Class warfare

These classes are published classes, simply I am using them now, but cannot even get them to compile. So for example I get the following errors relating to member functions not being found. I have simplified the code such that one can see the point of failure. Does anyone know what is happening here to cause the failure.

Code:

Edge.cpp
========


#include "Point.h"
#include "Edge.h"


Edge::Edge(Point &_org, Point &_dest) :
 org(_org), dest(_dest)
{
}

/*Edge::Edge(void) :
  org(Point(0,0)), org(Point(1,0))
 {
 }
*/


Edge &Edge::rot(void)
{
  Point m; 
  m = 0.5 * (org + dest);
  Point v = dest - org;
  Point n(v.y, -v.x);
  org = m - 0.5 * n;
  dest = m + 0.5 * n;
  return *this;
}



Edge.h file
========



class Point;

class Edge {
 public:
   Point org;
   Point dest;
   Edge(Point &_org, Point &_dest);
   Edge(void);
   Edge &rot(void);
   //Edge &flip(void);
   //Point point(double);
   //int intersect(Edge&, double&);
   //int cross(Edge&, double&);
   //bool isVertical(void);
   //double slope(void);
   //double y(double);
 };


Point.cpp
========



#include "Point.h"

Point::Point(double _x, double _y):
x(_x), y(_y)
{
}

Point Point::operator+(Point &p)
{
  return Point(x+p.x, y+p.y);
}

Point Point::operator-(Point &p)
{
  return Point(x - p.x, y-p.y);
}

Point operator*(double s, Point &p)
{
  return Point(s *p.x, s *p.y);
}

double Point::operator[](int i)
{
  return (i ==0) ? x : y;
}

int Point::operator==(Point &p)
{
  return (x == p.x) && (y == p.y);
}

int Point::operator!=(Point &p)
{
  return !(*this == p);
}

int Point::operator<(Point &p)
{
  return ((x < p.x) || ((x == p.x) && (y < p.y)));
}

int Point::operator>(Point &p)
{
  return ((x > p.x) || ((x == p.x) && (y > p.y)));
}


/*int main(void)
{

return (0);
}*/



Point.h
========

class Point{
public:
  double x;
  double y;
  Point(double _x = 0.0, double _y = 0.0);
  Point operator+(Point&);
  Point operator-(Point&);
  friend Point operator*(double, Point&);
  double operator[](int);
  int operator==(Point&);
  int operator!=(Point&);
  int operator<(Point&);
  int operator>(Point&);
  int classify(Point&, Point&);
  //int classify(Edge&);
  double polarAngle(void);
  double length(void);
  //double distance(Edge&);
};

Error on compilation.
============

g++ -c Edge.cpp -o Edge.o
Edge.cpp: In member function ‘Edge& Edge::rot()’:
Edge.cpp:20: error: no match for ‘operator*’ in ‘5.0e-1 * ((Edge*)this)->Edge:rg. Point:perator+(((Point&)(&((Edge*)this)->Edge::dest)))’
Point.h:10: note: candidates are: Point operator*(double, Point&)
Edge.cpp:23: error: no match for ‘operator-’ in ‘m - operator*(5.0e-1, ((Point&)(& n)))’
Point.h:9: note: candidates are: Point Point:perator-(Point&)
Edge.cpp:24: error: no match for ‘operator+’ in ‘m + operator*(5.0e-1, ((Point&)(& n)))’
Point.h:8: note: candidates are: Point Point:perator+(Point&)

Code:

The Makefile is as follows:


all: Poly
	
Poly:    Point.o Edge.o Polygons.o 

	g++  -o Poly Point.o Edge.o Polygons.o

Point.o: Point.cpp
	g++    -c Point.cpp -o Point.o	

Edge.o: Edge.cpp
	g++    -c Edge.cpp -o Edge.o	
	
Polygons.o: Polygons.cpp
	g++    -c Polygons.cpp -o Polygons.o

clean:
	find . -name '*.o' | xargs rm -f ;

Thanks

Your operator functions should take a const reference (or be called by value), otherwise you can't pass temporary values into the operator. So for example:

Code:

  Point operator+(const Point&);
  Point operator-(const Point&);
  friend Point operator*(double, const Point&);

--
Mats

>> m = 0.5 * (org + dest);

The result of (org + dest) is a temporary object. Some compilers may allow passing it to a function that takes a non-const reference as an extension (or a bug), and that is probably why the classes were published.

Originally Posted by Daved

>> m = 0.5 * (org + dest);

The result of (org + dest) is a temporary object. Some compilers may allow passing it to a function that takes a non-const reference as an extension (or a bug), and that is probably why the classes were published.

It indeed works on VC++ 6.0...

So, as a conclusion, the published code is either not tested with a modern compiler, or written a while ago. Use with new compiler will require adding const.

--
Mats

>> Use with new compiler will require adding const.

It actually works with VC++ 2005 as well. It's probably more accurate to say that if you want it to be standards conforming (and therefore more portable) then it requires the const.

Also, to the OP, a common approach is to make operator+= a member, and operator+ a friend that is implemented in terms of +=:

Code:

Foo & operator+=(const Foo & rhs) { ... ; return *this; }
friend Foo operator+(const Foo & lhs, const Foo & rhs)
{
    Foo result = lhs;
    result += rhs;

    return result;
}

don't class members that are forwardly-declared types have to be pointers?

Originally Posted by m37h0d

don't class members that are forwardly-declared types have to be pointers?

Sort of. As long as the compiler doesn't need to know the size or content, it's fine to forward declare classes. This means that you can, realistically, use forward declared classes as either pointers or references, but not much else.

Edit: Oh, and I think forward declared classes can be used as parameters to function prototypes, even if they are not reference/pointer paramters.

--
Mats

>> don't class members that are forwardly-declared types have to be pointers?
In this case it probably only "works" because Point.h is #include'd before Edge.h which means the forward declaration is ignored.

You are right that Edge.h should #include "Point.h" here, since Edge has Point members and the compiler needs to know how big the Point is to know the size of Edge. If the #include order changes the build would break.

Also, I don't know if they were omitted for brevity, but I noticed there are no include file guards in the headers posted.

Great, thats the ticket!

Thanks for that, now for my next question...got the following class:

Code:

class Vertex : public Node, public Point {
public:
  Vertex(double x, double y);
  Vertex(const Point&);
  Vertex *cw(void);
  Vertex *ccw(void);
  Vertex *neighbor(int rotation);
  Point point(void);
  Vertex *insert(const Vertex*);
  Vertex *remove(void);
  void splice(const Vertex*);
  Vertex *split(const Vertex*);
  friend class Polygon;
};

However I am getting the following error:

Vertex.h:3: error: expected class-name before ‘,’ token

Which is referring to the line "class Vertex : public Node, public Point"

Now all the classes have hopefully been defined correctly....

Any idea why this is failing?? Again it should be standard code, but does not compile...

Thanks

Post the smallest and simplest program that demonstrates the problem.

Originally Posted by disruptivetech

Great, thats the ticket!

Thanks for that, now for my next question...got the following class:

Code:

class Vertex : public Node, public Point {
public:
  Vertex(double x, double y);
  Vertex(const Point&);
  Vertex *cw(void);
  Vertex *ccw(void);
  Vertex *neighbor(int rotation);
  Point point(void);
  Vertex *insert(const Vertex*);
  Vertex *remove(void);
  void splice(const Vertex*);
  Vertex *split(const Vertex*);
  friend class Polygon;
};

However I am getting the following error:

Vertex.h:3: error: expected class-name before ‘,’ token

Which is referring to the line "class Vertex : public Node, public Point"

Now all the classes have hopefully been defined correctly....

Any idea why this is failing?? Again it should be standard code, but does not compile...

Thanks

You need to make sure that Node and point are declared before you declare Vertex. This probably means that you need to include Point.h and Node.h at the top of Vertex.h.