g ++"因为以下虚函数是纯粹的"与抽象基类

Question

这是我的示例代码,它产生错误:

struct Impl
{
  int data_size_;
  int find(int var){return 0;}
  int get(int rowid){return 0;}
};

class Container
{
public:
  Container() {}
  virtual ~Container() {}
  virtual int get_size() = 0;
  virtual int get(int rowid) = 0;
};


class SortedContainer : virtual public Container {
public:
  virtual int find(int var) = 0;
};

class ContainerImpl : public Container
{
protected:
  Impl impl_;
public:
  int get_size() {return impl_.data_size_;}
  int get(int rowid) {return impl_.get(rowid);}
};

class SortedContainerImpl
  : public SortedContainer, public ContainerImpl
{
private:
  typedef ContainerImpl Base;
public:
  int find(int var){return Base::impl_.find(var);}
};

ContainerImpl ci;
SortedContainerImpl sci;

似乎"SortedContainerImpl"出错,而"ContainerImpl"没问题.

g ++抱怨:

example_b.cpp:42:21: error: cannot declare variable ‘sci’ to be of abstract type ‘SortedContainerImpl’
example_b.cpp:32:7: note:   because the following virtual functions are pure within ‘SortedContainerImpl’:
example_b.cpp:13:15: note:  virtual int Container::get_size()
example_b.cpp:14:15: note:  virtual int Container::get(int)

我从ContainerImpl获取SortedContainerImpl以重用get_size()和get(int)

我不熟悉c ++,这个问题的本质是什么,我该如何解决？

谢谢大家.

Answer 1

在C++中有了一个纯虚拟成员函数,你的类就变成了抽象类,你不能创建它的任何对象.
这样的类本身并不是可实例化的.它意味着充当接口.可以从这样的抽象类派生,并提供派生类中所有纯虚函数的实现.

请注意,您的类SortedContainerImpl派生自两个类SortedContainer和ContainerImpl.
SortedContainer反过来派生自Container但它永远不会实现纯虚函数.

Answer 2

您的SortedContainerImpl类有两个单独的Container基类.一个是虚拟的(通过SortedContainer类),另一个是非虚​​拟的(通过ContainerImpl类).

SortedContainerImpl具有具体实现Container::get_size()和Container::get(int)用于从来自在基ContainerImpl,但不为经由来自在虚拟基础SortedContainer.

解决问题的一种方法是在以下方面给出具体实现SortedContainerImpl:

class SortedContainerImpl
  : public SortedContainer, public ContainerImpl
{
private:
  typedef ContainerImpl Base;
public:
  int find(int var){return Base::impl_.find(var);}

  int get_size() {return ContainerImpl::get_size();}
  int get(int rowid) {return ContainerImpl::get(rowid);}
};

另一种方法是创建Container一个虚拟基类ContainerImpl,所以SortedContainerImpl只能获得一个虚拟基类Container:

class ContainerImpl : virtual public Container
{
protected:
  Impl impl_;
public:
  int get_size() {return impl_.data_size_;}
  int get(int rowid) {return impl_.get(rowid);}
};