C中的高阶函数“filter”
发布时间:2020-12-16 10:16:51 所属栏目:百科 来源:网络整理
导读:我想用C编写一个更高阶的函数滤波器.我到目前为止提出的代码如下: #include iostream#include string#include functional#include algorithm#include vector#include list#include iteratorusing namespace std;bool isOdd(int const i) { return i % 2 !=
我想用C编写一个更高阶的函数滤波器.我到目前为止提出的代码如下:
#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(),ostream_iterator<int const>(cout," ")); } 但是,在编译此代码时,我收到以下错误消息,我无法理解,因此摆脱: /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> >,__gnu_cxx::__normal_iterator<typename std::_Vector_base<_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,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 = __gnu_cxx::__normal_iterator<const int*,_OI = __gnu_cxx::__normal_iterator<const int*,std::allocator<const int> > >]’ /usr/include/c++/4.3/bits/stl_algobase.h:466: instantiated from ‘_OI std::copy(_II,_OI) [with _II = __gnu_cxx::__normal_iterator<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 = std::allocator<const int>]’ Filter.cpp:23: instantiated from ‘Container<A,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的正确方法是什么. 谢谢. 编辑: 谢谢大家.这是我在应用你的建议后得到的新代码(它现在可以使用了,是的!) #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> filtered(container); filtered.erase(remove_if(filtered.begin(),filtered.end()); return filtered; } int main() { int a[] = {23,64}; vector<int> v(a,a + 6); vector<int> filtered = filter(v,ostream_iterator<int>(cout," ")); } 解决方法
为什么容器参数化了?
template <typename C,typename P> C filter(C const & container,P pred) { C filtered(container); filtered.erase(remove_if(filtered.begin(),filtered.end()); return filtered; } 也适用.请注意,我通过值而不是const引用传递P,正如Meyers在Effective C中所建议的那样(迭代器和仿函数应该通过值传递). (编辑:李大同) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |