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

"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