Question

I’m learning c++. I have written a small program which should compute the energy
of a N particle system. Up to now I have three small files:

data.h:

    class Particle {                                                                                                                      
        public:                                                                                                                       
        double mass;                                                                                                                  
        double charge;                                                                                                                
        double posx,posy,posz;                                                                                                        
};                                                                                                                                    

        Particle part[2];  

main.cpp:

#include <iostream>                                                                                                                   
#include "data.h"                                                                                                                     
using namespace std;                                                                                                                  

double energy(Particle part );                                                                                                        

int main ()                                                                                                                           
{                                                                                                                                     
        double sd;                                                                                                                    
        part[0].mass = 10.0;                                                                                                          
        part[4].mass = 90.0;                                                                                                          
        cout << part[0].mass << "\n";                                                                                                 
        cout << part[4].mass << "\n";                                                                                                 

        sd = energy(part);                                                                                                            
        cout << "sd" << sd <<  "\n" ;                                                                                                 
        return 0;                                                                                                                     
}    

energy.cpp:

#include <iostream>                                                                                                                   
using namespace std;                                                                                                                  

double energy(Particle part)                                                                                                          
{                                                                                                                                     
        cout << part[0].mass << "\n";                                                                                                 
        double dummy;                                                                                                                 
        dummy = 2.0;                                                                                                                  
        return (dummy);                                                                                                               
}   

I have two questions:

1)I want to make visible the Class particle in the function “energy”. In other words,
I want to use the variables of the class function (with the values given in “main”)
in the energy function.
I have tried as you see energy(Particle part) but it seems Particle is not defined
in that scope.

2)As you see in “data.h” I declared “part” as an array with two members. However,
in “main” I can use more than two members, for instance part[3],part[4]… Why I
could use more members than those I declared?

I am compiling with g++ -o test energy.cpp main.cpp

thanks.

Answer 1

1)I want to make visible the Class particle in the function “energy”. In other words, I want to use the variables of the class function (with the values given in “main”) in the energy function. I have tried as you see energy(Particle part) but it seems Particle is not defined in that scope.

If I understand you right.. You want to have

Particle part[2];

to be use able in main.cpp and in energy.cpp ?
If yes.. change this to:

extern Particle part[2];

and in energy.cpp add this:

#include "data.h"
Particle part[2];

and you will be able to use

double energy()                                                                                                          
{              
        //main.cpp will have same part                                                                                                                       
        cout << part[0].mass << "\n";                                                                                                 
        double dummy;                                                                                                                 
        dummy = 2.0;                                                                                                                  
        return (dummy);                                                                                                               
}

2)As you see in “data.h” I declared “part” as an array with two members. However, in “main” I can use more than two members, for instance part[3],part[4]… Why I could use more members than those I declared?

Because it’s C/C++ ? no range checks. You can do what ever you want.
But if you do, the result will be unexpected.