问题
I want to get the unique elements from a vector<foo> based on a member of foo. I am using boost::adaptors::transform to select the member, then sorting, then using boost::adaptors::unique. I'm having trouble getting the sort step to work. Leaving aside the unique call for now, I've tried the below code on Coliru.
#include <iostream>
#include <string>
#include <vector>
#include <boost/range/adaptor/transformed.hpp>
#include <boost/range/algorithm/sort.hpp>
struct foo
{
foo(std::string a) : bar(a) {}
std::string bar;
bool operator<(const foo& rhs) const {return bar < rhs.bar;}
};
int main()
{
std::vector<foo> words = { foo("z"), foo("d"), foo("b"), foo("c") };
#if 1
{
auto asString = boost::adaptors::transform(words, +[](const foo& x) {return x.bar;});
auto sortedStrings = boost::range::sort(asString);
for (const auto& el : sortedStrings)
std::cout << el << std::endl;
}
#else
{
auto asString = boost::adaptors::transform(words, +[](const foo& x) {return x.bar;});
std::sort(asString.begin().base(), asString.end().base());
for (const auto& el : asString)
std::cout << el << std::endl;
}
{
auto sortedStrings = boost::range::sort(words);
for (const auto& el : sortedStrings)
std::cout << el.bar << std::endl;
}
#endif
return 0;
}
The #if section doesn't work:
In file included from /usr/local/include/c++/6.3.0/bits/char_traits.h:39:0,
from /usr/local/include/c++/6.3.0/ios:40,
from /usr/local/include/c++/6.3.0/ostream:38,
from /usr/local/include/c++/6.3.0/iostream:39,
from main.cpp:1:
/usr/local/include/c++/6.3.0/bits/stl_algobase.h: In instantiation of 'void std::iter_swap(_ForwardIterator1, _ForwardIterator2) [with _ForwardIterator1 = boost::iterators::transform_iterator<std::__cxx11::basic_string<char> (*)(const foo&), __gnu_cxx::__normal_iterator<foo*, std::vector<foo> >, boost::iterators::use_default, boost::iterators::use_default>; _ForwardIterator2 = boost::iterators::transform_iterator<std::__cxx11::basic_string<char> (*)(const foo&), __gnu_cxx::__normal_iterator<foo*, std::vector<foo> >, boost::iterators::use_default, boost::iterators::use_default>]':
/usr/local/include/c++/6.3.0/bits/stl_algo.h:84:20: required from 'void std::__move_median_to_first(_Iterator, _Iterator, _Iterator, _Iterator, _Compare) [with _Iterator = boost::iterators::transform_iterator<std::__cxx11::basic_string<char> (*)(const foo&), __gnu_cxx::__normal_iterator<foo*, std::vector<foo> >, boost::iterators::use_default, boost::iterators::use_default>; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/usr/local/include/c++/6.3.0/bits/stl_algo.h:1918:34: required from '_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = boost::iterators::transform_iterator<std::__cxx11::basic_string<char> (*)(const foo&), __gnu_cxx::__normal_iterator<foo*, std::vector<foo> >, boost::iterators::use_default, boost::iterators::use_default>; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/usr/local/include/c++/6.3.0/bits/stl_algo.h:1950:38: required from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = boost::iterators::transform_iterator<std::__cxx11::basic_string<char> (*)(const foo&), __gnu_cxx::__normal_iterator<foo*, std::vector<foo> >, boost::iterators::use_default, boost::iterators::use_default>; _Size = long int; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/usr/local/include/c++/6.3.0/bits/stl_algo.h:1965:25: required from 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = boost::iterators::transform_iterator<std::__cxx11::basic_string<char> (*)(const foo&), __gnu_cxx::__normal_iterator<foo*, std::vector<foo> >, boost::iterators::use_default, boost::iterators::use_default>; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/usr/local/include/c++/6.3.0/bits/stl_algo.h:4707:18: required from 'void std::sort(_RAIter, _RAIter) [with _RAIter = boost::iterators::transform_iterator<std::__cxx11::basic_string<char> (*)(const foo&), __gnu_cxx::__normal_iterator<foo*, std::vector<foo> >, boost::iterators::use_default, boost::iterators::use_default>]'
/usr/local/include/boost/range/algorithm/sort.hpp:33:14: required from 'RandomAccessRange& boost::range::sort(RandomAccessRange&) [with RandomAccessRange = boost::range_detail::transformed_range<std::__cxx11::basic_string<char> (*)(const foo&), std::vector<foo> >]'
main.cpp:27:57: required from here
/usr/local/include/c++/6.3.0/bits/stl_algobase.h:148:11: error: no matching function for call to 'swap(boost::iterators::detail::iterator_facade_base<boost::iterators::transform_iterator<std::__cxx11::basic_string<char> (*)(const foo&), __gnu_cxx::__normal_iterator<foo*, std::vector<foo> >, boost::iterators::use_default, boost::iterators::use_default>, std::__cxx11::basic_string<char>, boost::iterators::random_access_traversal_tag, std::__cxx11::basic_string<char>, long int, false, false>::reference, boost::iterators::detail::iterator_facade_base<boost::iterators::transform_iterator<std::__cxx11::basic_string<char> (*)(const foo&), __gnu_cxx::__normal_iterator<foo*, std::vector<foo> >, boost::iterators::use_default, boost::iterators::use_default>, std::__cxx11::basic_string<char>, boost::iterators::random_access_traversal_tag, std::__cxx11::basic_string<char>, long int, false, false>::reference)'
swap(*__a, *__b);
Whereas the std::sort with the base iterator type in the #else section does work. I don't understand why. In my real use case, I am also using boost::adaptors::indirect and boost::adaptors::filter (and I'd prefer to do the filtering before doing the sorting, or at least try to do so and see how it performs), so that's why I'm not simply sorting words with a lambda predicate before doing the transform.
回答1:
Problem is that you have a view of temporary strings that you cannot swap. you may fix you code with the following:
auto asString = boost::adaptors::transform(words, +[](foo& x) -> std::string& {return x.bar;});
Demo
Note that with that, you sort the string directly, and not the class.
来源：https://stackoverflow.com/questions/42727801/how-do-i-boostrangesort-a-boosttransformed-range