Re: Do you like this use of STL accumulate

Hi!

firelotus schrieb:


In my eyes "accumulate" with own function just has a bad name. In
functional programming this function is called "fold" and is used quite
often because most things can be expressed as a fold (in functional
programming you use lists often, thus you can use fold often). From this
point of view the use of accumulate to build up a string is ok.

Building a string this way creates lots of copies of the strings in C++.
This is no good, as it transforms simple linear algorithm into a
quadratic one (in terms of runtime required). So better use a
std::ostringstream. So how could we use accumulate to do so while not
building functions that are too special?

A short and simple implementation would be like Maciej showed, just
replaced string concatenation by stream operations and replaced the
for-loop from using an index to using iterators:

void CoffeeFundCgiView::setMembers(const std::vector<std::string>&
members)
{
      std::ostringstream stream;
      BOOST_FOREACH(std::string const& member, members);
      {
          stream << "<OPTION>" << member << "</OPTION>";
      }
      memberList_ = stream.str();
}

The other approach using stringstreams and accumulate would require some
hackery to handle the reference to the streamobject (which cannot be
copied). std algos do not easily support passing references around.

Maybe (untested):

std::ostream& buildOption(
    std::ostream& stream,
    std::string const& member
    )
{
    stream << "<OPTION>" << member << "</OPTION>";
    return stream;
}

void CoffeeFundCgiView::setMembers(const std::vector<std::string>&
members)
{
    std::ostringstream stream;
    std::accumulate(members.begin(), members.end(),
        boost::bind(&buildOption, boost::ref(stream), _1));
    memberList_ = stream.str();
}

Frank

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