Having trouble with Classes

I went through the chapter of Classes and I though i had understood this pretty well, with class, private, public, constructors, destructors ... So I tried to do a program from an exercise that seemed simple, but It compiling a bunch of errors, if anyone could put me on the right track to understand this, it would be great.
the exercise is:

Define a class called Plot that has private members of length and width. Include a constructor and a public function that calculates the area and the length of the boundary of the field. Use public functions in a program that computes and displays the area and the length of the boundary of the plot where the length and width are 7 and 9 respectively. Hint: The length of the boundary is 2 * (l + w).

This is what I have done as far as the program goes:

Code:

#include <iostream>
#include <stdlib.h>

using namespace std;

class Plot
{
public:

	double setlength( double l );
	double setwidth( double w );

	double getlength();
	double getwidth();

	void area();
	void perimeter();
private:
	double length;
	double width;
};
Plot::Plot(double , double )
{
  length = l;
  width = w;
}
Plot::setlength()
{
length = ( l >= 0.0 && l <= 7.0 ) ? l = 0;
}
Plot::setwidth()
{
	width = ( w >= 0.0 && w <= 9.0 ) ? w = 0;
}
void Plot::area()
{
	return (l * w);
}
void Plot::perimeter()
{
	return ( 2 * ( l + w ));
}

int main()
{
Plot rect( 7,  9 );
cout << " The area of the rectangle is " << Plot.area() << endl;
cout << " The perimeter of the rectangle is " << Plot.perimeter() << endl;
	return 0;
}

These are the error codes I'm getting:

Code:

(27) : error C2511: 'Plot::Plot(double,double)' : overloaded member function not found in 'Plot'
(11) : see declaration of 'Plot'
(32) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
(32) : error C2511: 'int Plot::setlength(void)' : overloaded member function not found in 'Plot'
(11) : see declaration of 'Plot'
(36) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
(36) : error C2511: 'int Plot::setwidth(void)' : overloaded member function not found in 'Plot'
(11) : see declaration of 'Plot'
(41) : error C2065: 'l' : undeclared identifier
(41) : error C2065: 'w' : undeclared identifier
(50) : error C2661: 'Plot::Plot' : no overloaded function takes 2 arguments
(51) : warning C4832: token '.' is illegal after UDT 'Plot'
(11) : see declaration of 'Plot'
(51) : error C2275: 'Plot' : illegal use of this type as an expression
(11) : see declaration of 'Plot'
(52) : warning C4832: token '.' is illegal after UDT 'Plot'
(11) : see declaration of 'Plot'
(52) : error C2275: 'Plot' : illegal use of this type as an expression
(11) : see declaration of 'Plot'

If anyone can help me understand what I'm doing wrong I would really appreciate it.
thanks.

change your code to this:

Code:

#include <iostream>
#include <stdlib.h>

using namespace std;

class Plot
{
public:
               Plot(double, double); //ADDED LINE

                void setlength( double l ); //SHOULDNT RETURN DOUBLE
	void setwidth( double w );  // SAME THING HERE VOID

	double getlength();
	double getwidth();

	float area();  //RETURN VALUE CHANGED
                float perimeter();  //SAME THING
private:
	double length;
	double width;
};

Plot::Plot(double , double )
{
  length = l;
  width = w;
}

void Plot::setlength()  //ALWAYS PUT THE RETURN TYPE!!!
{
length = ( l >= 0.0 && l <= 7.0 ) ? l = 0;
}
void Plot::setwidth()  //RETURN TYPE AGAIN!
{
	width = ( w >= 0.0 && w <= 9.0 ) ? w = 0;
}
float Plot::area()  //YOUR RETURNING WHY WAS THIS VOID
{
	return (l * w);
}
float Plot::perimeter()  //SAME THING HERE
{
	return ( 2 * ( l + w ));
}

int main()
{
Plot rect = new Plot(7,9);  //INSTANTIATE PLOT
cout << " The area of the rectangle is " << rect.area() << endl;
cout << " The perimeter of the rectangle is " << rect.perimeter() << endl;  //YOU CANT CALL CLASS VARIABLES THAT DONT EXIST, USE THE VARIABLE
	return 0;
}

I think that's it, phew!

Durban thanks I've made the changes but I'm still getting a bunch of errors that I don't understand. If anyone can give a hand to figure this out it would be great.

Code:

#include <iostream>
#include <stdlib.h>

using namespace std;

class Plot
{
public:
               Plot(double, double); //ADDED LINE

                void setlength( double l ); //SHOULDNT RETURN DOUBLE
	void setwidth( double w );  // SAME THING HERE VOID

	double getlength();
	double getwidth();

	float area();  //RETURN VALUE CHANGED
                float perimeter();  //SAME THING
private:
	double length;
	double width;
};

Plot::Plot(double , double )
{
  length = l;
  width = w;
}

void Plot::setlength()  //ALWAYS PUT THE RETURN TYPE!!!
{
length = ( l >= 0.0 && l <= 7.0 ) ? l = 0;
}
void Plot::setwidth()  //RETURN TYPE AGAIN!
{
	width = ( w >= 0.0 && w <= 9.0 ) ? w = 0;
}
float Plot::area()  //YOUR RETURNING WHY WAS THIS VOID
{
	return (l * w);
}
float Plot::perimeter()  //SAME THING HERE
{
	return ( 2 * ( l + w ));
}

int main()
{
Plot rect = new Plot(7,9);  //INSTANTIATE PLOT
cout << " The area of the rectangle is " << Plot.area() << endl;
cout << " The perimeter of the rectangle is " << Plot.perimeter() << endl;  //YOU CANT CALL CLASS VARIABLES THAT DONT EXIST, USE THE VARIABLE
	return 0;
}

Here are the errors I get:

Code:

 (30) : error C2065: 'l' : undeclared identifier
(31) : error C2065: 'w' : undeclared identifier
 (35) : error C2511: 'void Plot::setlength(void)' : overloaded member function not found in 'Plot'
 (11) : see declaration of 'Plot'
 (39) : error C2511: 'void Plot::setwidth(void)' : overloaded member function not found in 'Plot'
 (11) : see declaration of 'Plot'
 (53) : error C2440: 'initializing' : cannot convert from 'Plot *' to 'Plot'
        No constructor could take the source type, or constructor overload resolution was ambiguous
 (54) : warning C4832: token '.' is illegal after UDT 'Plot'
 (11) : see declaration of 'Plot'
 (54) : error C2275: 'Plot' : illegal use of this type as an expression
 (11) : see declaration of 'Plot'
 (55) : warning C4832: token '.' is illegal after UDT 'Plot'
 (11) : see declaration of 'Plot'
 (55) : error C2275: 'Plot' : illegal use of this type as an expression
 (11) : see declaration of 'Plot'

Thanks.

Code:

void Plot::setlength( double l )
{
        length = ( l >= 0.0 && l <= 7.0 ) ? l = 0;
}
void Plot::setwidth( double w )
{
	width = ( w >= 0.0 && w <= 9.0 ) ? w = 0;
}
float Plot::area()  //you should change this to return double instead
{
	return (length * width);
}
float Plot::perimeter()  //you should change this to return double instead
{
	return ( 2 * ( length + width ));
}

It would work this way. But you should change the returntypes for area() and perimeter() to double.

Code:

int main()
{
   Plot rect = new Plot(7,9);  // you create a pointer to Plot here
   cout << " The area of the rectangle is " << rect->area() << endl;
   cout << " The perimeter of the rectangle is " << rect->perimeter() << endl;
   delete rect. // you have to free the memory
    // you could also use Plot this way 
   Plot orect(7,9);  // you create a Plot on the stack
   cout << " The area of the rectangle is " << orect.area() << endl;
   cout << " The perimeter of the rectangle is " << orect.perimeter() << endl;
   return 0;
}

Kurt