Удалить элементы из неупорядоченной карты, выполняя предикат

Question

Я хочу удалить элементы (ячейки гистограммы) из std::unordered_map (гистограммы), которая выполняет предикат (ячейки гистограммы с нулевым счетом), заданные в виде лямбда-выражения следующим образом

std::remove_if(begin(m_map), end(m_map), [](const Bin & bin) { return bin.second == 0; });

, но GCC-4.6.1 жалуется следующим образом

/usr/include/c++/4.6/bits/stl_pair.h:156:2: error: assignment of read-only member ‘std::pair<const unsigned char, unsigned char>::first’
/usr/include/c++/4.6/bits/stl_pair.h: In member function ‘std::pair<_T1, _T2>& std::pair<_T1, _T2>::operator=(std::pair<_T1, _T2>&&) [with _T1 = const unsigned char, _T2 = long unsigned int, std::pair<_T1, _T2> = std::pair<const unsigned char, long unsigned int>]’:
/usr/include/c++/4.6/bits/stl_algo.h:1149:13:   instantiated from ‘_FIter std::remove_if(_FIter, _FIter, _Predicate) [with _FIter = std::__detail::_Hashtable_iterator<std::pair<const unsigned char, long unsigned int>, false, false>, _Predicate = pnw::histogram<V, C, H>::pack() [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]::<lambda(const Bin&)>]’
tests/../histogram.hpp:68:13:   instantiated from ‘void pnw::histogram<V, C, H>::pack() [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]’
tests/../histogram.hpp:85:13:   instantiated from ‘void pnw::histogram<V, C, H>::normalize(uint) [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >, uint = unsigned int]’
tests/../histogram.hpp:121:51:   instantiated from ‘H& pnw::histogram<V, C, H>::add(It, It) [with It = __gnu_cxx::__normal_iterator<const unsigned char*, std::vector<unsigned char> >, V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]’
tests/../histogram.hpp:129:55:   instantiated from ‘H& pnw::histogram<V, C, H>::add(const V&) [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]’
tests/../histogram.hpp:57:60:   instantiated from ‘pnw::histogram<V, C, H>::histogram(const V&, pnw::histogram<V, C, H>::TYPE_t) [with V = std::vector<unsigned char>, C = long unsigned int, H = std::unordered_map<unsigned char, long unsigned int, std::hash<unsigned char>, std::equal_to<unsigned char>, std::allocator<std::pair<const unsigned char, long unsigned int> > >]’
tests/t_histogram.cpp:38:61:   instantiated from ‘void test_dense_histogram() [with T = unsigned char, C = long unsigned int]’
tests/t_histogram.cpp:64:5:   instantiated from ‘void test_histograms() [with C = long unsigned int]’
tests/t_histogram.cpp:200:29:   instantiated from here
/usr/include/c++/4.6/bits/stl_pair.h:156:2: error: assignment of read-only member ‘std::pair<const unsigned char, long unsigned int>::first’
make: *** [tests/t_histogram.o] Error 1

Не применимо ли std::remove_if к std::unordered_map?

Answer 1

Ответ - нет (вы не можете использовать remove_if для ассоциативных контейнеров). Вам нужно сделать простой цикл; erase(iterator) член теперь возвращает следующий действительный итератор - так что ваш цикл становится:

for(auto it = begin(m_map); it != end(m_map);)
{
  if (it->second == 0)
  {
    it = m_map.erase(it); // previously this was something like m_map.erase(it++);
  }
  else
    ++it;
}

Answer 2

Имеется предложение Стефана Т. Лававея о едином уничтожении контейнера :

template <class K, class T, class H, class P, class A, class Predicate>
void erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);

т.е.

std::erase_if(m_map, [](const Bin& bin) { return bin.second == 0; });

К сожалению, это не сделало это C ++ 17. Я полагаю, это связано с принятием TS диапазонов.