (brain?) Have a des9na9on: ear (brain?) You can not choose to hear later words before earlier. You can not choose to hear a word again. It s a stream! Hey! What if... I have a different source? ipad, Compact Disc Sure, some specific sources have addi-onal capabili9es, more than the standard stream. I ask the stream to replay? NO. Will refuse to replay. Some9mes it can let you peek on what s coming, but don t depend on it. I record the speech? Sure, you can record any stream (the sound), but you can not copy the stream itself (the one who talk)! C++ streams Variables of composed, object, type One type fits all istream (in to program, program is des9na9on) ostream (out from program, program is source) Three kinds of specific streams General I/O: cin, cout, cerr, clog File streams (file on disk act as source or des9na9on) S9ng streams (string variable in memory act as source or des9na9on) File stream input Must be connected to a file on disk before use. #include <fstream> ifstream infile; // create stream object infile.open( data.txt ); // connect stream // object ifstream in( data.txt ); // create and // connect int a, b; in >> a; // read from in infile >> b; // what do you get?? in.close(); // disconnect File stream output Must be connected to a file on disk before use. #include <fstream> ofstream ofile; // create stream object ofile.open( data.txt ); // connect stream // object ofstream out( data.txt ); // create and // connect int a4711, b512; out << a; // write to out ofile << b; // what is in the file?? out.close(); // disconnect String stream input Must be connected to a string variable before use. #include <sstream> string data ; istringstream instr; // create stream instr.str(data); // connect stream istringstream in(data); // create, connect int a, b; in >> a; // read from in instr >> b; // what do you get?? 2

3 String stream output #include <sstream> ostringstream outstr; // create stream int a4711, b512; outstr << a << b; string stroutstr.str(); // what is in str? Stream error sources Every stream will SILENTLY REFUSE further opera9on afer ANY failure: End of file A file stream reach end of file A string stream reach end of file (string) User press Ctrl- D to signal end of input Logic failure Conversion from characters to number fail Serious error Like hardware failure Error checking Must be performed just acer each opera9on Provide possibility to detect error Almost never used explicitly! Four opera9ons available: bool eof(); // slammed into eof? bool fail(); // conversion failed? bool bad(); // unrecoverable? bool good(); // no error set? Peculiari9es (that make sense) bad() and good() are not opposites A stream may be!good() even if it s!bad() fail() and good() are not opposites A stream may be!good() even if it s!fail() eof() can be set when fail() is not set Input can succeed despite slamming into eof! Error checking as usual int a, b; cin >> a >> b; if (cin) // formatted input did not fail while (cin >> a) // formatted input did not fail Error clearing Every stream will REFUSE further opera9on afer ANY failure! Errors must be cleared before stream can be operated again! Clearing an error will NOT EVER fix the problem! // clear all errorflags void clear(); 3

4 Fixing the problem When a stream encounter an error the situa9on or data that caused the error will remain! Step one: detect that you have a problem, just acer each opera-on if (! (cin >> i) ) Step two: clear the stream so it will cooperate cin.clear(); Step three: remove the data that caused the error! cin.ignore(1024, \n ); char c; int max; string s; Remove offending data while ( cin.get(c) && c!= \n ) ; // no operation max = numeric_limits<streamsize>::max(); cin.ignore(max, \n ); getline(cin, s, \n ); cin >> s; Fail- safe reading cout << enter number < 10 ; cin >> number; if (!cin.fail() ) if ( number < 10 ) // success else // good read but stupid user else if ( cin.bad() ) // unrecoverable else if ( cin.eof() ) // failed due to end of file else // conversion failed, recover Hey! What really happens? int a, b; (cin >> a) >> b; if (cin) // formatted input did not fail while (cin >> a) // formatted input did not fail Formahed I/O Applies conversion from stream byte sequence to requested data type, or from given data type to stream byte sequence. Formahed input may fail! Returns reference (lvalue!) to stream you used! // formatted input istream& operator>>(...) // formatted output ostream& operator<<(...) Unformahed input No conversion take place. Does not fail (unless you hit end of file or failed to clear previous errors!) istream& get(char& c); istream& ignore(streamsize, int); istream& get(char& c); istream& read(char*, streamsize); istream& getline(istream&, string&, int); 4

5 Unformahed output No conversion take place. Does not fail (unless you hit end of file or fail to clear previous errors!) ostream& put(char c); ostream& flush(); ostream& write(const char* s, streamsize n); Seeking in filestreams Filestreams can perform next opera9on at any posi9on in the file: istream& tellg(); istream& seekg(streampos pos, seekdir dir); ostream& tellp(); ostream& seekp(streampos pos, seekdir dir); Three seek reference posi9ons: ios::beg // beginning of file ios::cur // current position ios::end // end of file Modes for filestreams Files can be opened in several modes. Files should be closed as soon as possible. void open(const string& name, openmode mode); void close(); Possible openmodes: ios::app ios::ate ios::trunc ios::out ios::in ios::binary Manipulators Can affect how formahed I/O conversion behave. #include <iomanip> setprecision(int) fixed, scientific setfill(char), setw(int) left, right oct, dec, hex, boolalpha skipws, flush, endl Stream references Streams can be sent to func9ons, and returned from func9ons as references! It must be reference! You can not copy! Use the generic istream&, ostream& types An ifstream is an istream An istringstream is also an istream cin is an istream Print table to any stream! ostream& print_table(ostream& os) for (int i0; i < 10; i = i + 1) os << setw(4) << i << endl; return os; 5

6 Using the print func9on! int main() ofstream file( table.txt ); if (! file ) throw ios::failure( open failed! ); print_table(file); file.close(); print_table(cout) <<. << endl; return 0; Read to end of file (1 of 3) This is the most common pilall. WRONG! int value; while (!cin.eof() ) cin >> value; cout << value; Read to end of file (2 of 3) This is beher but STILL WRONG! int value; cin >> value; while (! cin.eof() ) cout << value; cin >> value; Read to end of file (3 of 3) This is the simplest and best solu9on int value; while (cin >> value) cout << value; Read line by line // read until failure (any reason) while ( getline(cin, line) ) cout << line << endl; // what happens here? while ( getline(cin >> number, line) ) cout << line << : << number << endl; Interpret line by line // Read line first, then interpret it! while ( getline(cin, line) ) istringstream iss(line); int count0, number; while (iss >> number) count = count + 1; cout << Line had << count << numbers! << endl; 6

7 Type conversion If you can read a type from cin you can also convert a string to that type! If you can write a type to cout you can also convert that type to a string! Just use a stringstream! istringstream iss(str_rep); double dbl_rep; iss >> dbl_rep; Note: This is likely to introduce rounding errors!! Handling files string filename; cout << Enter filename: ; cin >> filename; ifstream fileobject(filename); if (! fileobject ) cerr << Could not open << filename <<. << endl; throw ios::failure( File open failed. ); // Use your open fileobject here! fileobject.close(); Binary files Some people store data to files without conver9ng it to strings first: saves storage space no conversion errors due to rounding efficient read and write (no conversion, right?) But: problema9c to move files between architectures not human readable files Byte order (endianness) Endian- mess really! lihle endian: x86, alpha, avr big endian: motorola, sparc, ibm Why all the fuzz? consider a 4 byte integer type placed at address X do you place the least significant bits in X+0? (lihle) or do you place the least significant bits in X+3? (big) answer: people do both! what happen when you write one way and read the other way? yes, you guessed it! you get weird values! Binary file example // Method only for fundamental types! Not string! double pi ; char* adr_to_pinullptr; adr_to_pi = reinterpret_cast<char*>(&pi); ofstream ofs( pi.bin, ios::binary); ofs.write(adr_to_pi, sizeof(pi)); double in; char* adr_to_innullptr; adr_to_in = reinterpret_cast<char*>(&in); ifstream ifs( pi.bin, ios::binary); ifs.read(adr_to_in, sizeof(in)); More on typecas9ng to_type(from_type) Usually simplest. static_cast<to_type>(from) For pointer cast between related types. reinterpret_cast<to_type>(from) For pointer cast between unrelated types (DANGER!). dynamic_cast<to_type>(from) With run- 9me type- check. Introduced later in course. const_cast<to_type>(from) DO NOT USE. Violates purpose of const. You will not encounter any excep9ons in the course. 7

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