如何提升:: range :: sort（）boost :: transformed_range？

Question

我想根据foo的成员获得vector<foo>中的独特元素。我使用boost::adaptors::transform来选择成员，然后排序，然后使用boost::adaptors::unique。我无法让排序步骤起作用。撇开unique现在呼吁，我已经尝试Coliru下面的代码。

#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; 
} 

的#if部分不起作用：

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); 

尽管std::sort与基迭代器类型的#else节不工作。我不明白为什么。在我的真实使用案例中，我也在使用boost::adaptors::indirect和boost::adaptors::filter（并且我希望在进行排序前进行过滤，或者至少尝试这样做，看看它是如何执行的），所以这就是为什么我不是简单的在执行转换之前用lambda谓词对words进行排序。

Answer 1

问题是您有临时字符串的视图，您不能交换。你可能会解决你的代码与以下：

auto asString = boost::adaptors::transform(words, +[](foo& x) -> std::string& {return x.bar;}); 

Demo

注意，对于这一点，你串直接进行排序，而不是类。