CODEWITHSUNDEEP
c++polymorphismabstract-classpure-virtual
user1665569
user1665569

Reputation: 191

pure virtual functions and inheritance

I'm having problems with polymorphism and pure virtual functions. My main class

#include<memory>

class Shape
{
  public:
  Gdiplus::Point start; 
  Gdiplus::Point end;

  std::shared_ptr<Gdiplus::Pen> m_pen;

  virtual void Draw(Gdiplus::Graphics & m_GraphicsImage) = 0;


  void setPen(std::shared_ptr<Gdiplus::Pen> pen2);


  void setStart(int xPos, int yPos);

  void setEnd(int xCor, int yCor);

};

Then i have this class that derives from Shape. Line.h

 #pragma once


 #include<memory>

class Line: public Shape
{
public:
  void Draw(Gdiplus::Graphics & m_GraphicsImage);
}

This is my line.cpp. 

#include "stdafx.h"
#include "Line.h"
#include "ShapeMaker.h"


void Line::Draw(Gdiplus::Graphics & m_GraphicsImage)
{

  m_GraphicsImage.DrawLine(m_pen.get(),start.X,start.Y,end.X,end.Y);
}

On my main i declare a shared pointer of type Shape for polymorphism reasons 

 std::shared_ptr<Shape> m_shape;

and then try and call functions for the Line.cpp but it doesn't work, 

LRESULT CDrawView::OnLButtonDown(UINT uMsg, WPARAM wParam, LPARAM lParam, BOOL& bHandled)

{
int xPos= GET_X_LPARAM(lParam);
int yPos = GET_Y_LPARAM(lParam);
start.X = xPos;
start.Y = yPos;


//Line line;
auto line = std::make_shared<Shape> (m_shape);
std::shared_ptr<Gdiplus::Pen> myPen(pen.Clone());
line->setPen(myPen);
line->setStart(xPos,yPos);
return 0;
}

LRESULT CDrawView::OnLButtonUp(UINT uMsg, WPARAM wParam, LPARAM lParam, BOOL& bHandled)
{
int xPos= GET_X_LPARAM(lParam);
int yPos = GET_Y_LPARAM(lParam);
end.X = xPos;
end.Y = yPos;

//Pen pen(Color(0, 0, 255));
//Line line;
auto line = std::make_shared<Shape> (m_shape);
line->setEnd(xPos,yPos);
line->Draw(m_GraphicsImage);
m_shape.reset();


RedrawWindow();


return 0;

} Now I'm getting drawview.cpp(54): error C2371: 'line' : redefinition; different basic types 1> \draw\drawview.cpp(53) : see declaration of 'line'

Upvotes: 0

Views: 273

Answers (3)

Uttam
Uttam

Reputation: 83

The error is for instantiating Shape, which is abstract class.

The above syntax 'auto line = std::make_shared (m_shape);' is the culprit.

In this statement, you are trying to instantiate Shape class.

Refer to this http://www.cplusplus.com/reference/memory/dynamic_pointer_cast/

for correct usage of casting between shared_ptr

Upvotes: 0

T. Kiley
T. Kiley

Reputation: 2802

I believe the problem is your constructor 

std::make_shared<Shape> (m_shape);

Which is essentially new Shape (an abstract class). Instead you should use 

std::make_shared<Line> (m_shape);

Upvotes: 2

NPE
NPE

Reputation: 500923

The line

void Draw(Gdiplus::Graphics & m_GraphicsImage)

should read

void Line::Draw(Gdiplus::Graphics & m_GraphicsImage)

Without the Line:: you are defining a free function called Draw() that happens to have the same signature as Line::Draw() but is otherwise unrelated to it.

Upvotes: 0

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