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* An example of a Reader which operates on input consisting
* of 'parts' returned from a iterator. You may then split the input
* string how you wish, and still use the reader for conversion.
#include "instream.h"
#include "outstream.h"
#include <string.h>
using namespace std;
using namespace stream;
template <class It>
class PartsReader: public Reader {
It start;
It finish;
// note Reader(stderr) - so that the reader's FILE* isn't NULL
PartsReader(It start, It finish)
: Reader(stderr), start(start),finish(finish) {
// only method that needs to be overriden
virtual int read_fmt(const char *fmt, va_list ap) {
if (start == finish) {
return 0;
const char *s = *start++;
if (fmt[0] == '%' and fmt[1] == 's') {
strncpy(va_arg(ap, char *),s,256);
return 1;
} else {
return vsscanf(s,fmt,ap);
// a helper function, until C++17 class template type deduction
// arrives
template <class It>
PartsReader<It> make_parts_reader(It start, It finish) {
return PartsReader<It>(start,finish);
int main()
int n;
double x;
string s;
auto parts = {"42","5.2","hello"};
auto rdr = make_parts_reader(parts.begin(),parts.end());
rdr (n) (x) (s);
return 0;