Virtual Functions

The virtual keyword is the function specifier, which provides a mechanism to select dynamically at runtime an appropriate function-member among the functions of basic and derived classes. Structures cannot have virtual functions. It can be used to change the declarations for function-members only.

The virtual function, like an ordinary function, must have an executable body. When called, its semantic is the same as that of other functions.

A virtual function may be overridden in a derived class. The choice of what function definition should be called for a virtual function is made dynamically (at runtime). A typical case is when a base class contains a virtual function, and derived classes have their own versions of this function.

The pointer to the base class can indicate either a base class object or the object of a derived class. The choice of the member-function to call will be performed at runtime and will depend on the type of the object, not the type of the pointer. If there is no member of a derived type, the virtual function of the base class is used by default.

Destructors are always virtual, regardless of whether they are declared with the virtual keyword or not.

Attention: it is not recommended to call virtual methods from constructors and desctructors, because the result is undefined in this case.

Let's consider the use of virtual functions on the example of Tetris.mq5. The base class CTetrisShape with the virtual function Draw is defined in the included file TetisShape.mqh.

//+------------------------------------------------------------------+
class CTetrisShape
  {
protected:
   int               m_type;
   int               m_xpos;
   int               m_ypos;
   int               m_xsize;
   int               m_ysize;
   int               m_prev_turn;
   int               m_turn;
   int               m_right_border;
public:
   void              CTetrisShape();
   void              SetRightBorder(int border) { m_right_border=border; }
   void              SetYPos(int ypos)          { m_ypos=ypos;           }
   void              SetXPos(int xpos)          { m_xpos=xpos;           }
   int               GetYPos()                  { return(m_ypos);        }
   int               GetXPos()                  { return(m_xpos);        }
   int               GetYSize()                 { return(m_ysize);       }
   int               GetXSize()                 { return(m_xsize);       }
   int               GetType()                  { return(m_type);        }
   void              Left()                     { m_xpos-=SHAPE_SIZE;    }
   void              Right()                    { m_xpos+=SHAPE_SIZE;    }
   void              Rotate()                   { m_prev_turn=m_turn; if(++m_turn>3) m_turn=0; }
   virtual void      Draw()                     { return;                }
   virtual bool      CheckDown(int& pad_array[]);
   virtual bool      CheckLeft(int& side_row[]);
   virtual bool      CheckRight(int& side_row[]);
  };

Further, for each derived class, this function is implemented in accordance with characteristics of a descendant class. For example, the first shape CTetrisShape1 has its own implementation of the Draw() function:

class CTetrisShape1 : public CTetrisShape
  {
public:
   //--- shape drawing
   virtual void      Draw()
     {
      int    i;
      string name;
      //---
      if(m_turn==0 || m_turn==2)
        {
         //--- horizontal
         for(i=0; i<4; i++)
           {
            name=SHAPE_NAME+(string)i;
            ObjectSetInteger(0,name,OBJPROP_XDISTANCE,m_xpos+i*SHAPE_SIZE);
            ObjectSetInteger(0,name,OBJPROP_YDISTANCE,m_ypos);
           }
        }
      else
        {
         //--- vertical
         for(i=0; i<4; i++)
           {
            name=SHAPE_NAME+(string)i;
            ObjectSetInteger(0,name,OBJPROP_XDISTANCE,m_xpos);
            ObjectSetInteger(0,name,OBJPROP_YDISTANCE,m_ypos+i*SHAPE_SIZE);
           }
        }
     }
  }

The Square shape is described by class CTetrisShape6 and has its own implementation of the Draw() method:

class CTetrisShape6 : public CTetrisShape
  {
public:
   //--- Shape drawing
   virtual void      Draw()
     {
      int    i;
      string name;
      //---
      for(i=0; i<2; i++)
        {
         name=SHAPE_NAME+(string)i;
         ObjectSetInteger(0,name,OBJPROP_XDISTANCE,m_xpos+i*SHAPE_SIZE);
         ObjectSetInteger(0,name,OBJPROP_YDISTANCE,m_ypos);
        }
      for(i=2; i<4; i++)
        {
         name=SHAPE_NAME+(string)i;
         ObjectSetInteger(0,name,OBJPROP_XDISTANCE,m_xpos+(i-2)*SHAPE_SIZE);
         ObjectSetInteger(0,name,OBJPROP_YDISTANCE,m_ypos+SHAPE_SIZE);
        }
     }
  };

Depending on the class, to which the created object belongs, it calls the virtual function of this or that derived class.

void CTetrisField::NewShape()
  {
//--- creating one of the 7 possible shapes randomly
   int nshape=rand()%7;
   switch(nshape)
     {
      case 0: m_shape=new CTetrisShape1; break;
      case 1: m_shape=new CTetrisShape2; break;
      case 2: m_shape=new CTetrisShape3; break;
      case 3: m_shape=new CTetrisShape4; break;
      case 4: m_shape=new CTetrisShape5; break;
      case 5: m_shape=new CTetrisShape6; break;
      case 6: m_shape=new CTetrisShape7; break;
     }
//--- draw
   m_shape.Draw();
//---
  }

OverloadStatic Members of a Class