C++ Header File Interaction -

ok, pretty new c++, have couple of questions header files...

1.) variables should declare, , should not in header file?

2.) when declare variable in header file, should use extern?

here 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 main.cpp

/*******************************************************************  * program take input initial velocity (fps), , launch angle  * based on information, current posotion of object thrown  * calculated until hits 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.  *  * function takes in angle in degrees, , converts  * radians.  * **/ double degtorad(double angle){     return angle * (pi/180);     }  /******************************************************************  * simulate function.  *  * takes in angle in radians, , velocity. calculates  * path of projectile, , displays in 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) @ time " << time << " (" << x << ", " << y << ")" <<  endl;         ++time;     } //while     cout << "leaving simulation" << endl; } //simulate  /***************************************************************************  * main function.  *  * produces output console in order coach user on input. **/ int main() {     cout << "execution beginning" << endl;     cout << "enter inital velocity (feet per second):" << endl;     cin >> initialvelocity;      if(initialvelocity > 0){         cout << "good. " << initialvelocity << " valid value initial velocity." << endl;     }     else{         cout << "error: " << initialvelocity << " not valid value initial velocity." <<endl;         return 0;     }      cout << "enter initial angle in degrees (from horizontal):" << endl;     cin >> initialangle;      if(initialangle >= 0 && initialangle <= 90){         cout << "good. " << initialangle << " valid value initial angle." << endl;     }     else{         cout << "error: " << initialangle << " not valid value initial angle." << endl;         return 0;     }      simulate(degtorad(initialangle), initialvelocity);      cout << "ending execution" << endl; return 0; } 

like said, new c++, can please explain how these 2 interact, or should make them interact more efficiently. program compiles, , runs correctly, unclear on protocol , uses of header files .cpp file. also, functions , variables should in private section of header, , should go in public? thank you.

i think need know 2 things:

• whats difference between declaration , definition.
  • you need declaration use class or function (for compiler know there)
  • the definition code gets compiled (function body, initialization)
• what #include do.
  • #include copies content of header source file including it.

that means, don’t want definitions in header files, because if multiple files include header, violate one-definition-rule, causing multiple-defintion-errors during linking stage.

which variables should declare, , should not in header file?

the header-file interface other source-files want call functions/use classes.
put in declarations things needed other source files work.

when declare variable in header file, should use extern?

the extern keyword specifies declaration variable rather definition. means variable-definition somewhere else (e.g. source file).

if want use gravity_factor in source file, includes header:

• in source: const int gravity_factor = -16;
• in hour header: extern const int gravity_factor;

remarks:

• what have in header file class definition
  • public , private accessors class’ members
  • you may rather want namespace here or plain functions
  • the main function has nothing inside class definition