多重继承中的纯虚函数“妙用”

在多重继承中，处于中间层的类可能有一些用，但又不会实例化。在最终实现类中需要调用中间类的方法，同时必须保证在最终类中也实现该方法。如何做到？

说的不清晰，举OceanBase的例子吧：

ObPhyOperator是一个纯虚基类，它要求每一个子类都实现reset()方法：

class ObPhyOperator
{
  public:
  virtual void reset() = 0;
};

ObSingleChildPhyOperator从ObPhyOperator继承过来，是所有“Single child Operator"的父类，它有一个成员”child_op_"。一些情况下为了能够重用operator，在set_child之前需要调用reset重置child_op_状态为NULL：

class ObSingleChildPhyOperator : public ObPhyOperator
{
  public:
    int set_child(ObPhyOperator *op);
    virtual void reset() = 0;  // 注意这个 "=0" !!!!

  private:
    ObPhyOperator *child_op_;
};

int ObSingleChildPhyOperator::set_child(ObPhyOperator *op)
{
  if (NULL != child_op_)
  {
     TBSYS_LOG(WARN, "operator already inited!");
     return OB_ERROR;
  }
  child_op_ = op;
  return OB_SUCCESS;
}

void ObSingleChildPhyOperator::reset()
{
  child_op_ = NULL;
}

ObFinalOperator继承自ObSingleChildPhyOperator，我们要求它一定有一个reset()方法，用于管理operator的内部状态，如内存、基本参数等。如何做到“一定”呢？上面ObSingleChildPhyOperator的

virtual void reset() = 0;  // 注意这个 "=0" !!!!

保证了这一点。如果没有“=0”，即使ObFinalOperator不实现reset()方法，语法上也不会报错，给代码维护带来困难。

下面通过一个实验来一步步学习这个技巧。

下面的代码能够编译通过吗？

#include <iostream>     /* cin, cout */
using namespace std;

class ObPhyOperator
{
  public:
  virtual void reset() = 0;
};

class ObSingleChildPhyOperator : public ObPhyOperator
{
  public:
  virtual void reset() = 0;
};

class ObFinalOperator: public ObSingleChildPhyOperator
{
  public:
};

int main()
{
  ObPhyOperator *phy = new ObFinalOperator();
  return 0;
}

报错如下：

virtual_structure.cpp: In function ‘int main()’:
virtual_structure.cpp:54: error: cannot allocate an object of abstract type ‘ObFinalOperator’
virtual_structure.cpp:35: note:   because the following virtual functions are pure within ‘ObFinalOperator’:
virtual_structure.cpp:31: note:         virtual void ObSingleChildPhyOperator::reset()

OK，为ObSingleChildPhyOperator加上reset实现呢？

#include <iostream>     /* cin, cout */
using namespace std;

/// Base virtual class ObPhyOperator
class ObPhyOperator
{
  public:
  virtual void reset() = 0;
};

/// Middle virtual class ObSingleChildPhyOperator
class ObSingleChildPhyOperator : public ObPhyOperator
{
  public:
  virtual void reset() = 0;
};

void ObSingleChildPhyOperator::reset()
{
  cout << "single" << endl;
}

/// ObFinalOperator
class ObFinalOperator: public ObSingleChildPhyOperator
{
  public:
};

int main()
{
  ObPhyOperator *phy = new ObFinalOperator();
  return 0;
}

报错依旧：

virtual_structure.cpp: In function ‘int main()’:
virtual_structure.cpp:52: error: cannot allocate an object of abstract type ‘ObFinalOperator’
virtual_structure.cpp:35: note:   because the following virtual functions are pure within ‘ObFinalOperator’:
virtual_structure.cpp:45: note:         virtual void ObSingleChildPhyOperator::reset()

看来，必须在ObFinalOperator中实现reset方法：

#include <iostream>     /* cin, cout */
using namespace std;

/// Base virtual class ObPhyOperator
class ObPhyOperator
{
  public:
  virtual void reset() = 0;
};

/// Middle virtual class ObSingleChildPhyOperator
class ObSingleChildPhyOperator : public ObPhyOperator
{
  public:
  virtual void reset() = 0;
};

void ObSingleChildPhyOperator::reset()
{
  cout << "single" << endl;
}

/// ObFinalOperator
class ObFinalOperator: public ObSingleChildPhyOperator
{
  public:
    virtual void reset();
};


void ObFinalOperator::reset()
{
  cout << "final" << endl;
  // ObSingleChildPhyOperator::reset();
}


int main()
{
  ObPhyOperator *phy = new ObFinalOperator();
  return 0;
}

编译通过，bingo~