streambuf_prot(3C++)

Synopsis

   #include <iostream.h>
   

   typedef long streamoff, streampos;
   class ios {
   public:
   	enum	seek_dir { beg, cur, end };
   	enum	open_mode { in, out, ate, app, trunc, nocreate, noreplace } ;
   	// and lots of other stuff, see ios(3C++) ...
   } ;
   

   class streambuf {
   public:
   		streambuf() ;
   		streambuf(char* p, int len);
   	void	dbp() ;
   protected:
   	int	allocate();
   	char*	base();
   	int	blen();
   	char*	eback();
   	char* 	ebuf();
   	char*	egptr();
   	char*	epptr();
   	void	gbump(int n);
   	char*	gptr();
   	char*	pbase();
   	void	pbump(int n);
   	char*	pptr();
   	void	setg(char* eb, char* g, char* eg);
   	void	setp(char* p, char* ep);
   	void	setb(char* b, char* eb, int a=0);
   	int	unbuffered();
   	void	unbuffered(int);
   

   	virtual int 	doallocate();
   	virtual	~streambuf() ;
   public:
   	virtual int	pbackfail(int c);
   	virtual int	overflow(int c=EOF);
   	virtual int	underflow();
   	virtual streambuf*
   		setbuf(char* p, int len);
   	streambuf*
   		setbuf(unsigned char* p, in len);
   	virtual streampos
   		seekpos(streampos, int =ios::in|ios:out);
   	virtual streampos
   		seekoff(streamoff, seek_dir, int =ios::in|ios:out);
   	virtual int	sync();
   };

Description

In the following descriptions assume:
-- sb is a streambuf*.
-- i and n are ints.
-- ptr, b, eb, p, ep, eb, g, and eg are char*s.
-- c is an int character (positive or EOF)).
-- pos is a streampos. (See streambuf_pub(3C++).)
-- off is a streamoff.
-- dir is a seekdir.
-- mode is an int representing an open_mode.

Constructors:

streambuf()

Constructs an empty buffer corresponding to an empty sequence.

streambuf(b,len)

Constructs an empty buffer and then sets up the reserve area to be the len bytes starting at b.

The get, put, and reserve areas

The protected members of streambuf present an interface to derived classes organized around three areas (arrays of bytes) managed cooperatively by the base and derived classes. They are the get area, the put area, and the reserve area (or buffer). The get and the put areas are normally disjoint, but they may both overlap the reserve area, whose primary purpose is to be a resource in which space for the put and get areas can be allocated. The get and the put areas are changed as characters are put into and gotten from the buffer, but the reserve area normally remains fixed. The areas are defined by a collection of char* values. The buffer abstraction is described in terms of pointers that point between characters, but the char* values must point at chars. To establish a correspondence, the char* values should be thought of as pointing just before the byte they really point at.

Functions to examine the pointers

ptr=sb->base()

Returns a pointer to the first byte of the reserve area. Space between sb->base() and sb->ebuf() is the reserve area.

ptr=sb->eback()

Returns a pointer to a lower bound on sb->gptr(). Space between sb->eback() and sb->gptr() is available for putback.

ptr=sb->ebuf()

Returns a pointer to the byte after the last byte of the reserve area.

ptr=sb->egptr()

Returns a pointer to the byte after the last byte of the get area.

ptr=sb->epptr()

Returns a pointer to the byte after the last byte of the put area.

ptr=sb->gptr()

Returns a pointer to the first byte of the get area. The available characters are those between sb->gptr() and sb->egptr(). The next character fetched will be *sb->gptr()) unless sb->egptr() is less than or equal to sb->gptr().

ptr=sb->pbase()

Returns a pointer to the put area base. Characters between sb->pbase() and sb->pptr() have been stored into the buffer and not yet consumed.

ptr=sb->pptr()

Returns a pointer to the first byte of the put area. The space between sb->pptr() and sb->epptr() is the put area and characters will be stored here.

Functions for setting the pointers

sb->setb(b, eb, i)

Sets base() and ebuf() to b and eb respectively. i controls whether the area will be subject to automatic deletion. If i is non-zero, then b will be deleted when base is changed by another call of setb(), or when the destructor is called for *sb. If b and eb are both null then we say that there is no reserve area. If b is non-null, there is a reserve area even if eb is less than b and so the reserve area has zero length.

sb->setp(p, ep)

Sets pptr() to p, pbase() to p, and epptr() to ep.

sb->setg(eb, g, eg)

Sets eback() to eb, gptr() to g, and egptr() to eg.

Other non-virtual members

i=sb->allocate()

Tries to set up a reserve area. If a reserve area already exists or if sb->unbuffered() is nonzero, allocate() returns 0 without doing anything. If the attempt to allocate space fails, allocate() returns EOF, otherwise (allocation succeeds) allocate() returns 0. allocate() is not called by any non-virtual member function of streambuf.

i=sb->blen()

Returns the size (in chars) of the current reserve area.

dbp()

Writes directly on file descriptor 1 information in ASCII about the state of the buffer. It is intended for debugging and nothing is specified about the form of the output. It is considered part of the protected interface because the information it prints can only be understood in relation to that interface, but it is a public function so that it can be called anywhere during debugging.

sb->gbump(n)

Increments gptr() by n which may be positive or negative. No checks are made on whether the new value of gptr() is in bounds.

sb->pbump(n)

Increments pptr() by n which may be positive or negative. No checks are made on whether the new value of pptr() is in bounds.

sb->unbuffered(i)

i=sb->unbuffered()

There is a private variable known as sb's buffering state. sb->unbuffered(i) sets the value of this variable to i and sb->unbuffered() returns the current value. This state is independent of the actual allocation of a reserve area. Its primary purpose is to control whether a reserve area is allocated automatically by allocate.

Virtual member functions

i=sb->doallocate()

Is called when allocate() determines that space is needed. doallocate() is required to call setb() to provide a reserve area or to return EOF if it cannot. It is only called if sb->unbuffered() is zero and sb->base() is zero.

i=overflow(c)

Is called to consume characters. If c is not EOF, overflow() also must either save c or consume it. Usually it is called when the put area is full and an attempt is being made to store a new character, but it can be called at other times. The normal action is to consume the characters between pbase() and pptr(), call setp() to establish a new put area, and if c!=EOF store it (using sputc()). sb->overflow() should return EOF to indicate an error; otherwise it should return something else.

i=sb->pbackfail(c)

Is called when eback() equals gptr() and an attempt has been made to putback c. If this situation can be dealt with (e.g., by repositioning an external file), pbackfail() should return c; otherwise it should return EOF.

pos=sb->seekoff(off, dir, mode)

Repositions the get and/or put pointers (i.e., the abstract get and put pointers, not pptr() and gptr()). The meanings of off and dir are discussed in streambuf_pub(3C++). mode specifies whether the put pointer (ios::out bit set) or the get pointer (ios::in bit set) is to be modified. Both bits may be set, in which case both pointers should be affected. A class derived from streambuf is not required to support repositioning. seekoff() should return EOF if the class does not support repositioning. If the class does support repositioning, seekoff() should return the new position or EOF on error.

pos=sb->seekpos(pos, mode)

Repositions the streambuf get and/or put pointer to pos. mode specifies which pointers are affected as for seekoff(). Returns pos (the argument) or EOF if the class does not support repositioning or an error occurs.

sb=sb->setbuf(ptr, len)

Offers the array at ptr with len bytes to be used as a reserve area. The normal interpretation is that if ptr or len are zero then this is a request to make the sb unbuffered. The derived class may use this area or not as it chooses. It may accept or ignore the request for unbuffered state as it chooses. setbuf() should return sb if it honors the request. Otherwise it should return 0.

i=sb->sync()

Is called to give the derived class a chance to look at the state of the areas, and synchronize them with any external representation. Normally sync() should consume any characters that have been stored into the put area, and if possible give back to the source any characters in the get area that have not been fetched. When sync() returns there should not be any unconsumed characters, and the get area should be empty. sync() should return EOF if some kind of failure occurs.

i=sb->underflow()

Is called to supply characters for fetching, i.e., to create a condition in which the get area is not empty. If it is called when there are characters in the get area it should return the first character. If the get area is empty, it should create a nonempty get area and return the next character (which it should also leave in the get area). If there are no more characters available, underflow() should return EOF and leave an empty get area.

The default definitions of the virtual functions:

i=sb->streambuf::doallocate()

Attempts to allocate a reserve area using operator new.

i=sb->streambuf::overflow(c)

Is compatible with the old stream package, but that behavior is not considered part of the specification of the iostream package. Therefore, streambuf::overflow() should be treated as if it had undefined behavior. That is, derived classes should always define it.

i=sb->streambuf::pbackfail(c)

Returns EOF.

pos=sb->streambuf::seekpos(pos, mode)

Returns sb->seekoff(streamoff(pos),ios::beg,mode). Thus, to define seeking in a derived class, it is frequently only necessary to define seekoff() and use the inherited streambuf::seekpos().

pos=sb->streambuf::seekoff(off, dir, mode)

Returns EOF.

sb=sb->streambuf::setbuf(ptr, len)

Will honor the request when there is no reserve area.

i=sb->streambuf::sync()

Returns 0 if the get area is empty and there are no unconsumed characters. Otherwise it returns EOF.

i=sb->streambuf::underflow()

Is compatible with the old stream package, but that behavior is not considered part of the specification of the iostream package. Therefore, streambuf::underflow() should be treated as if it had undefined behavior. That is, it should always be defined in derived classes.

Notices

The interface for unbuffered actions is awkward. It's hard to write underflow() and overflow() virtuals that behave properly for unbuffered streambuf()s without special casing. Also there is no way for the virtuals to react sensibly to multi-character gets or puts.

Although the public interface to streambufs deals in characters and bytes, the interface to derived classes deals in chars. Since a decision had to be made on the types of the real data pointers, it seemed easier to reflect that choice in the types of the protected members than to duplicate all the members with both plain and unsigned char versions. But perhaps all these uses of char* ought to have been with a typedef.

The implementation contains a variant of setbuf() that accepts a third argument. It is present only for compatibility with the old stream package.

References