Re: [Boost-users] Adapters library: using back_insertor with make_transform_iterator

From: David Abrahams (dave_at_[hidden])
Date: 2005-07-01 02:24:38

"Robert Mathews" <rmathews_at_[hidden]> writes:

> How does this work?
>
> I'm trying to use the make_transform_iterator to make a writeable output
> iterator (looks like it is referred to as a "Writeable LValue" in the
> iterator template code

Don't look at the code; read the documentation.  The meaning of
"writeable lvalue iterator" is detailed in the paper here:
http://www.boost.org/libs/iterator/doc/index.html#new-style-iterators

> ), but I can't figure out how to do it.
>
> Here's the example: (I know that I could have used the
> make_transform_iterators on the source iterators in this toy example, but
> that won't do for my real application)
>
> typedef map<int,string> Map;
> typedef vector<int> V1;
> typedef vector<string> V2;
> string convert(int iValue)
> {
>  return str(ostringstream() << iValue);
> }
>
> int _tmain(int argc, _TCHAR* argv[])
> {
>  V1 aSourceVector;
>  V2 aTargetVector;
>  copy(aSourceVector.begin(), aSourceVector.end(),
>   make_transform_iterator(back_inserter(aTargetVector), boost::bind(convert,
> _1));
> }

Okay, let's review what a transform_iterator does, from
http://www.boost.org/libs/iterator/doc/transform_iterator.html:

  The transform iterator adapts an iterator by modifying the operator*
  to apply a function object to the result of dereferencing the
  iterator and returning the result.

So your transform_iterator's operator* is going to apply convert to
the result of dereferencing a back_insert_iterator.  Okay, that's your
first problem.  Do you know what you get when you dereference a
back_insert_iterator?

According to my copy of the standard, back_insert_iterator's operator*
has the following signature:

   back_insert_iterator<Container>& operator*();

In other words, it returns a reference to a back_insert_iterator.
Since convert takes an int, you have an impedance mismatch.

Sounds like you want something more like function_output_iterator:
http://www.boost.org/libs/iterator/doc/function_output_iterator.html

It doesn't adapt an iterator, and I'm guessing you don't really want
to adapt an iterator anyway (you could just use push_back, or more
likely in your case, operator<<), but if I'm wrong, you can always
embed the iterator in the function object you adapt.

HTH,

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com