Help a beginner

P

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(int, int);
};

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

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

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

Line::Line(Point initp1, Point initp2)
{
start_point=initp1;
end_point=initp2;
return;
}

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

Rolf Magnus

PhilB said:
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?
Click to expand...

Isn't it obvious from the error messages?
Thanks!
PhilB

myprog2.cpp: In method `Line::Line (Point, Point)':
myprog2.cpp:32: no matching function for call to `Point::point ()'
Click to expand...

You try to use the default constructor (i.e. `Point::point ()'), but
your Point class doesn't have one.
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(int, int);
};

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

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

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

Line::Line(Point initp1, Point initp2)
{
start_point=initp1;
end_point=initp2;
return;
}
Click to expand...

Since you have not specified an initializer for your Point objects, the
default constructor will be used to create them, but you don't have
one. Try this instead:

Line::Line(Point initp1, Point initp2)
: start_point(initp1),
end_point(initp2)
{
}

This will use the copy constructor (which is generated by the compiler
automatically) to create your two Point members.
 
L

lallous

PhilB said:
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(int, int);
};

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

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

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

Line::Line(Point initp1, Point initp2)
{
start_point=initp1;
end_point=initp2;
return;
}

main()
{
Point p1(10,20);
Point p2(20,40);
Line l1(p1,p2);
}
//------------------------
Click to expand...
Hello,

In the class as:
class Point
{
protected:
int x;
int y;
public:
Point::point(int, int); ^^^^remove the "Point::"
};
Click to expand...

Same for the other class.
 
K

Kevin Saff

PhilB said:
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 ()'
Click to expand...

The compiler is trying to tell you there is no default constructor for
Point.
myprog2.cpp:12: candidates are: Point::point (int, int)
Click to expand...

The compiler says it only sees this constructor, to create a point from two
ints.
[snip]
class Point
{
protected:
int x;
int y;
Click to expand...

Protected data is usually a mistake; it tends to introduce confusing
dependencies between a base and its derived classes.

Also, if you intend to use Point as a base class, you should declare a
virtual destructor, (virtual ~Point()) or else derived objects may not be
properly destroyed.
public:
Point::point(int, int);
Click to expand...

(Note: the "Point::" is unnecessary inside the class definition, it is only
needed when you define the function outside of the class definition.)

This declares a constructor from two ints, which hides the
compiler-generated default constructor. If you want the default constructor
as well, you would need to define one:

Point () {}

This is not really necessary though, there's a better solution to your
immediate problem.
};

Point::point(int initx, int inity)
{
x=initx;
y=inity;
return;
}
Click to expand...

You don't need to explicitly return here. This function would usually be
written (for reasons explained later):

Point (int initx, int inity) : x (initx), y (inity) {}

This uses initialization syntax to assign the values of x and y. Your
syntax instead, will first create the two ints and then assign new values
for them. This isn't too big a deal for ints, but see below.

(Also, many C++ coders prefer to mark member data in some way to
differentiate between member data and function arguments. Popular ways
include m_x ("m_" for member) and x_.)
class Line:public Point
{
public:
Point start_point;
Point end_point;
Click to expand...

As someone trying to break old C habits it will probably be better for you
to only use private data until you learn the exceptions to this rule.
Instead, make functions that act on Lines and Points members of those
classes.
public:
Line::Line(Point, Point);
};

Line::Line(Point initp1, Point initp2)
{
start_point=initp1;
end_point=initp2;
return;
}
Click to expand...

OK, here's the compiler's problem; not just a style one :). This tells the
compiler to first default construct two points, and then assign the new
point values to them. However, the Point's constructor has hidden the
default constructor, so this won't work. Instead, you should write:

Line (Point initp1, Point initp2) : start_point (initp1), end_point
(initp2) {}

This will use the compiler-generated Point copy constructor (which isn't
hidden by the user-declared constructor), instead. In general it is good
practice to use initialization syntax where possible, since
1) The compiler may be able to optimize it better.
2) It's a familiar idiom to C++ coders.
3) It removes the necessity of defining default constructors for objects.

As you get more used to OO programming, you will find that default objects
do not make sense for all kinds of classes, and often require
special-purpose code to handle the default-constructed case. I think
"Point" is a class where a default constructor may or may not be
appropriate, depending on your other requirements.

HTH
 
V

Vardhan Prabhakar N [C]

PhilB said:
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 &)
Click to expand...
You dont have a default constructor, so convert the existing one to one.

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

Point::point(int initx, int inity)
Click to expand...
make this
Point::point(int initx=0, int inity=0)
 
G

Gary Labowitz

PhilB said:
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(int, int);
};

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

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

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

Line::Line(Point initp1, Point initp2)
{
start_point=initp1;
end_point=initp2;
return;
}

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

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
};

class Linex
{
Pointx startPoint, endPoint; //points for line
public:
void show( );
Linex (Pointx start, Pointx end)
{
startPoint = start;
endPoint = end;
}

};

void Linex::show( )
{
cout << "Start=(" << startPoint.x << ", " << startPoint.y
<< ") : End=(" << endPoint.x << ", " << endPoint.y
<< ")" << endl;
}

int main( )
{
Pointx point1(10,20);
Pointx point2(20,30);
Linex myLine(point1, point2);
cout << "Line is ";
myLine.show( );
cout << endl;

return 0;
}

Note: I made the entire class Linex a friend because my compiler is
objecting to making single function a friend. Don't know why.
MingW32: Is this a known bug?
 
J

jeffc

lallous said:
In the class as:


Same for the other class.
Click to expand...


Actually, I believe that's fine. Anyway, that's not the main problem. The
main problem is that he's trying to create Points with the default
constructor, and there's no default constructor.
 
J

jeffc

Gary Labowitz said:
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).
Click to expand...

No, you missed the public.
 
J

jeffc

Marko Becirevic said:
not Point::point(int, int);
but just Point(int, int);

same in Line class.
Click to expand...

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.
 
M

Marko Becirevic

//------------------------
class Point
{
protected:
int x;
int y;
public:
Point::point(int, int);
};
Click to expand...

not Point::point(int, int);
but just Point(int, int);

same in Line class.
 
P

PhilB

Gary Labowitz said:
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
Click to expand...

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
 
S

Steven Green

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(int, int);
};

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

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

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

Line::Line(Point initp1, Point initp2)
{
start_point=initp1;
end_point=initp2;
return;
}

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

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(int initx, int inity)
{
x=initx;
y=inity;
return;
}

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

Line::Line(Point* initp1, Point* initp2)
{
start_point=initp1;
end_point=initp2;
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
 
P

PhilB

Steven Green said:
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(int initx, int inity)
{
x=initx;
y=inity;
return;
}

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

Line::Line(Point* initp1, Point* initp2)
{
start_point=initp1;
end_point=initp2;
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
Click to expand...
Hi Steve,
Thanks a lot for your crystal-clear example,
and thanks for sharing your expertise.
PhilB
 
M

Michael Kale

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

Line::Line(Point* initp1, Point* initp2)
{
start_point=initp1;
end_point=initp2;
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
Click to expand...

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
 

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