Re: [Boost-users] [Boost.Iterator] transform_iteratorequivalent for output iterators

Jeff Flinn writes:

On 7/31/15 8:40 AM, dariomt wrote:

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Edward Diener writes:

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On 7/30/2015 7:06 AM, dariomt <at> gmail.com wrote:

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Here is my problem: for any given algorithm that writes its output

into

...

...

an output iterator, I'd like to apply a transformation *before* writing to the output iterator.

e.g. inside the algorithm the output iterator is dereferenced and incremented to write each result, typically like this:

*output++ = ith_result;

I'd like to wrap the output iterator with a unary function func

such

...

...

...

that the effect is equivalent to:

*output++ = func( ith_result );

I'm looking at the adaptors in Boost.Iterator but I cannot find a suitable one.

Am I missing anything? Is there a better approach?

Can you not use the function_output_iterator to accomplish what you want to do ?

IIUC the function_output_iterator "fakes" an output iterator with a function object that gets called for each element written to the iterator.

I need to wrap *both* an actual output iterator and a function object, and I need to apply the function *before* writing to the actual output iterator.

Given that the output iterator is written by dereference and assignment, I think I would need some kind of proxy reference, that when assigned to applies the function before doing the actual assignment.

That looks ugly, so I'm asking here for a more elegant solution :)

how is what you want to do different from:

boost::copy( myrange | transformed(func), outputitr );

Jeff

What if myrange is the output of an algorithm, but I don't want to copy it into an intermediate range? e.g. template void my_algo(InputIter first, InputIter last, OutputIter out) { // complex loop traversing input sequence ... // some complicated code to compute each result ... // write each result to output iterator *out++ = ith_result; } I know I could do it easily in two steps storing the intermediate results. intermediate_storage tmp; my_algo(input.begin(), input.end(), back_inserter(tmp)); transform(tmp.begin(), tmp.end(), output); But how to do it in one go, without requiring storage for intermediate results?