Virtual functions - MQL4

MQL4 Virtual Functions

Overview

**Purpose:** Provides a mechanism to dynamically select an appropriate function-member at runtime among functions of basic and derived classes.

**Keyword:** virtual is used as a function specifier.

**Applicability:** Can be used to change declarations for function-members only. Structures cannot have virtual functions.

**Function Body:** A virtual function must have an executable body.

**Invocation:** When called, its semantic is the same as other functions.

**Overriding:** A virtual function may be overridden in a derived class.

**Selection:** The choice of function definition to call is made dynamically at runtime, based on the actual type of the object, not the type of the pointer.

**Default Behavior:** If no member of a derived type exists, the virtual function of the base class is used by default.

**Destructors:** Destructors are always virtual, regardless of explicit declaration.

**Caution:** Calling virtual methods from constructors and destructors is not recommended; the result is undefined.

Example Scenario (Tetris)

**Base Class:** CTetrisShape contains a virtual function Draw().

**Derived Classes:** Classes like CTetrisShape1 and CTetrisShape6 provide their own implementations of the Draw() function.

**Dynamic Dispatch:** In CTetrisField::NewShape(), objects of derived classes are created and assigned to a base class pointer (m_shape). Calling m_shape.Draw() invokes the specific Draw() implementation of the actual object's derived class at runtime.

Code Snippets

**Base Class CTetrisShape:**

class CTetrisShape {
protected:
  int m_type, m_xpos, m_ypos, m_xsize, m_ysize, m_prev_turn, m_turn, m_right_border;
public:
  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[]);
};

**Derived Class CTetrisShape1:**

class CTetrisShape1 : public CTetrisShape {
public:
  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);
      }
    }
  }
};

**Derived Class CTetrisShape6:**

class CTetrisShape6 : public CTetrisShape {
public:
  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);
    }
  }
};

**Object Creation and Call:**

void CTetrisField::NewShape() {
  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;
  }
  m_shape.Draw(); // Dynamic dispatch occurs here
}

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