Question

例如:

abstract class goal
class priority
class childgoal

multimap<priority, goal> mm;
mm.insert(make_pair(priority(), childgoal());

我发现错误:

cannot declare field  std::pair<priority, goal>::second 
to be of abstract type  goal

如果我正确记住,Pair利用违约建筑商在分配之前创造第二位,这是造成痛苦的原因。我可能不正确,但会解释错误。

我怎么做呢? 在哪一种类型是抽象的时,我怎么能够把这插入一个多图(或可能是地图)。

Answer 1

你们可以绘制一个要点地图:

#include <memory>
#include <map>

class Base { };
class Derived1 : public Base { };
class Derived2 : public Base { };

typedef std::shared_ptr<Base> MyPtr;
typedef std::multimap<Key, MyPtr> MyMap;

int main()
{
  MyMap m { { Key(1), MyPtr(new Derived1) }, { Key(2), MyPtr(new Derived2) } };
}

统一初始化需要C++11支持。在旧版本中,你可以改为:

m.insert(MyMap::value_type(Key(1), MyPtr(new Derived1)));

如果没有人需要被绘制的物体,那么你可能会拿走<条码>:unique_ptr<Base>。一个简单的问题,是改变类型。

Answer 2

你们显然想要使用多变(因为你想要把一个衍生的阶层只算一个基级)。

但是,每当你想使用多吗).时,你就应当拥有使用点对基的自动化(如果我正确回顾,参考对基准也是有益的)。假设孩子们公开继承目标,在发言之后是:

childgoal is-a goal
a pointer to goal can point to goal or to childgoal (or to any derived class)
but a goal instance is-NOT-a childgoal

因此,如果你能够把你的童子团划入一个目标(,因为你试图传出一个抽象的“”),那么你就可以把在童年的所有非成员rid掉,从而使你们的原始目标破灭。我认为这不是你的意图。

If you are afraid about the memory management issue that would arise with classic pointers, you could consider using shared_pointers here :

abstract class goal;
class priority;
class childgoal : public goal;

typedef std::tr1::shared_ptr<goal> goalPtr; //sadly I think std::tr1 is not a multiplatform namespace 

multimap<priority, goalPtr> mm;
goalPtr tmpChildGoal(new childgoal()); //always use named shared pointer !
mm.insert(make_pair(priority(), tmpChildGoal);

Answer 3

Very simply said: abstract classes can only be used as pointers or references. If you need a container of a polymorphic instance of your class you need to store a pointer or, better, a smart pointer, possibly a unique_ptr.

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