初始化多态指针容器

Question

我可以用boost::ptr_vector初始化多态boost::assign::list_of吗？

#include <boost/ptr_container/ptr_vector.hpp>
#include <boost/assign/list_of.hpp>

boost::ptr_vector<Animal> ls = boost::assign::list_of(new Ant)(new Bee)(new Cat);

这不能编译：

error: no match for call to '(boost::assign_detail::generic_list<Ant*>) (Bear*)'

将boost::ptr_vector<Animal>替换为std::vector<Animal*>会产生同样的错误。

两个人建议手动将模板参数Animal*提供给list_of

boost::assign::list_of<Animal*>(new Ant)(new Bear)(new Cat)

但它仍然不起作用：

boost/ptr_container/ptr_sequence_adapter.hpp: In static member function 'static boost::ptr_container_detail::sequence_config<T, VoidPtrSeq>::U* boost::ptr_container_detail::sequence_config<T, VoidPtrSeq>::get_const_pointer(Iter) [with Iter = std::_Deque_iterator<Animal*, Animal*&, Animal**>, T = Animal, VoidPtrSeq = std::vector<void*, std::allocator<void*> >, boost::ptr_container_detail::sequence_config<T, VoidPtrSeq>::U = Animal]':
boost/ptr_container/detail/reversible_ptr_container.hpp:95:71:   instantiated from 'static boost::ptr_container_detail::reversible_ptr_container<Config, CloneAllocator>::Ty_* boost::ptr_container_detail::reversible_ptr_container<Config, CloneAllocator>::null_clone_allocator<allow_null_values>::allocate_clone_from_iterator(Iter) [with Iter = std::_Deque_iterator<Animal*, Animal*&, Animal**>, bool allow_null_values = false, Config = boost::ptr_container_detail::sequence_config<Animal, std::vector<void*, std::allocator<void*> > >, CloneAllocator = boost::heap_clone_allocator, boost::ptr_container_detail::reversible_ptr_container<Config, CloneAllocator>::Ty_ = Animal]'
boost/ptr_container/detail/scoped_deleter.hpp:70:21:   instantiated from 'boost::ptr_container_detail::scoped_deleter<T, CloneAllocator>::scoped_deleter(InputIterator, InputIterator) [with InputIterator = std::_Deque_iterator<Animal*, Animal*&, Animal**>, T = Animal, CloneAllocator = boost::ptr_container_detail::reversible_ptr_container<boost::ptr_container_detail::sequence_config<Animal, std::vector<void*, std::allocator<void*> > >, boost::heap_clone_allocator>::null_clone_allocator<false>]'
boost/ptr_container/detail/reversible_ptr_container.hpp:212:44:   instantiated from 'void boost::ptr_container_detail::reversible_ptr_container<Config, CloneAllocator>::clone_back_insert(ForwardIterator, ForwardIterator) [with ForwardIterator = std::_Deque_iterator<Animal*, Animal*&, Animal**>, Config = boost::ptr_container_detail::sequence_config<Animal, std::vector<void*, std::allocator<void*> > >, CloneAllocator = boost::heap_clone_allocator]'
boost/ptr_container/detail/reversible_ptr_container.hpp:303:13:   instantiated from 'void boost::ptr_container_detail::reversible_ptr_container<Config, CloneAllocator>::constructor_impl(I, I, std::forward_iterator_tag) [with I = std::_Deque_iterator<Animal*, Animal*&, Animal**>, Config = boost::ptr_container_detail::sequence_config<Animal, std::vector<void*, std::allocator<void*> > >, CloneAllocator = boost::heap_clone_allocator]'
boost/ptr_container/detail/reversible_ptr_container.hpp:378:13:   instantiated from 'boost::ptr_container_detail::reversible_ptr_container<Config, CloneAllocator>::reversible_ptr_container(InputIterator, InputIterator, boost::ptr_container_detail::reversible_ptr_container<Config, CloneAllocator>::allocator_type&) [with InputIterator = std::_Deque_iterator<Animal*, Animal*&, Animal**>, Config = boost::ptr_container_detail::sequence_config<Animal, std::vector<void*, std::allocator<void*> > >, CloneAllocator = boost::heap_clone_allocator, boost::ptr_container_detail::reversible_ptr_container<Config, CloneAllocator>::allocator_type = std::allocator<void*>]'
boost/ptr_container/ptr_sequence_adapter.hpp:178:36:   instantiated from 'boost::ptr_sequence_adapter<T, VoidPtrSeq, CloneAllocator>::ptr_sequence_adapter(InputIterator, InputIterator) [with InputIterator = std::_Deque_iterator<Animal*, Animal*&, Animal**>, T = Animal, VoidPtrSeq = std::vector<void*, std::allocator<void*> >, CloneAllocator = boost::heap_clone_allocator]'
boost/ptr_container/ptr_vector.hpp:45:9:   instantiated from 'boost::ptr_vector<T, CloneAllocator, Allocator>::ptr_vector(InputIterator, InputIterator) [with InputIterator = std::_Deque_iterator<Animal*, Animal*&, Animal**>, T = Animal, CloneAllocator = boost::heap_clone_allocator, Allocator = std::allocator<void*>]'
boost/assign/list_of.hpp:163:46:   instantiated from 'Container boost::assign_detail::converter<DerivedTAssign, Iterator>::convert(const Container*, boost::assign_detail::default_type_tag) const [with Container = boost::ptr_vector<Animal>, DerivedTAssign = boost::assign_detail::generic_list<Animal*>, Iterator = std::_Deque_iterator<Animal*, Animal*&, Animal**>]'
boost/assign/list_of.hpp:142:54:   instantiated from 'Container boost::assign_detail::converter<DerivedTAssign, Iterator>::convert_to_container() const [with Container = boost::ptr_vector<Animal>, DerivedTAssign = boost::assign_detail::generic_list<Animal*>, Iterator = std::_Deque_iterator<Animal*, Animal*&, Animal**>]'
boost/assign/list_of.hpp:436:81:   instantiated from 'boost::assign_detail::generic_list<T>::operator Container() const [with Container = boost::ptr_vector<Animal>, T = Animal*]'

Answer 1

我试试看。我认为list_of是一个正在推导模板参数的模板。因为您的参数类型是一个派生类，所以它认为它是容器类型。兄弟班不匹配。

您可能必须显式提供模板参数，如下所示：

boost::ptr_vector<Animal> ls = boost::assign::list_of<Animal*>(new Ant)(new Bee)(new Cat);

Answer 2

您不应该使用list_of来分配以提升指针集合。如果其中一个构造函数抛出，已经构造的实例尚未添加到容器中，也不会被list_of删除(它需要值，而不是多态类型)。

Boost.Assignment提供了ptr_list_of()。但是，该元素只能添加同构元素(相同类型的元素)。

因此，我认为Boost.Assignment对于异构集合并不真正有用。

Answer 3

如果允许两步初始化，boost::assign::push_back可能满足以下目的：

ptr_vector<Animal> ls;
assign::push_back( ls )(new Ant)(new Bee)(new Cat);

不幸的是，我不知道一个简单的方法来完成一步初始化。如果我们像下面这样准备一些转换器，可能可以完成一个步骤的初始化。

template< class T >
struct converter : std::vector< T > {
  template< class Container >
  operator Container() const {
    Container c;
    for ( const_iterator i = begin(), e = end();  i != e;  ++ i )
      c.push_back( *i );
    return c;
  }
};

template< class T >
converter< typename T::value_type > convert( T const& x ) {
  converter< typename T::value_type > c;
  c.assign( x.begin(), x.end() );
  return c;
}

ptr_vector<Animal> ls =
  convert( assign::list_of<Animal*>(new Ant)(new Bee)(new Cat) );

但这可能会很长..。