Streams and files

Enum io_state

{

goodbit = 0x00;

eofbit = 0x01;

failbit = 0x02;

badbit = 0x04;

hardfail = 0x80;

};

For access to error status following functions can be used:

int rdstate(); returns current error status;

void clear(int i); sets error bit;

int good(); returns nono-zero when no errors;

int eof(); returns non-zero when end of file is reached;

int bad() return non-zero if badbit==1 or hardfail==1.

More simply the status of a stream can be checked in this way:

if (cin>>x) {		if (!cout) {
// successful input	or	// unsuccessful ouput
...		cerr<<”Something wwrong”<<”\n”;
}		}

This way is more elegant. It is possible due to overloading of operators! and void*:

int operator!();

operator void*();

Operator! returns non--zero value if bits failbit, badbit or hardfail are set.

Operator void*() converts the stream into the pointer. If failbit, badbit or hardfail are set this pointer will be NULL.

For streams of files the following hierarchy of cvlasses is used:

istream		ifstream
				fstream
ostream		ofstream

Classes ifstream and ofstream are described in <fstream.h> headr file.

EXAMPLE: program that copies text from file “complex.cpp” into file “target.cpp”, character by character.

#include <stdio.h>
#include <fstream.h>
main()
{
char ch;
ifstream f1("complex.cpp");
if (!f1) {
cerr<<"Can't open file f1"<<"\n";
}else{
ofstream f2("target.cpp");
if (!f2) {
cerr<<"Can't create file f2"<<"\n";
}else{
while (f1.get(ch)) {f2.put(ch);};
}
}
return 0;
}

If constructor failes to open file, it returns zero (!f1 or!f2 in our case). Destructors automatically will close files “complex.cpp” and “target.cpp”. Of course, destructors ~istream() and ~ofstream() are called automatically when program main() terminates.

The stream can be created with no connection to file. File name can be connected to stream later, during file opening:

ofstream of;
of.open(“target.txt”);	connection with file on disk
...
of.close();	broke this connection

Following functions can be used for opening streams:

void ofstream::open(char* name, int=ios::out);

void ifstream::open(char* name, int=ios::in);

The first parameter is MS DOS file name. The second parameter defines opening mode:

ios::app	append record to the end of file;
ios::ate	open file and go to the end of file;
ios::in	open file for input
ios::out	open file for output
ios::trunc	truncate file (make file length equal to zero)
ios::nocreate	if file does not exists, opening doesn’t create it;

These modes are defined in class ios. Programmer can define several opening modes as disjunction of base modes:

fstream inout(“data.txt”,ios::in || ios::out);

inout<<i;

Дата добавления: 2014-11-28