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A Generic Ostream Iterator

2 replies on 1 page. Most recent reply: Sep 10, 2009 1:26 AM by Jono Pfaff

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Christopher Diggins

Posts: 1215
Nickname: cdiggins
Registered: Feb, 2004

A Generic Ostream Iterator (View in Weblogs)
Posted: Nov 11, 2005 10:00 AM
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Summary
Ostream iterators are a handy but under-utilized tool for using the STL to output containers and ranges. Here I provide an alternative, which has a more pleasing syntax.
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An ostream iterator, allows you to use STL algorithms, such as std::copy to output a container or range to an output stream. A simple example is:
int main() {
  int array[] = { 1, 2, 4, 8, 16, 32, 64 };
  std::copy(array, array + 7, std::ostream_iterator<int>(std::cout, "\n"));
}
The declaration for an ostream_iterator is clunky because you to know the kind of elements it will be reading. You can make the code more readable as follows:
int main() {
  int array[] = { 1, 2, 4, 8, 16, 32, 64 };
  typedef std::ostream_iterator<int> oiter;
  std::copy(array, array + 7, oiter(std::cout, "\n"));
}
This is still more complex than strictly neccessary. The problem is that the ostream_iterator is a template. An alternative implementation would be to make the operator=() member function a template.
#include <iostream>

struct putter {
  putter(const putter& x) : o(x.o), delim(x.delim) { }
  putter(std::ostream& x = std::cout, const char* s = "") : o(x), delim(s) { }
  template<typename T>
  putter& operator=(const T& x) { o << x << delim; return *this; }
  putter& operator*() { return *this; }
  putter& operator++() { return *this; }
  putter& operator++(int) { return *this; }
  mutable std::ostream& o;
  const char* delim;
};

putter put(std::ostream& o = std::cout, const char* delim = "") {
  return putter(o, delim);
}
Now you can output the contents of any iterator pair with less work.
int main() {
  int array[] = { 1, 2, 4, 8, 16, 32, 64 };
  std::copy(array, array + 7, put(std::cout, "\n"));
}


Elizabeth Wiethoff

Posts: 89
Nickname: ewiethoff
Registered: Mar, 2005

Re: A Generic Ostream Iterator Posted: Nov 30, 2005 12:42 AM
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<pre>
int main() {
int array[] = { 1, 2, 4, 8, 16, 32, 64 };
std::copy(array, array + 7, put(std::cout, "\n"));
}
</pre>

This is starting to look like functional programming in Python. :-) Is your generic ostream iterator in the <i>C++ Cookbook</i>?

Jono Pfaff

Posts: 1
Nickname: shiprat
Registered: Sep, 2009

Re: A Generic Ostream Iterator Posted: Sep 10, 2009 1:26 AM
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Hi,

I liked your generic ostream iterator, but i felt compelled to gild the lily, so this version automatically dereferences pointers. It might not be what everybody wants all the time, but its often what i want a lot of the time!

Jono
Auckland, NZ

struct putter {

template<typename U>
struct Getter{
const U& operator()(const U& u){return u;}
const U& operator()(U*const&){}
};

template<typename U>
struct Getter<U*>{
const U& operator()(U const&){}
const U& operator()(U*const&u){return *u;}
};

putter(const putter& x) : o(x.o), delim(x.delim) { }
putter(std::ostream& x = std::cout, const char* s = "") : o(x), delim(s) { }
template<typename T>
putter& operator=(const T& x) {
Getter<T> getter;
o << getter(x) << delim; return *this;
}
putter& operator*() { return *this; }
putter& operator++() { return *this; }
putter& operator++(int) { return *this; }
mutable std::ostream& o;
const char* delim;
};

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