Help a beginner

[PhilB]

Hello experts,
I am a complete beginner in C++ (although I know C).
I am trying to compile the code below, and I get
the following error.
Can anyone explain to me my mistake?
Thanks!
PhilB

myprog2.cpp: In method `Line::Line (Point, Point)':
myprog2.cpp:32: no matching function for call to `Point::Point ()'
myprog2.cpp:12: candidates are: Point::Point (int, int)
myprog2.cpp:9: Point::Point (const Point &)
myprog2.cpp:32: no matching function for call to `Point::Point ()'
myprog2.cpp:12: candidates are: Point::Point (int, int)
myprog2.cpp:9: Point::Point (const Point &)
myprog2.cpp:32: no matching function for call to `Point::Point ()'
myprog2.cpp:12: candidates are: Point::Point (int, int)
myprog2.cpp:9: Point::Point (const Point &)

//------------------------
class Point
{
protected:
int x;
int y;
public:
Point::Point(in t, int);
};

Point::Point(in t initx, int inity)
{
x=initx;
y=inity;
return;
}

class Line:public Point
{
public:
Point start_point;
Point end_point;

public:
Line::Line(Poin t, Point);
};

Line::Line(Poin t initp1, Point initp2)
{
start_point=ini tp1;
end_point=initp 2;
return;
}

main()
{
Point p1(10,20);
Point p2(20,40);
Line l1(p1,p2);
}
//------------------------

[jeffc]

"Gary Labowitz" <gl*******@comc ast.net> wrote in message
news:K9******** ************@co mcast.com...

Please note it one of my top ten that a number of you objected to having on the list.

To poster: Your Point constructor is protected, which means it can only be
called by code of the Point class or one of its subclasses (Line).

No, you missed the public.

[jeffc]

"Marko Becirevic" <ma************ *******@zg.tel. hr> wrote in message
news:br******** **@ls219.htnet. hr...

//------------------------
class Point
{
protected:
int x;
int y;
public:
Point::Point(in t, int);
};

not Point::Point(in t, int);
but just Point(int, int);

same in Line class.

That is not the problem. The problem is that he has Points in his Line.
There is no way to construct those Points when he goes to create a Line in
his main program.

[PhilB]

"Gary Labowitz" <gl*******@comc ast.net> wrote in message news:<K9******* *************@c omcast.com>...

Please note it one of my top ten that a number of you objected to having on
the list.

To poster: Your Point constructor is protected, which means it can only be
called by code of the Point class or one of its subclasses (Line). You are
calling it from main.
When you create a Line object, C++ will automatically construct the Point
inherited portion by calling a constructor for a Point using a default
constructor. And there isn't one.
In addition, I'd say your design is flawed: A Line is not a kind of Point so
you should not be using an is-a design. Lines have Points which is a has-a
design. This is one way it could look:

#include <iostream>

using namespace std;

class Linex;
class Pointx
{
friend class Linex;
int x, y; //coordinates
public:
Pointx (int x, int y):x(x), y(y){};
Pointx ( ){}; //This is the missing constructor

Hello Gary
Thank you for your reply; I just added the line
Pointx ( ){};
and it compiles with no error!
Thanks to all for sharing your expertise.
PhilB

[Steven Green]

On Tue, 16 Dec 2003 05:51:02 -0800, PhilB wrote:

Hello experts,
I am a complete beginner in C++ (although I know C).
I am trying to compile the code below, and I get
the following error.
Can anyone explain to me my mistake?
Thanks!
PhilB

myprog2.cpp: In method `Line::Line (Point, Point)':
myprog2.cpp:32: no matching function for call to `Point::Point ()'
myprog2.cpp:12: candidates are: Point::Point (int, int)
myprog2.cpp:9: Point::Point (const Point &)
myprog2.cpp:32: no matching function for call to `Point::Point ()'
myprog2.cpp:12: candidates are: Point::Point (int, int)
myprog2.cpp:9: Point::Point (const Point &)
myprog2.cpp:32: no matching function for call to `Point::Point ()'
myprog2.cpp:12: candidates are: Point::Point (int, int)
myprog2.cpp:9: Point::Point (const Point &)

//------------------------
class Point
{
protected:
int x;
int y;
public:
Point::Point(in t, int);
};

Point::Point(in t initx, int inity)
{
x=initx;
y=inity;
return;
}

class Line:public Point
{
public:
Point start_point;
Point end_point;

public:
Line::Line(Poin t, Point);
};

Line::Line(Poin t initp1, Point initp2)
{
start_point=ini tp1;
end_point=initp 2;
return;
}

main()
{
Point p1(10,20);
Point p2(20,40);
Line l1(p1,p2);
}
//------------------------

The answer has been covered over and over, here so I will only include
my alternate implementation that doesn't require definition of the
default constructor

class Point
{
protected:
int x;
int y;
public:
Point(int, int);
};

Point::Point(in t initx, int inity)
{
x=initx;
y=inity;
return;
}

class Line
{
public:
Point* start_point;
Point* end_point;
public:
Line(Point*, Point*);
};

Line::Line(Poin t* initp1, Point* initp2)
{
start_point=ini tp1;
end_point=initp 2;
return;
}

main()
{
Point p1(10,20);
Point p2(20,40);
Line l1(&p1,&p2);
}

I removed the line is-a point relationship which makes little sence.
Additionally, I used pointers to avoid the calling of the default
constructor.

--Steve

[PhilB]

Steven Green <st************ @verizon.net> wrote in message news:<pa******* *************** ******@verizon. net>...

The answer has been covered over and over, here so I will only include
my alternate implementation that doesn't require definition of the
default constructor

class Point
{
protected:
int x;
int y;
public:
Point(int, int);
};

Point::Point(in t initx, int inity)
{
x=initx;
y=inity;
return;
}

class Line
{
public:
Point* start_point;
Point* end_point;
public:
Line(Point*, Point*);
};

Line::Line(Poin t* initp1, Point* initp2)
{
start_point=ini tp1;
end_point=initp 2;
return;
}

main()
{
Point p1(10,20);
Point p2(20,40);
Line l1(&p1,&p2);
}

I removed the line is-a point relationship which makes little sence.
Additionally, I used pointers to avoid the calling of the default
constructor.

--Steve

Hi Steve,
Thanks a lot for your crystal-clear example,
and thanks for sharing your expertise.
PhilB

[Michael Kale]

Steven Green <st************ @verizon.net> wrote in message news:<pa******* *************** ******@verizon. net>...

class Line
{
public:
Point* start_point;
Point* end_point;
public:
Line(Point*, Point*);
};

Line::Line(Poin t* initp1, Point* initp2)
{
start_point=ini tp1;
end_point=initp 2;
return;
}

main()
{
Point p1(10,20);
Point p2(20,40);
Line l1(&p1,&p2);
}

I removed the line is-a point relationship which makes little sence.
Additionally, I used pointers to avoid the calling of the default
constructor.

--Steve

Now, though, Line doesn't own its Points anymore ... in this example,
if you pass l1 as a return value or do anything that extends its
lifetime beyond p1 and p2, you'll have problems. Unless Points are
really expensive to create and/or copy, this may not be the best way
to do it.

My two cents at least,
Michael