Using header file in C++ program - c++

Question

I have some question for coding C++ program with header file.
This is my header.h file :
#include <iostream>
using namespace std;
class Rectangle
{
public:
Rectangle(double width, double length);
double get_perimeter();
double get_area();
void resize(double factor);
private:
double width;
double length;
double area;
double factor;
};
And this is my Question1.cpp file which store all the methods:
#include <iostream>
using namespace std;
class Rectangle
{
public:
Rectangle(double width, double length)
{
width = width; //I have no idea how to use this.something as its in Java
length = length; //Problem probably occurs at here
}
double Rectangle::get_perimeter()
{
return ((width * 2) + (length * 2)) ;
}
double Rectangle::get_area()
{
return (width * length);
}
void Rectangle::resize(double factor)
{
width *= factor;
length *= factor;
}
private:
double width;
double length;
double area;
double factor;
};
And lastly, here is my main method.cpp :
#include <iostream>
#include "header.h";
using namespace std;
int main()
{
Rectangle rectangle1(2.5,7.0);
cout << rectangle1.get_perimeter();
cout << rectangle1.get_area();
system("PAUSE");
return 0;
}
However, when I try to run the program, the system told me that there was build errors and unresolved externals which I have no idea why is it so. Could somebody please help me fix it?
Thanks in advance.

Answer 1

Your implementations should not look like
class Rectangle
{
public:
Rectangle(double width, double length) { .... }
but like
Rectangle::Rectangle(double width, double length) : width(width), length(length) {}
You need to include header.h in the implementation .cpp file and in any file that needs the definition of the Rectangle class. You also need include guards in your headers. And do not put using namespace std in a header. In fact, don't put it anywhere.

Answer 2

Change .h to ->
#include <iostream>
using namespace std;
class Rectangle
{
public:
Rectangle(double width, double length);
double get_perimeter();
double get_area();
void resize(double factor);
private:
double width;
double length;
double area;
double factor;
};
Then .cpp to->
#include <iostream>
#include "header.h"
using namespace std;
Rectangle::Rectangle(double width, double length)
{
this->width = width; //I have no idea how to use this.something as its in Java
this->length = length; //Problem probably occurs at here
}
double Rectangle::get_perimeter()
{
return ((this->width * 2) + (this->length * 2)) ;
}
double Rectangle::get_area()
{
return (this->width * this->length);
}
void Rectangle::resize(double factor)
{
this->width *= factor;
this->length *= factor;
}
This should work then.
Regards,
Luka

Answer 3

Few things to unpick here.
1) use this->width which is equivalent to java's this.width (In C++, this is a pointer). Actually some C++ programmers (including me) prefix member variables with m_. Then you can just write m_width = width.
2) include "header.h" at the top of Question1.cpp
3) avoid putting "using namespace std" in a header file, or you could get unintended namespace
clashes as your code expands. OK to put it in the separate source files though although some folk even discourage this.
4) depending on your compiler and linker, you'll need to link to various lib's that the iostream library uses. Perhaps if you tell us the compiler you're using, we can help you here.
5) you need to surround your header with
#ifndef ARBITRARY_TEXT_BUT_DISTINCT_FROM_ANY_OTHER_IN_YOUR_PROGRAM
#define ARBITRARY_TEXT_BUT_DISTINCT_FROM_ANY_OTHER_IN_YOUR_PROGRAM
...your code here
#endif
This is an include guard - helps prevent multiple inclusion of a header file contents.

Answer 4

In Question1.cpp you have to include header.h
And don't forget include guards in header.h
Also in Question1.cpp you must change
Rectangle(double width, double length)
to
Rectangle::Rectangle(double width, double length)

Answer 5

You need to tell your build system to compile "question1.cpp". Usually there is an "add existing file to project" menu item somewhere under "File".
And typically, your class and constructor would use a different name than the input parameter. Many people put a prefix at the begginning (mLength, iLength) or postfix at the end (length_ is common).
The other solution is to prefix the member variable with this->length, but that can get pretty messy after a while.

Answer 6

the big mistake that in the .cpp file you are supposed to implement only the methods not rewrite full class implementation try the following in .cpp file
Rectangle::Rectangle(double width, double length)
{
width = width; //I have no idea how to use this.something as its in Java
length = length; //Problem probably occurs at here
}
and don't include the methods in class{}; block and don't redefine your member variables
also don't forget to include the header file in the .cpp file
thanks

Answer 7

I wonder if you are getting linker errors as your cpp file is little weird
In the cpp file include the .h file and only implement the functions like
Rectangle::Rectangle(double width, double length){
//implement
}
double Rectangle::get_perimeter(){
//implement
}
double Rectangle::get_area(){
//implement
}
void Rectangle::resize(double factor){
//implement
}

Answer 8

When you want to split your class file into a *.cpp and a *.h file it always has the following form:
rectangle.h:
#ifndef __RECTANGLE_H_
#define __RECTANGLE_H_
#include <iostream>
using namespace std;
class Rectangle
{
public:
Rectangle(double width, double length);
double get_perimeter();
double get_area();
void resize(double factor);
private:
double width;
double length;
double area;
double factor;
};
#endif
now the rectangle.cpp should have the following form:
#include <iostream>
#include "rectangle.h"
using namespace std;
Rectangle(double width, double length)
{
width = width; //I have no idea how to use this.something as its in Java
length = length; //Problem probably occurs at here
}
double Rectangle::get_perimeter()
{
return ((width * 2) + (length * 2)) ;
}
double Rectangle::get_area()
{
return (width * length);
}
void Rectangle::resize(double factor)
{
width *= factor;
length *= factor;
}
so when as an explanation:
The header file tells you which fields and methods are available and the *.cpp file implements the methods.
In your main.cpp you just need o include the rectangle.h

Answer 9

this code below works
#include<iostream>
#include<iomanip>
using namespace std;
class student
{
private:
int area;//hours;
float perimeter;//gpa;
public:
void addcourse(int len, float wid)
{
float sum;
sum = area * perimeter;
area += len;
sum += wid * len;
perimeter = sum / area;
}
student() // constructor
{
area = 0;
perimeter= 0.0;
}
};
int main()
{
student s;
int length;//classhours;//l
float width;//classgpa;//w
cout << "Enter length ( 1 to end): ";
cin >> length;
while(length != 1)
{
cout << "Enter width: ";
cin >> width;
s.addcourse(length, width);
cout << "Enter length ( 1 to end): ";
cin >> length;
}
// cout << "Rectangle's length = " << r.getlength() << endl;
// cout << "Rectangle's width = " << r.getwidth() << endl;
// cout << "Rectangle's area = " << r.getarea() << endl;
// cout << "Rectangle's perimeter = " << r.getperimeter() << endl;
}