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Aug 14th, 2008

My class is not a modifiable lvalue?

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I have a very simple class

C++ Syntax (Toggle Plain Text)
class Point
{
 
public:
Point();
Point(double x, double y);
  double x_;
double y_;
}
class Point
{

public:
Point();
Point(double x, double y);
  double x_;
double y_;
}

the problem is, I can't assign my class to another instance, ie:

C++ Syntax (Toggle Plain Text)
Point A(2,4);
Point B();
B = A;
Point A(2,4);
Point B();
B = A;

It says the lvalue must be modifiable. Why is it not?

Thanks,

Dave

daviddoria

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Aug 14th, 2008

Re: My class is not a modifiable lvalue?

Try this.

C++ Syntax (Toggle Plain Text)
class Point {
public:
 
   double x;
   double y;
 
   Point() {
      x = 0;
      y = 0;
   }
 
   Point(double _x, double _y);
      x = _x;
      y = _y;
   }
 
   Point &operator =(const Point &p) {
      x = p.x;
      y = p.y;
      return *this;
   }
};
class Point {
public:

   double x;
   double y;

   Point() {
      x = 0;
      y = 0;
   }

   Point(double _x, double _y);
      x = _x;
      y = _y;
   }

   Point &operator =(const Point &p) {
      x = p.x;
      y = p.y;
      return *this;
   }
};

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Aug 14th, 2008

Re: My class is not a modifiable lvalue?

Point B(); is the declaration of a function called 'B' which takes no arguments and returns a Point.
modify to Point B ; and you have the definition of a variable called 'B' which is of type Point (initialized via the default constructor).

Last edited by vijayan121; Aug 14th, 2008 at 5:08 pm.

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Aug 14th, 2008

Re: My class is not a modifiable lvalue?

ahhh that was the key ! Needed to use B instead of B()

Thanks!

daviddoria

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Aug 15th, 2008

Re: My class is not a modifiable lvalue?

Click to Expand / Collapse Quote originally posted by vijayan121 ...

Point B(); is the declaration of a function called 'B' which takes no arguments and returns a Point.
modify to Point B ; and you have the definition of a variable called 'B' which is of type Point (initialized via the default constructor).

Are you completely sure about this?
As far as i know if you have declared the Point class' constructor Point() and then you state Point B(); then B is an instance of Point instantiated via the default constructor.
I also thought that that if you declare the copy constructor the assignation B = A should propery work

C++ Syntax (Toggle Plain Text)
 
class Point{
private:
  int x_;
  int y_;
public:
  Point(){/* whatever*/}
  Point(int x,int y){/* whatever*/}
  Point(const Point& p){ 
     x_ = p.x_;
     y_ = p.y_;
  }
};
class Point{
private:
  int x_;
  int y_;
public:
  Point(){/* whatever*/}
  Point(int x,int y){/* whatever*/}
  Point(const Point& p){ 
     x_ = p.x_;
     y_ = p.y_;
  }
};

I consider myself a begginer and I learn as I go. Then this you posted doesn't seems to be correct for me. Pleas correct me.
Kind regards

Narf!!!

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Aug 15th, 2008

Re: My class is not a modifiable lvalue?

Click to Expand / Collapse Quote originally posted by Narf!!! ...

Are you completely sure about this?
As far as i know if you have declared the Point class' constructor Point() and then you state Point B(); then B is an instance of Point instantiated via the default constructor.
I also thought that that if you declare the copy constructor the assignation B = A should propery work
C++ Syntax (Toggle Plain Text)
 
class Point{
private:
  int x_;
  int y_;
public:
  Point(){/* whatever*/}
  Point(int x,int y){/* whatever*/}
  Point(const Point& p){ 
     x_ = p.x_;
     y_ = p.y_;
  }
};
class Point{ private: int x_; int y_; public: Point(){/* whatever*/} Point(int x,int y){/* whatever*/} Point(const Point& p){ x_ = p.x_; y_ = p.y_; } };
I consider myself a begginer and I learn as I go. Then this you posted doesn't seems to be correct for me. Pleas correct me.
Kind regards

Yes, vijayan121 is correct. Try this program below and see what happens:

C++ Syntax (Toggle Plain Text)
#include <iostream>
using namespace std;
 
class Point {
public:
 
   double x;
   double y;
 
   Point() 
   {
      cout << "I'm in the constructor\n";
      x = 0;
      y = 0;
   }
 
   Point(double _x, double _y)
   {
      x = _x;
      y = _y;
   }
 
   Point &operator =(const Point &p) 
   {
      x = p.x;
      y = p.y;
      return *this;
   }
};
 
 
 
int main ()
{
    Point B();  // nothing displayed
    Point C; // constructor call
    B();     // function call
    Point D (); // nothing displayed
    return 0;
}
 
 
Point B ()
{
      cout << "I'm in the function\n";
      Point* A = new Point ();  // constructor call
      return *A;
}
#include <iostream>
using namespace std;

class Point {
public:

   double x;
   double y;

   Point() 
   {
      cout << "I'm in the constructor\n";
      x = 0;
      y = 0;
   }

   Point(double _x, double _y)
   {
      x = _x;
      y = _y;
   }

   Point &operator =(const Point &p) 
   {
      x = p.x;
      y = p.y;
      return *this;
   }
};



int main ()
{
    Point B();  // nothing displayed
    Point C; // constructor call
    B();     // function call
    Point D (); // nothing displayed
    return 0;
}


Point B ()
{
      cout << "I'm in the function\n";
      Point* A = new Point ();  // constructor call
      return *A;
}

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Aug 15th, 2008

Re: My class is not a modifiable lvalue?

Thanks a lot!!!

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