3

filter我想用 C++写一个高阶函数。到目前为止，我提出的代码如下：

#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，因此摆脱了：

/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（现在可以使用了，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, " "));
}
4

4 回答 4

5

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;
}

效果也一样。请注意，正如 Meyers 在 Effective C++ 中所建议的那样，我通过P值而不是通过 const 引用传递（迭代器和仿函数应该按值传递）。

于 2010-05-09T09:20:18.947 回答
4

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

于 2010-05-09T09:27:08.627 回答
3

错误不在 中filter，而是在：

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

你不能有 const stuff 的向量。删除内部常量：

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

（当然filtered.erase，不是container.erase。）

于 2010-05-09T09:22:24.247 回答
0

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

filtered.erase(remove_if(filtered.begin(), filtered.end(), pred), filtered.end());
于 2010-05-09T09:35:15.683 回答