Re: C++ FAQ

Jerry Coffin <>
Thu, 2 Jul 2009 22:28:18 CST
In article <>, says...

[ ... ]

template <typename Iter>
void join_print(Iter begin, Iter end, std::ostream& os, const char* delim)
     if (begin == end) return; else goto print_element;
     for (; begin != end; ++begin) {
         os << delim;
         os << *begin;

where a goto-free version would incur a check at each iteration of
the loop.

First of all, given that each iteration of the loop does an output
operation, there's essentially no chance at all that the conditional
in the loop is going to make a measurable difference in speed -- just
in really round numbers, conditional tests take on the order of
nanoseconds and I/O operations take on the order of microseconds to

Second, it's not really true anyway. This basic type of example (a
loop and a half) is well known, and it's equally well known that
avoiding the goto involves nothing more than duplicating a bit of
code. If the code you have to duplicate is sufficiently substantial,
it can be worthwhile, but in this case it's quite trivial:

template <typename Iter>
void join_print(
    Iter begin,
    Iter end,
    std::ostream& os,
    const char* delim)
    if (begin!=end) {
        os << *begin++;
        for (;begin!=end; ++begin)
            os << delim << *begin;

Of course, I'd prefer to separate concerns, and use std::copy with
the infix_ostream_iterator that was recent posted to comp.lang.c++:

// Warning: only minimally tested.
// infix_iterator.h
// Lifted from Jerry Coffin's 's prefix_ostream_iterator
#if !defined(INFIX_ITERATOR_H_)
#include <ostream>
#include <iterator>

template <class T,
class charT=char,
class traits=std::char_traits<charT> >

class infix_ostream_iterator :
    public std::iterator
    std::basic_ostream<charT,traits> *os;
    charT const* delimiter;
    bool first_elem;
    typedef charT char_type;
    typedef traits traits_type;
    typedef std::basic_ostream<charT,traits> ostream_type;

    infix_ostream_iterator(ostream_type& s)
        : os(&s),delimiter(0), first_elem(true)
    infix_ostream_iterator(ostream_type& s, charT const *d)
        : os(&s),delimiter(d), first_elem(true)
    infix_ostream_iterator<T,charT,traits>& operator=(T const &item)
        if (!first_elem && delimiter != 0)
            *os << delimiter;
        *os << item;
        first_elem = false;
        return *this;

    infix_ostream_iterator<T,charT,traits> &operator*() {
        return *this;
    infix_ostream_iterator<T,charT,traits> &operator++() {
        return *this;
    infix_ostream_iterator<T,charT,traits> &operator++(int) {
        return *this;

Using this, a call to join_print like this:

    join_print(vect.begin(), vect.end(), std::cout, "|");

would be replaced with a call to copy, like this:

    std::copy(vect.begin(), vect.end(),
        std::infix_ostream_iterator<whatever>(std::cout, "|"));

While that's slightly longer, it's also considerably more flexible:
it's an ostream_iterator that can be used with any algorith that
would work with another ostream_iterator.


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