Re: [Boost-users] Adapters library: using back_insertor withmake_transform_iterator

1 Jul 2005

      Yes, that's what I needed, thank you very much. FWIW, I couldn't find
make_function_output_iterator when I first looked at the documentation, and
then I ended out trying to read the code in the iterators library instead
... which was about when I gave up and posted.

I rewrote it as follows, binding to the convert function and then sending
that result to the push_back, and it now looks like the following.
Unfortunately, vector::push_back is overloaded, so that the cast is really
ugly. But it works.

int _tmain(int argc, _TCHAR* argv[])
{
 V1 aSourceVector;
 V2 aTargetVector;
 aSourceVector.push_back(43);

 copy(aSourceVector.begin(),
          aSourceVector.end(),
          make_function_output_iterator(
                         boost::bind((void (vector<string>::*)(const
string&))&vector<string>::push_back,
                                           boost::ref(aTargetVector),
                                           boost::bind(convert,_1))));
}

"David Abrahams"  wrote in message
news:u4qbfc7k9.fsf@boost-consulting.com...
...
"Robert Mathews"  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 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