This is Info file iostream.info, produced by Makeinfo-1.55 from the input file /kalessin/giga/bothner/dist/devo/libio/iostream.texi. START-INFO-DIR-ENTRY * iostream: (iostream). The C++ input/output facility. END-INFO-DIR-ENTRY This file describes libio, the GNU library for C++ iostreams and C stdio. libio includes software developed by the University of California, Berkeley. Copyright (C) 1993 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions.  File: iostream.info, Node: Top, Next: Introduction, Prev: (DIR), Up: (DIR) The GNU C++ Iostream Library **************************** This file provides reference information on the GNU C++ iostream library (`libio'), version 0.64. * Menu: * Introduction:: * Operators:: Operators and default streams. * Streams:: Stream classes. * Files and Strings:: Classes for files and strings. * Streambuf:: Using the streambuf layer. * Stdio:: C input and output. * Index::  File: iostream.info, Node: Introduction, Next: Operators, Prev: Top, Up: Top Introduction ************ The iostream classes implement most of the features of AT&T version 2.0 iostream library classes, and most of the features of the ANSI X3J16 library draft (which is based on the AT&T design). This manual is meant as a reference; for tutorial material on iostreams, see the corresponding section of any recent popular introduction to C++. * Menu: * Copying:: Special GNU licensing terms for libio. * Acknowledgements:: Contributors to GNU iostream.  File: iostream.info, Node: Copying, Next: Acknowledgements, Up: Introduction Licensing terms for `libio' =========================== Since the `iostream' classes are so fundamental to standard C++, the Free Software Foundation has agreed to a special exception to its standard license, when you link programs with `libio.a': As a special exception, if you link this library with files compiled with a GNU compiler to produce an executable, this does not cause the resulting executable to be covered by the GNU General Public License. This exception does not however invalidate any other reasons why the executable file might be covered by the GNU General Public License. The code is under the GNU General Public License (version 2) for all other purposes than linking with this library; that means that you can modify and redistribute the code as usual, but remember that if you do, your modifications, and anything you link with the modified code, must be available to others on the same terms. These functions are also available as part of the `libg++' library; if you link with that library instead of `libio', the GNU Library General Public License applies.  File: iostream.info, Node: Acknowledgements, Prev: Copying, Up: Introduction Acknowledgements ================ Per Bothner wrote most of the `iostream' library, but some portions have their origins elsewhere in the free software community. Heinz Seidl wrote the IO manipulators. The floating-point conversion software is by David M. Gay of AT&T. Some code was derived from parts of BSD 4.4, which was written at the University of California, Berkeley. The iostream classes are found in the `libio' library. An early version was originally distributed in `libg++', and they are still included there as well, for convenience if you need other `libg++' classes. Doug Lea was the original author of `libg++', and some of the file-management code still in `libio' is his. Various people found bugs or offered suggestions. Hongjiu Lu worked hard to use the library as the default stdio implementation for Linux, and has provided much stress-testing of the library.  File: iostream.info, Node: Operators, Next: Streams, Prev: Introduction, Up: Top Operators and Default Streams ***************************** The GNU iostream library, `libio', implements the standard input and output facilities for C++. These facilities are roughly analogous (in their purpose and ubiquity, at least) with those defined by the C `stdio' functions. Although these definitions come from a library, rather than being part of the "core language", they are sufficiently central to be specified in the latest working papers for C++. You can use two operators defined in this library for basic input and output operations. They are familiar from any C++ introductory textbook: `<<' for output, and `>>' for input. (Think of data flowing in the direction of the "arrows".) These operators are often used in conjunction with three streams that are open by default: - Variable: ostream cout The standard output stream, analogous to the C `stdout'. - Variable: istream cin The standard input stream, analogous to the C `stdin'. - Variable: ostream cerr An alternative output stream for errors, analogous to the C `stderr'. For example, this bare-bones C++ version of the traditional "hello" program uses `<<' and `cout': #include int main(int argc, char **argv) { cout << "Well, hi there.\n"; return 0; } Casual use of these operators may be seductive, but--other than in writing throwaway code for your own use--it is not necessarily simpler than managing input and output in any other language. For example, robust code should check the state of the input and output streams between operations (for example, using the method `good'). *Note Checking the state of a stream: States. You may also need to adjust maximum input or output field widths, using manipulators like `setw' or `setprecision'. - Operator on ostream: << Write output to an open output stream of class `ostream'. Defined by this library on any OBJECT of a C++ primitive type, and on other classes of the library. You can overload the definition for any of your own applications' classes. Returns a reference to the implied argument `*this' (the open stream it writes on), permitting statements like cout << "The value of i is " << i << "\n"; - Operator on istream: >> Read input from an open input stream of class `istream'. Defined by this library on primitive numeric, pointer, and string types; you can extend the definition for any of your own applications' classes. Returns a reference to the implied argument `*this' (the open stream it reads), permitting multiple inputs in one statement.  File: iostream.info, Node: Streams, Next: Files and Strings, Prev: Operators, Up: Top Stream Classes ************** The previous chapter referred in passing to the classes `ostream' and `istream', for output and input respectively. These classes share certain properties, captured in their base class `ios'. * Menu: * Ios:: Shared properties. * Ostream:: Managing output streams. * Istream:: Managing input streams. * Iostream:: Input and output together.  File: iostream.info, Node: Ios, Next: Ostream, Up: Streams Shared properties: class `ios' ============================== The base class `ios' provides methods to test and manage the state of input or output streams. `ios' delegates the job of actually reading and writing bytes to the abstract class `streambuf', which is designed to provide buffered streams (compatible with C, in the GNU implementation). *Note Using the `streambuf' layer: Streambuf, for information on the facilities available at the `streambuf' level. - Constructor: ios::ios ([streambuf* SB [, ostream* TIE]) The `ios' constructor by default initializes a new `ios', and if you supply a `streambuf' SB to associate with it, sets the state `good' in the new `ios' object. It also sets the default properties of the new object. You can also supply an optional second argument TIE to the constructor: if present, it is an initial value for `ios::tie', to associate the new `ios' object with another stream. - Destructor: ios::~ios () The `ios' destructor is virtual, permitting application-specific behavior when a stream is closed--typically, the destructor frees any storage associated with the stream and releases any other associated objects. * Menu: * States:: Checking the state of a stream. * Format Control:: Choices in formatting. * Manipulators:: Convenient ways of changing stream properties. * Extending:: Extended data fields. * Synchronization:: Synchronizing related streams. * Streambuf from Ios:: Reaching the underlying streambuf.  File: iostream.info, Node: States, Next: Format Control, Up: Ios Checking the state of a stream ------------------------------ Use this collection of methods to test for (or signal) errors and other exceptional conditions of streams: - Method: ios::operator void* () const You can do a quick check on the state of the most recent operation on a stream by examining a pointer to the stream itself. The pointer is arbitrary except for its truth value; it is true if no failures have occurred (`ios::fail' is not true). For example, you might ask for input on `cin' only if all prior output operations succeeded: if (cout) { // Everything OK so far cin >> new_value; ... } - Method: ios::operator ! () const In case it is more convenient to check whether something has failed, the operator `!' returns true if `ios::fail' is true (an operation has failed). For example, you might issue an error message if input failed: if (!cin) { // Oops cerr << "Eh?\n"; } - Method: iostate ios::rdstate () const Return the state flags for this stream. The value is from the enumeration `iostate'. You can test for any combination of `goodbit' There are no indications of exceptional states on this stream. `eofbit' End of file. `failbit' An operation has failed on this stream; this usually indicates bad format of input. `badbit' The stream is unusable. - Method: void ios::setstate (iostate STATE) Set the state flag for this stream to STATE *in addition to* any state flags already set. Synonym (for upward compatibility): `ios::set'. See `ios::clear' to set the stream state without regard to existing state flags. See `ios::good', `ios::eof', `ios::fail', and `ios::bad', to test the state. - Method: int ios::good () const Test the state flags associated with this stream; true if no error indicators are set. - Method: int ios::bad () const Test whether a stream is marked as unusable. (Whether `ios::badbit' is set.) - Method: int ios::eof () const True if end of file was reached on this stream. (If `ios::eofbit' is set.) - Method: int ios::fail () const Test for any kind of failure on this stream: *either* some operation failed, *or* the stream is marked as bad. (If either `ios::failbit' or `ios::badbit' is set.) - Method: void ios::clear (iostate STATE) Set the state indication for this stream to the argument STATE. You may call `ios::clear' with no argument, in which case the state is set to `good' (no errors pending). See `ios::good', `ios::eof', `ios::fail', and `ios::bad', to test the state; see `ios::set' or `ios::setstate' for an alternative way of setting the state.  File: iostream.info, Node: Format Control, Next: Manipulators, Prev: States, Up: Ios Choices in formatting --------------------- These methods control (or report on) settings for some details of controlling streams, primarily to do with formatting output: - Method: char ios::fill () const Report on the padding character in use. - Method: char ios::fill (char PADDING) Set the padding character. You can also use the manipulator `setfill'. *Note Changing stream properties in expressions: Manipulators. Default: blank. - Method: int ios::precision () const Report the number of significant digits currently in use for output of floating point numbers. Default: `6'. - Method: int ios::precision (int SIGNIF) Set the number of significant digits (for input and output numeric conversions) to SIGNIF. You can also use the manipulator `setprecision' for this purpose. *Note Changing stream properties using manipulators: Manipulators. - Method: int ios::width () const Report the current output field width setting (the number of characters to write on the next `<<' output operation). Default: `0', which means to use as many characters as necessary. - Method: int ios::width (int NUM) Set the input field width setting to NUM. Return the *previous* value for this stream. This value resets to zero (the default) every time you use `<<'; it is essentially an additional implicit argument to that operator. You can also use the manipulator `setw' for this purpose. *Note Changing stream properties using manipulators: Manipulators. - Method: fmtflags ios::flags () const Return the current value of the complete collection of flags controlling the format state. These are the flags and their meanings when set: `ios::dec' `ios::oct' `ios::hex' What numeric base to use in converting integers from internal to display representation, or vice versa: decimal, octal, or hexadecimal, respectively. (You can change the base using the manipulator `setbase', or any of the manipulators `dec', `oct', or `hex'; *note Changing stream properties in expressions: Manipulators..) On input, if none of these flags is set, read numeric constants according to the prefix: decimal if no prefix (or a `.' suffix), octal if a `0' prefix is present, hexadecimal if a `0x' prefix is present. Default: `dec'. `ios::fixed' Avoid scientific notation, and always show a fixed number of digits after the decimal point, according to the output precision in effect. Use `ios::precision' to set precision. `ios::left' `ios::right' `ios::internal' Where output is to appear in a fixed-width field; left-justified, right-justified, or with padding in the middle (e.g. between a numeric sign and the associated value), respectively. `ios::scientific' Use scientific (exponential) notation to display numbers. `ios::showbase' Display the conventional prefix as a visual indicator of the conversion base: no prefix for decimal, `0' for octal, `0x' for hexadecimal. `ios::showpoint' Display a decimal point and trailing zeros after it to fill out numeric fields, even when redundant. `ios::showpos' Display a positive sign on display of positive numbers. `ios::skipws' Skip white space. (On by default). `ios::stdio' Flush the C `stdio' streams `stdout' and `stderr' after each output operation (for programs that mix C and C++ output conventions). `ios::unitbuf' Flush after each output operation. `ios::uppercase' Use upper-case characters for the non-numeral elements in numeric displays; for instance, `0X7A' rather than `0x7a', or `3.14E+09' rather than `3.14e+09'. - Method: fmtflags ios::flags (fmtflags VALUE) Set VALUE as the complete collection of flags controlling the format state. The flag values are described under `ios::flags ()'. Use `ios::setf' or `ios::unsetf' to change one property at a time. - Method: fmtflags ios::setf (fmtflags FLAG) Set one particular flag (of those described for `ios::flags ()'; return the complete collection of flags *previously* in effect. (Use `ios::unsetf' to cancel.) - Method: fmtflags ios::setf (fmtflags FLAG, fmtflags MASK) Clear the flag values indicated by MASK, then set any of them that are also in FLAG. (Flag values are described for `ios::flags ()'.) Return the complete collection of flags *previously* in effect. (See `ios::unsetf' for another way of clearing flags.) - Method: fmtflags ios::unsetf (fmtflags FLAG) Make certain FLAG (a combination of flag values described for `ios::flags ()') is not set for this stream; converse of `ios::setf'. Returns the old values of those flags.  File: iostream.info, Node: Manipulators, Next: Extending, Prev: Format Control, Up: Ios Changing stream properties using manipulators --------------------------------------------- For convenience, MANIPULATORS provide a way to change certain properties of streams, or otherwise affect them, in the middle of expressions involving `<<' or `>>'. For example, you might write cout << "|" << setfill('*') << setw(5) << 234 << "|"; to produce `|**234|' as output. - Manipulator: ws Skip whitespace. - Manipulator: flush Flush an output stream. For example, `cout << ... <'. - Manipulator: setw (int N) You can change the value of `ios::width' in `<<' expressions with the manipulator `setw(N)'; for example, cout << setw(5) << 234; prints ` 234' with two leading blanks. Requires `#include '. - Manipulator: setbase (int BASE) Where BASE is one of `10' (decimal), `8' (octal), or `16' (hexadecimal), change the base value for numeric representations. Requires `#include '. - Manipulator: dec Select decimal base; equivalent to `setbase(10)'. - Manipulator: hex Select hexadecimal base; equivalent to `setbase(16)'. - Manipulator: oct Select octal base; equivalent to `setbase(8)'. - Manipulator: setfill (char PADDING) Set the padding character, in the same way as `ios::fill'. Requires `#include '.  File: iostream.info, Node: Extending, Next: Synchronization, Prev: Manipulators, Up: Ios Extended data fields -------------------- A related collection of methods allows you to extend this collection of flags and parameters for your own applications, without risk of conflict between them: - Method: static fmtflags ios::bitalloc () Reserve a bit (the single bit on in the result) to use as a flag. Using `bitalloc' guards against conflict between two packages that use `ios' objects for different purposes. This method is available for upward compatibility, but is not in the ANSI working paper. The number of bits available is limited; a return value of `0' means no bit is available. - Method: static int ios::xalloc () Reserve space for a long integer or pointer parameter. The result is a unique nonnegative integer. You can use it as an index to `ios::iword' or `ios::pword'. Use `xalloc' to arrange for arbitrary special-purpose data in your `ios' objects, without risk of conflict between packages designed for different purposes. - Method: long& ios::iword (int INDEX) Return a reference to arbitrary data, of long integer type, stored in an `ios' instance. INDEX, conventionally returned from `ios::xalloc', identifies what particular data you need. - Method: long ios::iword (int INDEX) const Return the actual value of a long integer stored in an `ios'. - Method: void*& ios::pword (int INDEX) Return a reference to an arbitrary pointer, stored in an `ios' instance. INDEX, originally returned from `ios::xalloc', identifies what particular pointer you need. - Method: void* ios::pword (int INDEX) const Return the actual value of a pointer stored in an `ios'.  File: iostream.info, Node: Synchronization, Next: Streambuf from Ios, Prev: Extending, Up: Ios Synchronizing related streams ----------------------------- You can use these methods to synchronize related streams with one another: - Method: ostream* ios::tie () const Report on what output stream, if any, is to be flushed before accessing this one. A pointer value of `0' means no stream is tied. - Method: ostream* ios::tie (ostream* ASSOC) Declare that output stream ASSOC must be flushed before accessing this stream. - Method: int ios::sync_with_stdio ([int SWITCH]) Unless iostreams and C `stdio' are designed to work together, you may have to choose between efficient C++ streams output and output compatible with C `stdio'. Use `ios::sync_with_stdio()' to select C compatibility. The argument SWITCH is a GNU extension; use `0' as the argument to choose output that is not necessarily compatible with C `stdio'. The default value for SWITCH is `1'. If you install the `stdio' implementation that comes with GNU `libio', there are compatible input/output facilities for both C and C++. In that situation, this method is unnecessary--but you may still want to write programs that call it, for portability.  File: iostream.info, Node: Streambuf from Ios, Prev: Synchronization, Up: Ios Reaching the underlying `streambuf' ----------------------------------- Finally, you can use this method to access the underlying object: - Method: streambuf* ios::rdbuf () const Return a pointer to the `streambuf' object that underlies this `ios'.  File: iostream.info, Node: Ostream, Next: Istream, Prev: Ios, Up: Streams Managing output streams: class `ostream' ======================================== Objects of class `ostream' inherit the generic methods from `ios', and in addition have the following methods available. Declarations for this class come from `iostream.h'. - Constructor: ostream::ostream () The simplest form of the constructor for an `ostream' simply allocates a new `ios' object. - Constructor: ostream::ostream (streambuf* SB [, ostream TIE]) This alternative constructor requires a first argument SB of type `streambuf*', to use an existing open stream for output. It also accepts an optional second argument TIE, to specify a related `ostream*' as the initial value for `ios::tie'. If you give the `ostream' a `streambuf' explicitly, using this constructor, the SB is *not* destroyed (or deleted or closed) when the `ostream' is destroyed. * Menu: * Writing:: Writing on an ostream. * Output Position:: Repositioning an ostream. * Ostream Housekeeping:: Miscellaneous ostream utilities.  File: iostream.info, Node: Writing, Next: Output Position, Up: Ostream Writing on an `ostream' ----------------------- These methods write on an `ostream' (you may also use the operator `<<'; *note Operators and Default Streams: Operators.). - Method: ostream& ostream::put (char C) Write the single character C. - Method: ostream& ostream::write (STRING, int LENGTH) Write LENGTH characters of a string to this `ostream', beginning at the pointer STRING. STRING may have any of these types: `char*', `unsigned char*', `signed char*'. - Method: ostream& ostream::form (const char *FORMAT, ...) A GNU extension, similar to `fprintf(FILE, FORMAT, ...)'. FORMAT is a `printf'-style format control string, which is used to format the (variable number of) arguments, printing the result on this `ostream'. See `ostream::vform' for a version that uses an argument list rather than a variable number of arguments. - Method: ostream& ostream::vform (const char *FORMAT, va_list ARGS) A GNU extension, similar to `vfprintf(FILE, FORMAT, ARGS)'. FORMAT is a `printf'-style format control string, which is used to format the argument list ARGS, printing the result on this `ostream'. See `ostream::form' for a version that uses a variable number of arguments rather than an argument list.  File: iostream.info, Node: Output Position, Next: Ostream Housekeeping, Prev: Writing, Up: Ostream Repositioning an `ostream' -------------------------- You can control the output position (on output streams that actually support positions, typically files) with these methods: - Method: streampos ostream::tellp () Return the current write position in the stream. - Method: ostream& ostream::seekp (streampos LOC) Reset the output position to LOC (which is usually the result of a previous call to `ostream::tellp'). LOC specifies an absolute position in the output stream. - Method: ostream& ostream::seekp (streamoff LOC, REL) Reset the output position to LOC, relative to the beginning, end, or current output position in the stream, as indicated by REL (a value from the enumeration `ios::seekdir'): `beg' Interpret LOC as an absolute offset from the beginning of the file. `cur' Interpret LOC as an offset relative to the current output position. `end' Interpret LOC as an offset from the current end of the output stream.  File: iostream.info, Node: Ostream Housekeeping, Prev: Output Position, Up: Ostream Miscellaneous `ostream' utilities --------------------------------- You may need to use these `ostream' methods for housekeeping: - Method: ostream& flush () Deliver any pending buffered output for this `ostream'. - Method: int ostream::opfx () `opfx' is a "prefix" method for operations on `ostream' objects; it is designed to be called before any further processing. See `ostream::osfx' for the converse. `opfx' tests that the stream is in state `good', and if so flushes any stream tied to this one. The result is `1' when `opfx' succeeds; else (if the stream state is not `good'), the result is `0'. - Method: void ostream::osfx () `osfx' is a "suffix" method for operations on `ostream' objects; it is designed to be called at the conclusion of any processing. All the `ostream' methods end by calling `osfx'. See `ostream::opfx' for the converse. If the `unitbuf' flag is set for this stream, `osfx' flushes any buffered output for it. If the `stdio' flag is set for this stream, `osfx' flushes any output buffered for the C output streams `stdout' and `stderr'.  File: iostream.info, Node: Istream, Next: Iostream, Prev: Ostream, Up: Streams Managing input streams: class `istream' ======================================= Class `istream' objects are specialized for input; as for `ostream', they are derived from `ios', so you can use any of the general-purpose methods from that base class. Declarations for this class also come from `iostream.h'. - Constructor: istream::istream () When used without arguments, the `istream' constructor simply allocates a new `ios' object and initializes the input counter (the value reported by `istream::gcount') to `0'. - Constructor: istream::istream (streambuf *SB [, ostream TIE]) You can also call the constructor with one or two arguments. The first argument SB is a `streambuf*'; if you supply this pointer, the constructor uses that `streambuf' for input. You can use the second optional argument TIE to specify a related output stream as the initial value for `ios::tie'. If you give the `istream' a `streambuf' explicitly, using this constructor, the SB is *not* destroyed (or deleted or closed) when the `ostream' is destroyed. * Menu: * Char Input:: Reading one character. * String Input:: Reading strings. * Input Position:: Repositioning an istream. * Istream Housekeeping:: Miscellaneous istream utilities.  File: iostream.info, Node: Char Input, Next: String Input, Up: Istream Reading one character --------------------- Use these methods to read a single character from the input stream: - Method: int istream::get () Read a single character (or `EOF') from the input stream, returning it (coerced to an unsigned char) as the result. - Method: istream& istream::get (char& C) Read a single character from the input stream, into `&C'. - Method: int istream::peek () Return the next available input character, but *without* changing the current input position.  File: iostream.info, Node: String Input, Next: Input Position, Prev: Char Input, Up: Istream Reading strings --------------- Use these methods to read strings (for example, a line at a time) from the input stream: - Method: istream& istream::get (char* C, int LEN [, char DELIM]) Read a string from the input stream, into the array at C. The remaining arguments limit how much to read: up to `len-1' characters, or up to (but not including) the first occurrence in the input of a particular delimiter character DELIM--newline (`\n') by default. (Naturally, if the stream reaches end of file first, that too will terminate reading.) If DELIM was present in the input, it remains available as if unread; to discard it instead, see `iostream::getline'. `get' writes `\0' at the end of the string, regardless of which condition terminates the read. - Method: istream& istream::get (streambuf& SB [, char DELIM]) Read characters from the input stream and copy them on the `streambuf' object SB. Copying ends either just before the next instance of the delimiter character DELIM (newline `\n' by default), or when either stream ends. If DELIM was present in the input, it remains available as if unread. - Method: istream& istream::getline (CHARPTR, int LEN [, char DELIM]) Read a line from the input stream, into the array at CHARPTR. cHARPTR may be any of three kinds of pointer: `char*', `unsigned char*', or `signed char*'. The remaining arguments limit how much to read: up to (but not including) the first occurrence in the input of a line delimiter character DELIM--newline (`\n') by default, or up to `len-1' characters (or to end of file, if that happens sooner). If `getline' succeeds in reading a "full line", it also discards the trailing delimiter character from the input stream. (To preserve it as available input, see the similar form of `iostream::get'.) If DELIM was *not* found before LEN characters or end of file, `getline' sets the `ios::fail' flag, as well as the `ios::eof' flag if appropriate. `getline' writes a null character at the end of the string, regardless of which condition terminates the read. - Method: istream& istream::read (POINTER, int LEN) Read LEN bytes into the location at POINTER, unless the input ends first. POINTER may be of type `char*', `void*', `unsigned char*', or `signed char*'. If the `istream' ends before reading LEN bytes, `read' sets the `ios::fail' flag. - Method: istream& istream::gets (char **S [, char DELIM]) A GNU extension, to read an arbitrarily long string from the current input position to the next instance of the DELIM character (newline `\n' by default). To permit reading a string of arbitrary length, `gets' allocates whatever memory is required. Notice that the first argument S is an address to record a character pointer, rather than the pointer itself. - Method: istream& istream::scan (const char *format ...) A GNU extension, similar to `fscanf(FILE, FORMAT, ...)'. The FORMAT is a `scanf'-style format control string, which is used to read the variables in the remainder of the argument list from the `istream'. - Method: istream& istream::vscan (const char *format, va_list args) Like `istream::scan', but takes a single `va_list' argument.  File: iostream.info, Node: Input Position, Next: Istream Housekeeping, Prev: String Input, Up: Istream Repositioning an `istream' -------------------------- Use these methods to control the current input position: - Method: streampos istream::tellg () Return the current read position, so that you can save it and return to it later with `istream::seekg'. - Method: istream& istream::seekg (streampos P) Reset the input pointer (if the input device permits it) to P, usually the result of an earlier call to `istream::tellg'. - Method: istream& istream::seekg (streamoff OFFSET, ios::seek_dir REF) Reset the input pointer (if the input device permits it) to OFFSET characters from the beginning of the input, the current position, or the end of input. Specify how to interpret OFFSET with one of these values for the second argument: `ios::beg' Interpret LOC as an absolute offset from the beginning of the file. `ios::cur' Interpret LOC as an offset relative to the current output position. `ios::end' Interpret LOC as an offset from the current end of the output stream.  File: iostream.info, Node: Istream Housekeeping, Prev: Input Position, Up: Istream Miscellaneous `istream' utilities --------------------------------- Use these methods for housekeeping on `istream' objects: - Method: int istream::gcount () Report how many characters were read from this `istream' in the last unformatted input operation. - Method: int istream::ipfx (int KEEPWHITE) Ensure that the `istream' object is ready for reading; check for errors and end of file and flush any tied stream. `ipfx' skips whitespace if you specify `0' as the KEEPWHITE argument, *and* `ios::skipws' is set for this stream. To avoid skipping whitespace (regardless of the `skipws' setting on the stream), use `1' as the argument. Call `istream::ipfx' to simplify writing your own methods for reading `istream' objects. - Method: void istream::isfx () A placeholder for compliance with the draft ANSI standard; this method does nothing whatever. If you wish to write portable standard-conforming code on `istream' objects, call `isfx' after any operation that reads from an `istream'; if `istream::ipfx' has any special effects that must be cancelled when done, `istream::isfx' will cancel them. - Method: istream& istream::ignore ([int N] [, int DELIM]) Discard some number of characters pending input. The first optional argument N specifies how many characters to skip. The second optional argument DELIM specifies a "boundary" character: `ignore' returns immediately if this character appears in the input. By default, DELIM is `EOF'; that is, if you do not specify a second argument, only the count N restricts how much to ignore (while input is still available). If you do not specify how many characters to ignore, `ignore' returns after discarding only one character. - Method: istream& istream::putback (char CH) Attempts to back up one character, replacing the character backed-up over by CH. Returns `EOF' if this is not allowed. Putting back the most recently read character is always allowed. (This method corresponds to the C function `ungetc'.) - Method: istream& istream::unget () Attempt to back up one character.  File: iostream.info, Node: Iostream, Prev: Istream, Up: Streams Input and output together: class `iostream' =========================================== If you need to use the same stream for input and output, you can use an object of the class `iostream', which is derived from *both* `istream' and `ostream'. The constructors for `iostream' behave just like the constructors for `istream'. - Constructor: iostream::iostream () When used without arguments, the `iostream' constructor simply allocates a new `ios' object, and initializes the input counter (the value reported by `istream::gcount') to `0'. - Constructor: iostream::iostream (streambuf* SB [, ostream* TIE]) You can also call a constructor with one or two arguments. The first argument SB is a `streambuf*'; if you supply this pointer, the constructor uses that `streambuf' for input and output. You can use the optional second argument TIE (an `ostream*') to specify a related output stream as the initial value for `ios::tie'. As for `ostream' and `istream', `iostream' simply uses the `ios' destructor. However, an `iostream' is not deleted by its destructor. You can use all the `istream', `ostream', and `ios' methods with an `iostream' object.  File: iostream.info, Node: Files and Strings, Next: Streambuf, Prev: Streams, Up: Top Classes for Files and Strings ***************************** There are two very common special cases of input and output: using files, and using strings in memory. `libio' defines four specialized classes for these cases: `ifstream' Methods for reading files. `ofstream' Methods for writing files. `istrstream' Methods for reading strings from memory. `ostrstream' Methods for writing strings in memory. * Menu: * Files:: Reading and writing files. * Strings:: Reading and writing strings in memory.  File: iostream.info, Node: Files, Next: Strings, Up: Files and Strings Reading and writing files ========================= These methods are declared in `fstream.h'. You can read data from class `ifstream' with any operation from class `istream'. There are also a few specialized facilities: - Constructor: ifstream::ifstream () Make an `ifstream' associated with a new file for input. (If you use this version of the constructor, you need to call `ifstream::open' before actually reading anything) - Constructor: ifstream::ifstream (int FD) Make an `ifstream' for reading from a file that was already open, using file descriptor FD. (This constructor is compatible with other versions of iostreams for POSIX systems, but is not part of the ANSI working paper.) - Constructor: ifstream::ifstream (const char* FNAME [, int MODE [, int PROT]]) Open a file `*FNAME' for this `ifstream' object. By default, the file is opened for input (with `ios::in' as MODE). If you use this constructor, the file will be closed when the `ifstream' is destroyed. You can use the optional argument MODE to specify how to open the file, by combining these enumerated values (with `|' bitwise or). (These values are actually defined in class `ios', so that all file-related streams may inherit them.) Only some of these modes are defined in the latest draft ANSI specification; if portability is important, you may wish to avoid the others. `ios::in' Open for input. (Included in ANSI draft.) `ios::out' Open for output. (Included in ANSI draft.) `ios::ate' Set the initial input (or output) position to the end of the file. `ios::app' Seek to end of file before each write. (Included in ANSI draft.) `ios::trunc' Guarantee a fresh file; discard any contents that were previously associated with it. `ios::nocreate' Guarantee an existing file; fail if the specified file did not already exist. `ios::noreplace' Guarantee a new file; fail if the specified file already existed. `ios::bin' Open as a binary file (on systems where binary and text files have different properties, typically how `\n' is mapped; included in ANSI draft). The last optional argument PROT is specific to Unix-like systems; it specifies the file protection (by default `644'). - Method: void ifstream::open (const char* FNAME [, int MODE [, int PROT]]) Open a file explicitly after the associated `ifstream' object already exists (for instance, after using the default constructor). The arguments, options and defaults all have the same meanings as in the fully specified `ifstream' constructor. You can write data to class `ofstream' with any operation from class `ostream'. There are also a few specialized facilities: - Constructor: ofstream::ofstream () Make an `ofstream' associated with a new file for output. - Constructor: ofstream::ofstream (int FD) Make an `ofstream' for writing to a file that was already open, using file descriptor FD. - Constructor: ofstream::ofstream (const char* FNAME [, int MODE [, int PROT]]) Open a file `*FNAME' for this `ofstream' object. By default, the file is opened for output (with `ios::out' as MODE). You can use the optional argument MODE to specify how to open the file, just as described for `ifstream::ifstream'. The last optional argument PROT specifies the file protection (by default `644'). - Destructor: ofstream::~ofstream () The files associated with `ofstream' objects are closed when the corresponding object is destroyed. - Method: void ofstream::open (const char* FNAME [, int MODE [, int PROT]]) Open a file explicitly after the associated `ofstream' object already exists (for instance, after using the default constructor). The arguments, options and defaults all have the same meanings as in the fully specified `ofstream' constructor. The class `fstream' combines the facilities of `ifstream' and `ofstream', just as `iostream' combines `istream' and `ostream'. The class `fstreambase' underlies both `ifstream' and `ofstream'. They both inherit this additional method: - Method: void fstreambase::close () Close the file associated with this object, and set `ios::fail' in this object to mark the event.  File: iostream.info, Node: Strings, Prev: Files, Up: Files and Strings Reading and writing in memory ============================= The classes `istrstream', `ostrstream', and `strstream' provide some additional features for reading and writing strings in memory--both static strings, and dynamically allocated strings. The underlying class `strstreambase' provides some features common to all three; `strstreambuf' underlies that in turn. - Constructor: istrstream::istrstream (const char* STR [, int SIZE]) Associate the new input string class `istrstream' with an existing static string starting at STR, of size SIZE. If you do not specify SIZE, the string is treated as a `NUL' terminated string. - Constructor: ostrstream::ostrstream () Create a new stream for output to a dynamically managed string, which will grow as needed. - Constructor: ostrstream::ostrstream (char* STR, int SIZE [,int MODE]) A new stream for output to a statically defined string of length SIZE, starting at STR. You may optionally specify one of the modes described for `ifstream::ifstream'; if you do not specify one, the new stream is simply open for output, with mode `ios::out'. - Method: int ostrstream::pcount () Report the current length of the string associated with this `ostrstream'. - Method: char* ostrstream::str () A pointer to the string managed by this `ostrstream'. Implies `ostrstream::freeze()'. Note that if you want the string to be nul-terminated, you must do that yourself (perhaps by writing ends to the stream). - Method: void ostrstream::freeze ([int N]) If N is nonzero (the default), declare that the string associated with this `ostrstream' is not to change dynamically; while frozen, it will not be reallocated if it needs more space, and it will not be deallocated when the `ostrstream' is destroyed. Use `freeze(1)' if you refer to the string as a pointer after creating it via `ostrstream' facilities. `freeze(0)' cancels this declaration, allowing a dynamically allocated string to be freed when its `ostrstream' is destroyed. If this `ostrstream' is already static--that is, if it was created to manage an existing statically allocated string--`freeze' is unnecessary, and has no effect. - Method: int ostrstream::frozen () Test whether `freeze(1)' is in effect for this string. - Method: strstreambuf* strstreambase::rdbuf () A pointer to the underlying `strstreambuf'.  File: iostream.info, Node: Streambuf, Next: Stdio, Prev: Files and Strings, Up: Top Using the `streambuf' Layer *************************** The `istream' and `ostream' classes are meant to handle conversion between objects in your program and their textual representation. By contrast, the underlying `streambuf' class is for transferring raw bytes between your program, and input sources or output sinks. Different `streambuf' subclasses connect to different kinds of sources and sinks. The GNU implementation of `streambuf' is still evolving; we describe only some of the highlights. * Menu: * Areas:: Areas in a streambuf. * Overflow:: Simple output re-direction * Formatting:: C-style formatting for streambuf objects. * Stdiobuf:: Wrappers for C stdio. * Procbuf:: Reading/writing from/to a pipe * Backing Up:: Marking and returning to a position. * Indirectbuf:: Forwarding I/O activity.  File: iostream.info, Node: Areas, Next: Overflow, Up: Streambuf Areas of a `streambuf' ====================== Streambuf buffer management is fairly sophisticated (this is a nice way to say "complicated"). The standard protocol has the following "areas": * The "put area" contains characters waiting for output. * The "get area" contains characters available for reading. The GNU `streambuf' design extends this, but the details are still evolving. The following methods are used to manipulate these areas. These are all protected methods, which are intended to be used by virtual function in classes derived from `streambuf'. They are also all ANSI/ISO-standard, and the ugly names are traditional. (Note that if a pointer points to the 'end' of an area, it means that it points to the character after the area.) - Method: char* streambuf::pbase () const Returns a pointer to the start of the put area. - Method: char* streambuf::epptr () const Returns a pointer to the end of the put area. - Method: char* streambuf::pptr () const If `pptr() < epptr ()', the `pptr()' returns a pointer to the current put position. (In that case, the next write will overwrite `*pptr()', and increment `pptr()'.) Otherwise, there is no put position available (and the next character written will cause `streambuf::overflow' to be called). - Method: void streambuf::pbump (int N) Add N to the current put pointer. No error checking is done. - Method: void streambuf::setp (char* P, char* E) Sets the start of the put area to P, the end of the put area to E, and the current put pointer to P (also). - Method: char* streambuf::eback () const Returns a pointer to the start of the get area. - Method: char* streambuf::egptr () const Returns a pointer to the end of the get area. - Method: char* streambuf::gptr () const If `gptr() < egptr ()', then `gptr()' returns a pointer to the current get position. (In that case the next read will read `*gptr()', and possibly increment `gptr()'.) Otherwise, there is no read position available (and the next read will cause `streambuf::underflow' to be called). - Method: void streambuf:gbump (int N) Add N to the current get pointer. No error checking is done. - Method: void streambuf::setg (char* B, char* P, char* E) Sets the start of the get area to B, the end of the get area to E, and the current put pointer to P.  File: iostream.info, Node: Overflow, Next: Formatting, Prev: Areas, Up: Streambuf Simple output re-direction by redefining `overflow' =================================================== Suppose you have a function `write_to_window' that writes characters to a `window' object. If you want to use the ostream function to write to it, here is one (portable) way to do it. This depends on the default buffering (if any). #include /* Returns number of characters successfully written to WIN. */ extern int write_to_window (window* win, char* text, int length); class windowbuf : public streambuf { window* win; public: windowbuf (window* w) { win = w; } int sync (); int overflow (int ch); // Defining xsputn is an optional optimization. // (streamsize was recently added to ANSI C++, not portable yet.) streamsize xsputn (char* text, streamsize n); }; int windowbuf::sync () { streamsize n = pptr () - pbase (); return (n && write_to_window (win, pbase (), n) != n) ? EOF : 0; } int windowbuf::overflow (int ch) { streamsize n = pptr () - pbase (); if (n && sync ()) return EOF; if (ch != EOF) { char cbuf[1]; cbuf[0] = ch; if (write_to_window (win, cbuf, 1) != 1) return EOF; } pbump (-n); // Reset pptr(). return 0; } streamsize windowbuf::xsputn (char* text, streamsize n) { return sync () == EOF ? 0 : write_to_window (win, text, n); } int main (int argc, char**argv) { window *win = ...; windowbuf wbuf(win); ostream wstr(&wbuf); wstr << "Hello world!\n"; }