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- /*=============================================================================
- Copyright (c) 2011 Joel de Guzman
- Distributed under the Boost Software License, Version 1.0. (See accompanying
- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
- Problem:
- So... you have an input sequence I and a target vector R. You want to
- copy I into R. But, I may have less elements than the result vector R.
- For those elements not in R, you want them to be default constructed.
- Here's a case:
-
- I: list<double, std::string>
- R: vector<double, std::string, int, short>
-
- You want the elements at the right of I not in R (i.e. int, short)
- default constructed. Those at the left, found in both I and R, you want
- to simply copy from I.
-
- Of course you want to be able to handle any type of I and R.
- ==============================================================================*/
- // We'll use these containers as examples
- #include <boost/fusion/container/list.hpp>
- #include <boost/fusion/container/vector.hpp>
- // For doing I/O
- #include <boost/fusion/sequence/io.hpp>
- // We'll use join and advance for processing
- #include <boost/fusion/algorithm/transformation/join.hpp>
- #include <boost/fusion/iterator/advance.hpp>
- // The fusion <--> MPL link header
- #include <boost/fusion/mpl.hpp>
- // Same-o same-o
- #include <iostream>
- #include <string>
- int
- main()
- {
- using namespace boost::fusion;
- using namespace boost;
-
- // Let's specify our own tuple delimeters for nicer printing
- std::cout << tuple_open('[');
- std::cout << tuple_close(']');
- std::cout << tuple_delimiter(", ");
- // Here's your input sequence
- typedef list<double, std::string> I;
- I i(123.456, "Hello");
-
- // Here's your output sequence. For now, it is just a typedef
- typedef vector<double, std::string, int, short> R;
- // Let's get the sizes of the sequences. Yeah, you already know that.
- // But with templates, you are simply given, say, R and I, corresponding
- // to the types of the sequences. You'll have to deal with it generically.
- static int const r_size = result_of::size<R>::value;
- static int const i_size = result_of::size<I>::value;
- // Make sure that I has no more elements than R
- // Be nice and catch obvious errors earlier rather than later.
- // Without this assert, the mistake will still be caught by Fusion,
- // but the error will point to somewhere really obscure.
- BOOST_STATIC_ASSERT(i_size <= r_size);
- // Let's get the begin and end iterator types of the output sequence
- // There's no actual vector yet. We just want to know the types.
- typedef result_of::begin<R>::type r_begin;
- typedef result_of::end<R>::type r_end;
- // Let's skip i_size elements from r_begin. Again, we just want to know the type.
- typedef result_of::advance_c<r_begin, i_size>::type r_advance;
- // Now, make MPL iterators from r_advance and r_end. Ditto, just types.
- typedef mpl::fusion_iterator<r_advance> mpl_r_advance;
- typedef mpl::fusion_iterator<r_end> mpl_r_end;
- // Make an mpl::iterator_range from the MPL iterators we just created
- // You guessed it! --just a type.
- typedef mpl::iterator_range<mpl_r_advance, mpl_r_end> tail;
- // Use join to join the input sequence and our mpl::iterator_range
- // Our mpl::iterator_range is 'tail'. Here, we'll actually instantiate
- // 'tail'. Notice that this is a flyweight object, typically just 1 byte
- // in size -- it doesn't really hold any data, but is a fully conforming
- // sequence nonetheless. When asked to return its elements, 'tail' returns
- // each element default constructed. Breeds like a rabbit!
- // Construct R from the joined sequences:
- R r(join(i, tail()));
-
- // Then finally, print the result:
- std::cout << r << std::endl;
- return 0;
- }
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