C ++中的高阶函数«过滤器»答案

【问题标题】：Higher order function « filter » in C++C ++中的高阶函数«过滤器»
【发布时间】：2010-05-09 09:08:28
【问题描述】：

我想用 C++ 编写一个高阶函数filter。到目前为止我想出的代码如下：

#include <iostream>
#include <string>
#include <functional>
#include <algorithm>
#include <vector>
#include <list>
#include <iterator>

using namespace std;

bool isOdd(int const i) {
  return i % 2 != 0;
}

template <
  template <class, class> class Container,
  class Predicate,
  class Allocator,
  class A
>
Container<A, Allocator> filter(Container<A, Allocator> const & container, Predicate const & pred) {
  Container<A, Allocator> filtered(container);
  container.erase(remove_if(filtered.begin(), filtered.end(), pred), filtered.end());
  return filtered;
}

int main() {
  int const a[] = {23, 12, 78, 21, 97, 64};
  vector<int const> const v(a, a + 6);
  vector<int const> const filtered = filter(v, isOdd);
  copy(filtered.begin(), filtered.end(), ostream_iterator<int const>(cout, " "));
}

但是在编译此代码时，我收到以下我无法理解的错误消息，因此删除了：

/usr/include/c++/4.3/ext/new_allocator.h: In instantiation of ‘__gnu_cxx::new_allocator<const int>’:
/usr/include/c++/4.3/bits/allocator.h:84:   instantiated from ‘std::allocator<const int>’
/usr/include/c++/4.3/bits/stl_vector.h:75:   instantiated from ‘std::_Vector_base<const int, std::allocator<const int> >’
/usr/include/c++/4.3/bits/stl_vector.h:176:   instantiated from ‘std::vector<const int, std::allocator<const int> >’
Filter.cpp:29:   instantiated from here
/usr/include/c++/4.3/ext/new_allocator.h:82: error: ‘const _Tp* __gnu_cxx::new_allocator<_Tp>::address(const _Tp&) const [with _Tp = const int]’ cannot be overloaded
/usr/include/c++/4.3/ext/new_allocator.h:79: error: with ‘_Tp* __gnu_cxx::new_allocator<_Tp>::address(_Tp&) const [with _Tp = const int]’
Filter.cpp: In function ‘Container<A, Allocator> filter(const Container<A, Allocator>&, const Predicate&) [with Container = std::vector, Predicate = bool ()(int), Allocator = std::allocator<const int>, A = const int]’:
Filter.cpp:30:   instantiated from here
Filter.cpp:23: error: passing ‘const std::vector<const int, std::allocator<const int> >’ as ‘this’ argument of ‘__gnu_cxx::__normal_iterator<typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer, std::vector<_Tp, _Alloc> > std::vector<_Tp, _Alloc>::erase(__gnu_cxx::__normal_iterator<typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer, std::vector<_Tp, _Alloc> >, __gnu_cxx::__normal_iterator<typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer, std::vector<_Tp, _Alloc> >) [with _Tp = const int, _Alloc = std::allocator<const int>]’ discards qualifiers
/usr/include/c++/4.3/bits/stl_algo.h: In function ‘_FIter std::remove_if(_FIter, _FIter, _Predicate) [with _FIter = __gnu_cxx::__normal_iterator<const int*, std::vector<const int, std::allocator<const int> > >, _Predicate = bool (*)(int)]’:
Filter.cpp:23:   instantiated from ‘Container<A, Allocator> filter(const Container<A, Allocator>&, const Predicate&) [with Container = std::vector, Predicate = bool ()(int), Allocator = std::allocator<const int>, A = const int]’
Filter.cpp:30:   instantiated from here
/usr/include/c++/4.3/bits/stl_algo.h:821: error: assignment of read-only location ‘__result.__gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* [with _Iterator = const int*, _Container = std::vector<const int, std::allocator<const int> >]()’
/usr/include/c++/4.3/ext/new_allocator.h: In member function ‘void __gnu_cxx::new_allocator<_Tp>::deallocate(_Tp*, size_t) [with _Tp = const int]’:
/usr/include/c++/4.3/bits/stl_vector.h:150:   instantiated from ‘void std::_Vector_base<_Tp, _Alloc>::_M_deallocate(_Tp*, size_t) [with _Tp = const int, _Alloc = std::allocator<const int>]’
/usr/include/c++/4.3/bits/stl_vector.h:136:   instantiated from ‘std::_Vector_base<_Tp, _Alloc>::~_Vector_base() [with _Tp = const int, _Alloc = std::allocator<const int>]’
/usr/include/c++/4.3/bits/stl_vector.h:286:   instantiated from ‘std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const _Alloc&) [with _InputIterator = const int*, _Tp = const int, _Alloc = std::allocator<const int>]’
Filter.cpp:29:   instantiated from here
/usr/include/c++/4.3/ext/new_allocator.h:98: error: invalid conversion from ‘const void*’ to ‘void*’
/usr/include/c++/4.3/ext/new_allocator.h:98: error:   initializing argument 1 of ‘void operator delete(void*)’
/usr/include/c++/4.3/bits/stl_algobase.h: In function ‘_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false, _II = const int*, _OI = const int*]’:
/usr/include/c++/4.3/bits/stl_algobase.h:435:   instantiated from ‘_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false, _II = __gnu_cxx::__normal_iterator<const int*, std::vector<const int, std::allocator<const int> > >, _OI = __gnu_cxx::__normal_iterator<const int*, std::vector<const int, std::allocator<const int> > >]’
/usr/include/c++/4.3/bits/stl_algobase.h:466:   instantiated from ‘_OI std::copy(_II, _II, _OI) [with _II = __gnu_cxx::__normal_iterator<const int*, std::vector<const int, std::allocator<const int> > >, _OI = __gnu_cxx::__normal_iterator<const int*, std::vector<const int, std::allocator<const int> > >]’
/usr/include/c++/4.3/bits/vector.tcc:136:   instantiated from ‘__gnu_cxx::__normal_iterator<typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer, std::vector<_Tp, _Alloc> > std::vector<_Tp, _Alloc>::erase(__gnu_cxx::__normal_iterator<typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer, std::vector<_Tp, _Alloc> >, __gnu_cxx::__normal_iterator<typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer, std::vector<_Tp, _Alloc> >) [with _Tp = const int, _Alloc = std::allocator<const int>]’
Filter.cpp:23:   instantiated from ‘Container<A, Allocator> filter(const Container<A, Allocator>&, const Predicate&) [with Container = std::vector, Predicate = bool ()(int), Allocator = std::allocator<const int>, A = const int]’
Filter.cpp:30:   instantiated from here
/usr/include/c++/4.3/bits/stl_algobase.h:396: error: no matching function for call to ‘std::__copy_move<false, true, std::random_access_iterator_tag>::__copy_m(const int*&, const int*&, const int*&)’

请告诉我我在这里做错了什么以及实现我想要的那种高阶多态性的正确方法是什么。

谢谢。

编辑：

谢谢大家。这是我在应用您的建议后得到的新代码（现在可以了，耶！）

#include <iostream>
#include <string>
#include <functional>
#include <algorithm>
#include <vector>
#include <list>
#include <iterator>

using namespace std;

bool isOdd(int const i) {
  return i % 2 != 0;
}

template <
  template <typename, typename> class Container,
  typename Predicate,
  typename Allocator,
  typename A
>
Container<A, Allocator> filter(Container<A, Allocator> const & container, Predicate const & pred) {
  Container<A, Allocator> filtered(container);
  filtered.erase(remove_if(filtered.begin(), filtered.end(), pred), filtered.end());
  return filtered;
}

int main() {
  int a[] = {23, 12, 78, 21, 97, 64};
  vector<int> v(a, a + 6);
  vector<int> filtered = filter(v, isOdd);
  copy(filtered.begin(), filtered.end(), ostream_iterator<int>(cout, " "));
}

【问题讨论】：

这个功能会不会很贵？您正在复制一个容器，过滤副本，然后按值返回过滤后的副本。为什么不直接在原始容器上使用remove_if()？或者，创建一个别名函数filter()，它接受迭代器，然后调用remove_if()？

标签： c++ templates stl higher-order-functions

【解决方案1】：

为什么你的Container 参数化了？

template <typename C, typename P>
C filter(C const & container, P pred) {
  C filtered(container);
  filtered.erase(remove_if(filtered.begin(), filtered.end(), pred), filtered.end());
  return filtered;
}

同样有效。请注意，我按值传递 P，而不是按 const 引用传递，正如 Meyers 在 Effective C++ 中所建议的那样（迭代器和仿函数应该按值传递）。

【讨论】：

如果要访问容器的类型参数怎么办？
@Red Hyena：那你就做typename C::value_type
还有一个问题......为什么不通过引用传递迭代器和仿函数？
@Red Hyena：至于迭代器，如果你引用它们，那么你不能传递临时变量（sort(vec.begin(), vec.end()) 不会编译）。如果你通过 const 引用传递它们，那么你将无法增加它们等等。

【解决方案2】：

remove_copy_if 呢？（使用 isEven()）。它已经为您构建好了。

【讨论】：

否则实现copy_if。在 C++ 中，使用迭代器比使用容器更惯用。

【解决方案3】：

错误不在filter，而是在：

int main() {
  int const a[] = {23, 12, 78, 21, 97, 64};
  vector<int const> const v(a, a + 6);
}

You can't have a vector of const stuff。删除内部 const：

int main() {
  int const a[] = {23, 12, 78, 21, 97, 64};
  vector<int> const v(a, a + 6);
}

（当然是filtered.erase，而不是container.erase。）

【讨论】：

【解决方案4】：

container 是一个常量引用。你不能打电话给erase()。你可能是想打电话

filtered.erase(remove_if(filtered.begin(), filtered.end(), pred), filtered.end());

【讨论】：