The following code example is taken from the book
The C++ Standard Library - A Tutorial and Reference, 2nd Edition
by Nicolai M. Josuttis, Addison Wesley Longman, 2012
Copyright © 2012 by Pearson Education, Inc. and Nicolai M. Josuttis
#include <cstdio>
#include <cstring>
#include <streambuf>
// for read():
#ifdef _MSC_VER
# include <io.h>
#else
# include <unistd.h>
#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());
}
};