/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference"
* by Nicolai M. Josuttis, Addison-Wesley, 1999
*
* (C) Copyright Nicolai M. Josuttis 1999.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include
#include
#include
#include
// for read():
#ifdef _MSC_VER
# include
#else
# include
#endif
class inbuf : public std::streambuf {
protected:
/* data buffer:
* - at most, four characters in putback area plus
* - at most, six characters in ordinary read buffer
*/
static const int bufferSize = 10; // size of the data buffer
char buffer[bufferSize]; // data buffer
public:
/* constructor
* - initialize empty data buffer
* - no putback area
* => force underflow()
*/
inbuf() {
setg (buffer+4, // beginning of putback area
buffer+4, // read position
buffer+4); // end position
}
protected:
// insert new characters into the buffer
virtual int_type underflow () {
// is read position before end of buffer?
if (gptr() < egptr()) {
return traits_type::to_int_type(*gptr());
}
/* process size of putback area
* - use number of characters read
* - but at most four
*/
int numPutback;
numPutback = gptr() - eback();
if (numPutback > 4) {
numPutback = 4;
}
/* copy up to four characters previously read into
* the putback buffer (area of first four characters)
*/
std::memmove (buffer+(4-numPutback), gptr()-numPutback,
numPutback);
// read new characters
int num;
num = read (0, buffer+4, bufferSize-4);
if (num <= 0) {
// ERROR or EOF
return EOF;
}
// reset buffer pointers
setg (buffer+(4-numPutback), // beginning of putback area
buffer+4, // read position
buffer+4+num); // end of buffer
// return next character
return traits_type::to_int_type(*gptr());
}
};
int main()
{
inbuf ib; // create special stream buffer
std::istream in(&ib); // initialize input stream with that buffer
char c;
for (int i=1; i<=20; i++) {
// read next character (out of the buffer)
in.get(c);
// print that character (and flush)
std::cout << c << std::flush;
// after eight characters, put two characters back into the stream
if (i == 8) {
in.unget();
}
}
std::cout << std::endl;
}
a
a
d