是否有任何非糟糕的方法来拥有多种类型的对象集合？我非常乐意从一个共同的基础中推导出每种类型。我需要合理的语义，因此可以复制，分配集合等等。
显然，我不能只使用基类的向量或列表。对象将被切片，复制根本不会起作用。使用指针或智能指针的向量或列表可以工作，但是你不会得到理智的复制语义。
要获得理智的复制语义，您需要使用像Boost ptr_vector这样的东西。但这需要一个痛苦且容易出错的基础设施。从本质上讲，您不能从基类派生一个新类，因为如果它进入集合，它将无法正确复制。
这似乎是一件很常见的事情，我所知道的所有解决方案都非常糟糕。看起来C ++基本上缺少一种创建与给定实例相同的对象的新实例的方法 - 即使该类型具有复制构造函数。制作clone或duplicate函数需要在每个派生类中仔细重载。如果你在创建一个派生自基础的新类（或从该基础派生的任何其他类）时没有这样做 - 繁荣，你的收藏就会中断。
真的没有更好的方法吗？
答案 0 :(得分：3)
我认为你可以使用std::vector<boost::any>完成大部分工作。
#include "boost/any.hpp"
#include <vector>
#include <iostream>
//Simple class so we can see what's going on
class MM {
public:
MM() { std::cout<<"Create @ "<<this<<std::endl; }
MM( const MM & o ) { std::cout<<"Copy "<<&o << " -> "<<this<<std::endl; }
~MM() { std::cout<<"Destroy @ "<<this<<std::endl; }
};
int main()
{
//Fill a vector with some stuff
std::vector<boost::any> v;
v.push_back(0);
v.push_back(0);
v.push_back(0);
v.push_back(0);
//Overwrite one entry with one of our objects.
v[0] = MM();
std::cout<<"Copying the vector"<<std::endl;
std::vector<boost::any> w;
w = v;
std::cout<<"Done"<<std::endl;
}
我得到了输出：
Create @ 0xbffff6ae
Copy 0xbffff6ae -> 0x100154
Destroy @ 0xbffff6ae
Copying the vector
Copy 0x100154 -> 0x100194
Done
Destroy @ 0x100194
Destroy @ 0x100154
我希望看到的是什么。
编辑：
根据您的要求，能够将成员视为一些常见的基类型，您需要与boost::any非常相似的内容，幸好这是一个相对简单的类。
template<typename BASE>
class any_with_base
{
// ... Members as for boost::any
class placeholder
{
virtual BASE * as_base() = 0;
//Other members as in boost::any::placeholder
};
template<typename ValueType>
class holder : public placeholder
{
virtual BASE * as_base() { return (BASE*)&held; }
//Other members as in boost::any::holder<T>
};
BASE* as_base() { return content?content->as_base():0; }
}
现在你应该能够做到这一点：
vector< any_with_base<Base> > v;
v.push_back( DerivedA() );
v.push_back( DerivedB() );
v[0].as_base()->base_fn();
v[1].as_base()->base_fn();
any_cast<DerivedA>(v[0])->only_in_a();
我实际上不喜欢any_cast的语法，并会利用这个机会添加一个“as”成员函数..这样我就可以把最后一行写成：
v[0].as<DerivedA>()->only_in_a();
答案 1 :(得分：0)
好的，为了跟进我的评论，有一种方法可以在不使用boost :: any的情况下做到这一点，在大多数情况下应该提供卓越的性能，但无可否认它更多涉及。我的解决方案结合了两个想法：使用不包含其内容类型的holder类，以及轻量级自定义RTTI。我们给持有者类提供有意义的复制和赋值语义，并使用持有者容器来管理对象的集合。我们使用轻量级RTTI在必要时发现对象的真实类型。这里有一些代码来展示我的建议：
#include <vector>
#include <cassert>
#include <iostream>
#include <boost/cast.hpp>
#include <boost/scoped_ptr.hpp>
#include <boost/static_assert.hpp>
/// This template makes it possible to enforce the invariant that every type in a
/// hierarchy defines the id( ) function, which is necessary for our RTTI. Using
/// a static assertion, we can force a compile error if a type doesn't provide id( ).
template< typename T >
struct provides_id {
typedef char one;
typedef long two;
template< typename U, std::string const &(U::*)( ) const = &U::id >
struct id_detector { };
template< typename U > static one test( id_detector< U > * );
template< typename U > static two test( ... );
enum { value = sizeof(test<T>(0)) == sizeof(one) };
};
/// Base class for the holder. It elides the true type of the object that it holds,
/// providing access only through the base class interface. Since there is only one
/// derived type, there is no risk of forgetting to define the clone() function.
template< typename T >
struct holder_impl_base {
virtual ~holder_impl_base( ) { }
virtual T *as_base( ) = 0;
virtual T const *as_base( ) const = 0;
virtual holder_impl_base *clone( ) const = 0;
};
/// The one and only implementation of the holder_impl_base interface. It stores
/// a derived type instance and provides access to it through the base class interface.
/// Note the use of static assert to force the derived type to define the id( )
/// function that we use to recover the instance's true type.
template< typename T, typename U >
struct holder_impl : public holder_impl_base< T > {
BOOST_STATIC_ASSERT(( provides_id< U >::value ));
holder_impl( U const &p_data )
: m_data( p_data )
{ }
virtual holder_impl *clone( ) const {
return new holder_impl( *this );
}
virtual T *as_base( ) {
return &m_data;
}
virtual T const *as_base( ) const {
return &m_data;
}
private:
U m_data;
};
/// The holder that we actually use in our code. It can be constructed from an instance
/// of any type that derives from T and it uses a holder_impl to elide the type of the
/// instance. It provides meaningful copy and assignment semantics that we are looking
/// for.
template< typename T >
struct holder {
template< typename U >
holder( U const &p_data )
: m_impl( new holder_impl< T, U >( p_data ))
{ }
holder( holder const &p_other )
: m_impl( p_other.m_impl -> clone( ))
{ }
template< typename U >
holder &operator = ( U const &p_data ) {
m_impl.reset( new holder_impl< T, U >( p_data ));
return *this;
}
holder &operator = ( holder const &p_other ) {
if( this != &p_other ) {
m_impl.reset( p_other.m_impl -> clone( ));
}
return *this;
}
T *as_base( ) {
return m_impl -> as_base( );
}
T const *as_base( ) const {
return m_impl -> as_base( );
}
/// The next two functions are what we use to cast elements to their "true" types.
/// They use our custom RTTI (which is guaranteed to be defined due to our static
/// assertion) to check if the "true" type of the object in a holder is the same as
/// as the template argument. If so, they return a pointer to the object; otherwise
/// they return NULL.
template< typename U >
U *as( ) {
T *base = as_base( );
if( base -> id( ) == U::static_id( )) {
return boost::polymorphic_downcast< U * >( base );
}
return 0;
}
template< typename U >
U const *as( ) const {
T *base = as_base( );
if( base -> id( ) == U::static_id( )) {
return boost::polymorphic_downcast< U const * >( base );
}
return 0;
}
private:
boost::scoped_ptr< holder_impl_base< T > > m_impl;
};
/// A base type and a couple derived types to demonstrate the technique.
struct base {
virtual ~base( )
{ }
virtual std::string const &id( ) const = 0;
};
struct derived1 : public base {
std::string const &id( ) const {
return c_id;
}
static std::string const &static_id( ) {
return c_id;
}
private:
static std::string const c_id;
};
std::string const derived1::c_id( "derived1" );
struct derived2 : public base {
std::string const &id( ) const {
return c_id;
}
static std::string const &static_id( ) {
return c_id;
}
private:
static std::string const c_id;
};
std::string const derived2::c_id( "derived2" );
/// A program to demonstrate that the technique works as advertised.
int main( ) {
std::vector< holder< base > > vector1;
vector1.push_back( derived1( ));
vector1.push_back( derived2( ));
std::vector< holder< base > > vector2 = vector1;
/// We see that we have true copies!
assert( vector1[0].as_base( ) != vector2[0].as_base( ));
assert( vector1[1].as_base( ) != vector2[0].as_base( ));
/// Easy assignment of container elements to new instances!
vector2[0] = derived2( );
vector2[1] = derived1( );
// Recovery of the "true" types!
std::vector< holder< base > >::iterator l_itr = vector1.begin( );
std::vector< holder< base > >::iterator l_end = vector1.end ( );
for( ; l_itr != l_end; ++l_itr ) {
if( derived1 *ptr = l_itr -> as< derived1 >( )) {
std::cout << ptr -> static_id( ) << std::endl;
}
else if( derived2 *ptr = l_itr -> as< derived2 >( )) {
std::cout << ptr -> static_id( ) << std::endl;
}
}
}
这是输出：
derived1
derived2