28

我想从std::unordered_map满足作为 lambda 表达式给出的预测（具有零计数的直方图箱）的（直方图）中删除元素（直方图箱），如下所示

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

但 GCC-4.6.1 抱怨如下

/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

不适std::remove_if用于std::unordered_map？

4

2 回答 2

45

答案是否定的（您不能remove_if在关联容器上使用）。你需要做一个简单的循环；该erase(iterator)成员现在返回下一个有效的迭代器 - 所以你的循环变成：

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;
}
于 2012-02-09T11:32:06.247 回答
6

Stephan T. Lavavej 提出了 Uniform Container Erasure 的建议：

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);

IE

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

不幸的是，它没有使它成为 C++17。我想这与 Ranges TS 的采用有关。

于 2017-01-13T11:22:27.327 回答