Question

I wanted to write a higher order function filter with C++. The code I have come up with so far is as follows:

#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, " "));
}

However on compiling this code, I get the following error messages that I am unable to understand and hence get rid of:

/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*&)’

Please tell me what I am doing wrong here and what is the correct way to achieve the kind of higher order polymorphism I want.

Thanks.

EDIT:

Thank you, everyone. Here’s the new code I got after applying your suggestions (and it works now, yay!)

#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, " "));
}

Answer 1

Why is your Container parametrized at all?

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;
}

Works just as well. Notice that I passed P by value rather than by const reference, as advised by Meyers in Effective C++ (iterators and functors should be passed by value).