C++ Header File Interaction - c++

Question

Ok, so I am pretty new to C++, I have a couple of questions about header files...
1.) Which variables should you declare, and which should you not in a header file?
2.) When you declare a variable in a header file, should you use extern?
Here is my header file:
#ifndef MAIN_H
#define MAIN_H
class Main
{
public:
int main(); //Constructor
virtual ~Main(); //Destructor
double initialVelocity;
double initialAngle;
private:
double degToRad(double angle);
void simulate(double angle, double velocity);
};
#endif
And here is my Main.cpp
/*******************************************************************
* This program will take input for initial velocity (fps), and a launch angle
* based on this information, the current posotion of the object thrown will be
* calculated until it hits the ground.
*
*
* Date: 30 August 2013
* Version 1.0
*
**/
# include "Main.h"
# include <iostream>
# include <fstream>
# include <cmath>
using namespace std;
/******************************************************************
* General Variables
**/
const int GRAVITY_FACTOR = -16;
const int GROUND = 0;
const double PI = atan(1.0)*4;
double initialVelocity;
double initialAngle;
/******************************************************************
* degToRad function.
*
* This function takes in an angle in degrees, and converts it to
* radians.
*
**/
double degToRad(double angle){
return angle * (PI/180);
}
/******************************************************************
* simulate function.
*
* Takes in the angle in radians, and the velocity. Calculates the
* path of the projectile, and displays it in the terminal.
*
**/
void simulate(double angle, double velocity){
cout << "Entering Simulation" << endl;
double time = 0;
double x = 1;
double y = 1;
double veloUp = 0;
double veloFo = 0;
veloUp = (velocity*sin(angle));
veloFo = (velocity*cos(angle));
cout << "Angle in radians: " << angle << endl;
cout << "Initial velocity upwards (fps): " << veloUp << endl;
cout << "Initial velocity forward (fps): " << veloFo << endl;
while(y >= GROUND){
x = veloFo * time;
y = GRAVITY_FACTOR*(time*time) + (veloUp * time);
cout << "(x, y) at time " << time << " is (" << x << ", " << y << ")" << endl;
++time;
} //while
cout << "Leaving Simulation" << endl;
} //simulate
/***************************************************************************
* The main function.
*
* Produces output to the console in order to coach the user on what to input.
**/
int main()
{
cout << "Execution Beginning" << endl;
cout << "Enter the inital velocity (feet per second):" << endl;
cin >> initialVelocity;
if(initialVelocity > 0){
cout << "Good. " << initialVelocity << " is a valid value for the initial velocity." << endl;
}
else{
cout << "ERROR: " << initialVelocity << " is not a valid value for the initial velocity." <<endl;
return 0;
}
cout << "Enter the initial angle in degrees (from the horizontal):" << endl;
cin >> initialAngle;
if(initialAngle >= 0 && initialAngle <= 90){
cout << "Good. " << initialAngle << " is a valid value for the initial angle." << endl;
}
else{
cout << "ERROR: " << initialAngle << " is not a valid value for the initial angle." << endl;
return 0;
}
simulate(degToRad(initialAngle), initialVelocity);
cout << "Ending Execution" << endl;
return 0;
}
Like I said, I am new to C++, can anyone please explain how these two interact, or what I should do to make them interact more efficiently. The program compiles, and runs correctly, but I am unclear on the protocol and uses of header files with a .cpp file. Also, which functions and variables should be in the private section of the header, and which should go in the public? Thank you.

Answer 1

I think you need to know 2 Things:
Whats the difference between declaration and definition.
you need a declaration to use a class or function (for the compiler to know its there)
the definition is the code that gets compiled (function body, initialization)
What does #include do.
#include just copies the content of the header into the source file including it.
That means, that you probably don’t want definitions in your header files, because if multiple files include your header, you violate the One-Definition-Rule, causing multiple-defintion-errors during the linking stage.
Which variables should you declare, and which should you not in a header file?
The header-file is your interface to other source-files that want to call your functions/use your classes.
So you put in declarations for things which are needed for other source files to work.
When you declare a variable in a header file, should you use extern?
The extern Keyword specifies a declaration for a variable rather than a definition.
Which means that your variable-definition is somewhere else (e.g. your source file).
If you want to use GRAVITY_FACTOR in another source file, which includes your header:
in your source: const int GRAVITY_FACTOR = -16;
in hour header: extern const int GRAVITY_FACTOR;
Remarks:
what you currently have in your header file is a class definition
public and private are accessors to your class’ members
you may rather want a namespace here or plain functions
the main function has nothing to do inside a class definition

Answer 2

In the header file,you can declare all class fields and functions,that you will implement in the cpp file.
You should'nt declare the variables that you will use inside your class functions(because there are like temp variables),But you can declare(and should) const varables,that you will use in your progamm.
You should'nt use extern,extern its for variables that you dont define them,and the compiler doesnt bother you about that,because when its extern,that means that the variable is defined somewhere else in your progamm.
extern is also used for functions that you want to treat your c++ code,as it were in C.
like:
extern "C" { //code. }
or extern "C" int func(int a);
In conclusion-
delcare const variables in .h file,and the classes.else - in cpp(unless its inline).
in your case:
const int GRAVITY_FACTOR = -16;
const int GROUND = 0;
const double PI = atan(1.0)*4;
are better to be in the .h file
+
you shouldnt declare it again:
double initialVelocity;
double initialAngle;
its already in .h file.
Hope it was useful..