Module: Standard C++ Library Library: Input/output
basic_ifstreambasic_istream
Class that supports reading from named files or other devices associated with a file descriptor
#include <fstream>
namespace std {
template<class charT, class traits = char_traits<charT> >
class basic_ifstream;
}
The class template basic_ifstream supports reading from named files or other devices associated with a file descriptor. It uses a basic_filebuf object to control the associated sequences. It inherits from basic_istream and can therefore use all the formatted and unformatted input functions.
namespace std {
template<class charT, class traits = char_traits<charT> >
class basic_ifstream
: public basic_istream<charT, traits>
{
public:
typedef traits traits_type;
typedef charT char_type;
typedef typename traits::int_type int_type;
typedef typename traits::pos_type pos_type;
typedef typename traits::off_type off_type;
basic_ifstream();
explicit basic_ifstream(const char*,
ios_base::openmode = ios_base::in);
// extensions:
explicit basic_ifstream(const char*,
ios_base::openmode, long);
explicit basic_ifstream(int, char_type* = 0,
streamsize = /* default size */);
explicit basic_ifstream(FILE*, char_type* = 0,
streamsize = /* default size */);
basic_filebuf<char_type, traits_type> *rdbuf() const;
bool is_open() const;
void open(const char*, ios_base::openmode mode =
ios_base::in);
// extensions:
void open(const char*, ios_base::openmode, long);
void open(int, char_type* = 0,
streamsize = /* default size */);
void open(FILE*, char_type* = 0,
streamsize = /* default size */);
void close();
};
char_type
The type char_type is a synonym for the template parameter charT.
int_type
The type int_type is a synonym of type traits::in_type.
off_type
The type off_type is a synonym of type traits::off_type.
pos_type
The type pos_type is a synonym of type traits::pos_type.
traits_type
The type traits_type is a synonym for the template parameter traits.
ifstream
The type ifstream is a specialization of class template basic_ifstream on type char:
typedef basic_ifstream<char> ifstream;
wifstream
The type wifstream is a specialization of class template basic_ifstream on type wchar_t:
typedef basic_ifstream<wchar_t> wifstream;
basic_ifstream();
Constructs an object of class basic_ifstream, initializing the base class basic_istream with the associated file buffer. This buffer is initialized by calling the basic_filebuf constructor basic_filebuf<char_type, traits_type>(rdbuf()). After construction, a file can be attached to the basic_ifstream object by using the open() member function.
basic_ifstream(const char* s,
ios_base::openmode mode = ios_base::in);
Constructs an object of class basic_ifstream, initializing the base class basic_istream with the associated file buffer. This buffer is initialized by calling the basic_filebuf constructor basic_filebuf<char_type, traits_type>(rdbuf()). The constructor then calls open(s,mode) in order to attach the file, whose name is pointed to by s, to the basic_ifstream object.
basic_ifstream(const char* s, ios_base::openmode mode,
long protection = 0666);
Constructs an object of class basic_ifstream, initializing the base class basic_istream with the associated file buffer. This buffer is initialized by calling the basic_filebuf constructor basic_filebuf<char_type, traits_type>(rdbuf()). The constructor then calls rdbuf()->open (s, mode, protection) in order to attach the file, whose name is pointed to by s, to the basic_ifstream object. The third argument, protection, holds file permissions. It determines the read/write/execute file permissions under UNIX, but is more limited elsewhere.
NOTE -- This function is not part of the C++ Standard, but is included here as a convenience extension. See Appendix B for a complete list of Rogue Wave extensions.
basic_ifstream(int fd, char_type *buf, streamsize n);
Constructs an object of class basic_ifstream, initializing the base class basic_istream with the associated file buffer. This buffer is initialized by calling the basic_filebuf constructor basic_filebuf<char_type, traits_type>(rdbuf()).The constructor then calls rdbuf()->open (fd, buf, n) in order to attach the file descriptor, fd, to the basic_ifstream object. If the function fails, it sets ios_base::failbit.
NOTE -- This function is not part of the C++ Standard, but is included here as an extension in order to manipulate pipes, sockets, or other UNIX devices that can be accessed through file descriptors. See Appendix B for a complete list of Rogue Wave extensions.
basic_ifstream(FILE *fp, char_type* buf, streamsize n);
Constructs an object of class basic_ifstream, initializing the base class basic_istream with the associated file buffer. This buffer is initialized by calling the basic_filebuf constructor basic_filebuf<char_type, traits_type>(rdbuf ()). The constructor then calls rdbuf()->open (fp, buf, n) in order to attach the file pointer, fp, to the basic_ifstream object. If the function fails, it sets ios_base::failbit.
NOTE -- This function is not part of the C++ Standard, but is provided here as a convenience extension. See Appendix B for a complete list of Rogue Wave extensions.
void close();
Calls rdbuf()->close(); if it fails, calls the basic_ios member function setstate(failbit).
bool is_open() const;
Returns rdbuf()->is_open().
void open(const char* s,ios_base::openmode = ios_base::in);
Calls rdbuf()->open(s, mode) to open the file. If the call fails, calls the basic_ios member function setstate(failbit).
void open(const char* s, ios_base::openmode, long protection);
Calls rdbuf()->open (s, mode, protection) to open the file. If the call fails, calls the basic_ios member function setstate(failbit). The third argument, protection, holds file permissions. protection determines the read/write/execute file permissions under UNIX. It is more limited under DOS, since files are always readable and do not have special execute permission.
NOTE -- The third argument, protection, is not part of the C++ Standard, but is provided here as an extension. See Appendix B for a complete list of Rogue Wave extensions to the Standard.
void
open(int fd, char_type *buf = 0,
streamsize n = /* default size */);
Calls rdbuf()->open (fd, buf, n) to open the file. If the call fails, calls the basic_ios member function setstate(failbit).
NOTE -- This function is not part of the C++ Standard, but is provided here as an extension. See Appendix B for a complete list of Rogue Wave extensions.
void
open(FILE *fp, char_type *buf = 0,
streamsize n = /* default size */);
Calls rdbuf()->open (fp, buf, n) to open the file. If that call fails, calls setstate (ios::failbit).
NOTE -- This function is not part of the C++ Standard, but is provided here as an extension. See Appendix B for a complete list of Rogue Wave extensions.
basic_filebuf<charT,traits>* rdbuf() const;
Returns a pointer to the basic_filebuf associated with the stream.
//
// ifstream.cpp
//
#include <fstream> // for fstream
#include <iomanip> // for setw
#include <iostream> // for cout, endl
int main ( )
{
const char *ntbs = "Le minot passait la piece a frotter.";
char c;
try {
// create a read/write file-stream object
std::ifstream in ("ifstream.results.out",
std::ios::in | std::ios::out |
std::ios::trunc);
if (!in.is_open ())
throw (std::ios::failure ("open error"));
// associate the ostream object's buffer with
// the ifstream object
std::ostream out (in.rdbuf ());
// output ntbs in out
out << ntbs << std::endl;
// seek to the beginning of the file
in.seekg (0);
// output each space-separated word on a separate line
while (in.get (c)) {
if (std::isspace (c, in.getloc ()))
std::cout << std::endl;
else
std::cout << c;
}
std::cout << std::endl;
// clear stream state, failbit | eofbit set by last call
// to get ()
in.clear ();
// move back to the beginning of the file
in.seekg (0);
char buf [40];
// guard against buffer overflow
in.width (sizeof buf);
// set right justification
std::cout << std::right;
// does the same thing as the previous code
// output each word on a separate line in a field
// of width 10
while (in >> buf) {
std::cout.width (10);
std::cout << buf << std::endl;
}
std::cout << std::endl;
// clear flags, last in >> buf set fail bit
// because of a newline at end of string
in.clear ();
// output the base info before each integer
out << std::showbase;
std::ostream::pos_type pos= out.tellp ();
const long l = 10;
// output l in hex with a field with of 10
out << std::hex << std::setw (10) << l << std::endl;
// output l in oct with a field with of 10
out << std::oct << std::setw (10) << l << std::endl;
// output l in dec with a field with of 10
out << std::dec << std::setw (10) << l << std::endl;
// move back to the beginning of the file
in.seekg (0);
// output the entire file
std::cout << in.rdbuf () << std::endl;
// clear the flags
in.clear ();
// seek the input sequence to pos
in.seekg (pos);
int a, b, d;
in.unsetf (std::ios::basefield);
// read the previous outputted integer
in >> a >> b >> d;
// output 3 times 10
std::cout << a << std::endl << b << std::endl << d
<< std::endl;
}
catch (std::ios::failure e) {
std::cout << e.what ();
}
return 0;
}
Program Output:
Le
minot
passait
la
piece
a
frotter.
Le
minot
passait
la
piece
a
frotter.
Le minot passait la piece a frotter.
0xa
012
10
10
10
10
char_traits, ios_base, basic_ios, basic_filebuf, basic_ofstream, basic_fstream
ISO/IEC 14882:1998 -- International Standard for Information Systems --Programming Language C++, Section 27.8.1.5
Copyright (c) 1994-2006 Rogue Wave Software, a Quovadx Division.
Licensed under the Apache License, Version 2.0.
Contact Rogue Wave about documentation or support issues. You can also seek help from other developers through the Apache stdcxx community (see below).