g ++“因为以下虚函数是纯净的"与抽象基类 [英] g++ "because the following virtual functions are pure" with abstract base class

问题描述

这是我的示例代码，会产生错误:

Here is my example code which produces the error:

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"出了问题，而"ContainerImpl"还可以.

it seems "SortedContainerImpl" went wrong while "ContainerImpl" is fine.

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中插入SortedContainerImpl，以便重用get_size()和get(int)

I inheret SortedContainerImpl from ContainerImpl in order to reuse get_size() and get(int)

我不熟悉c ++，此问题的本质是什么，该如何解决?

I'm not familiar with c++, What's the nature of this problem and How can I fix it?

谢谢.

推荐答案

您的SortedContainerImpl类具有两个单独的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).

Your SortedContainerImpl class has two separate Container base classes. One is virtual (via the SortedContainer class) and the other is non-virtual (via the ContainerImpl class).

SortedContainerImpl具有Container::get_size()和Container::get(int)的具体实现，但是对于通过SortedContainer进入的虚拟基础，没有实现.

SortedContainerImpl has concrete implementations of Container::get_size() and Container::get(int) for the base that comes in from ContainerImpl, but not for the virtual base that comes in via SortedContainer.

解决此问题的一种方法是在SortedContainerImpl中给出具体的实现:

One way to fix the problem is to give concrete implementations in 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);}
};

另一种方法是将Container设为ContainerImpl的虚拟基类，因此SortedContainerImpl仅获得一个虚拟的基类Container:

Another way would be to make Container a virtual base class of ContainerImpl, so SortedContainerImpl would only get the one, virtual, base 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);}
};

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