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Heron-Centric: Ruminations of a Language Designer
A Generic Ostream Iterator
by Christopher Diggins
November 11, 2005
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"));
}

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About the Blogger

Christopher Diggins is a software developer and freelance writer. Christopher loves programming, but is eternally frustrated by the shortcomings of modern programming languages. As would any reasonable person in his shoes, he decided to quit his day job to write his own ( www.heron-language.com ). Christopher is the co-author of the C++ Cookbook from O'Reilly. Christopher can be reached through his home page at www.cdiggins.com.

This weblog entry is Copyright © 2005 Christopher Diggins. All rights reserved.

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