The Standard Librarian: Stringstreams and Their Friends
By Matthew H. Austern, April 01, 2001
Stringstreams work much the same as other kinds of streams: you can insert and extract values in type-safe fashion and expect to have them formatted according to the currently defined locale. But there is another similarity between stringstreams and other streams; stringstreams serve as wrappers around buffers. Every stringstream contains a stringbuf, and that stringbuf can be customized to handle special kinds of string storage (e.g., arrays of char, vector, etc.). Matt shows how to create your own customized stringbufs, in keeping with the notion that to use the standard library effectively is to extend it.
April 2001 C++ Experts Forum/The Standard Librarian/Listing 1
#include <streambuf>
#include <ios>
#include <algorithm>
#include <iterator>
template <class FwIter>
class rangebuf : public std::streambuf {
private:
FwIter first;
FwIter last;
FwIter current;
enum { bufsize = 128 };
char buf[bufsize];
void fill_buffer();
public:
rangebuf(FwIter f, FwIter l);
protected:
pos_type seekoff(off_type off, std::ios::seekdir way,
std::ios::openmode which);
pos_type seekpos(pos_type pos, std::ios::openmode which);
int underflow();
int pbackfail(int c);
};
template <class FwIter>
rangebuf<FwIter>::rangebuf(FwIter f, FwIter l)
: std::streambuf(),
first(f), last(l), current(f)
{
fill_buffer();
}
template <class FwIter>
void rangebuf<FwIter>::fill_buffer()
{
ptrdiff_t len = std::distance(current, last);
ptrdiff_t n = std::min(len, ptrdiff_t(bufsize));
FwIter tmp = current;
std::advance(tmp, n);
std::copy(current, tmp, buf);
setg(buf, buf, buf + n);
}
template <class FwIter>
int rangebuf<FwIter>::underflow() {
std::advance(current, egptr() - eback());
fill_buffer();
return gptr() != egptr()
? static_cast<int>(static_cast<unsigned char>(*gptr()))
: EOF;
}
template <class FwIter>
int rangebuf<FwIter>::pbackfail(int c) {
if (gptr() == eback() && current != first) {
FwIter tmp = first;
std::advance(tmp, std::distance(first, current) - 1);
current = tmp;
fill_buffer();
gbump(1);
}
if (gptr() == eback() || (c != EOF && c != *(gptr() - 1)))
return EOF;
else {
gbump(-1);
return static_cast<unsigned char>(*gptr());
}
}
template <class FwIter>
std::streambuf::pos_type
rangebuf<FwIter>::seekoff(off_type off, std::ios::seekdir way,
std::ios::openmode which) {
switch(way) {
case std::ios::beg:
return seekpos(pos_type(off), which);
case std::ios::cur:
return seekpos(pos_type(off + std::distance(first, current)),
which);
case std::ios::end:
return seekpos(pos_type(off + std::distance(first, last)),
which);
default:
return pos_type(off_type(-1));
}
}
template <class FwIter>
std::streambuf::pos_type
rangebuf<FwIter>::seekpos(pos_type pos, std::ios::openmode which) {
if (which != std::ios::in)
return pos_type(off_type(-1));
off_type offset = off_type(pos);
if (offset < 0 || offset > std::distance(first, last))
return pos_type(off_type(-1));
FwIter tmp = first;
std::advance(tmp, offset);
current = tmp;
fill_buffer();
return pos;
}
