C++ - Using multidimensional arrays with variable arguments in a header file

Question

I declare and define several variables that should be read and write by all the other files in my program in a header called "variables.h".

variables.h

const int N;
const int M;
extern double x[M][N]; // x position of the particles
extern double y[M][N]; // y position of the particles
const double dt; // Time step size in MD simulation
const double totalStep; // Total number of MD steps

variables.cpp

N = 30;
M = 2;
dt = 1e-10; // Time step size in MD simulation
totalStep = 1e+7; // Total number of MD steps
double x[M][N];
double y[M][N];

Then I use these variables in "calc.cpp" with header "calc.h" like this:

calc.cpp

#include "calc.h"
#include "variables.h"
void myCalculation(double x[][N], double y[][N]){
    for(int n = 0; n < M; n++){
        for(int i = 0; i < N; i++){
            double a; // This is just to show you guys
            double b;
            a = ( x[n][i+1] - x[n][i] );
            b = ( y[n][i+1] - y[n][i] );
        }
    }
}

calc.h

void myCalculation(double x[][N], double y[][N]);

However, at this point I get the error: "use of undeclared identifier N in calc.h", since N is not defined there. When I include "variables.h" in "calc.h", I get "redefinition of variables" error. I assume it is related to using a header in a header file somehow.

How can use my array in my function in the "calc.h"?

Answer 1

order of declaration is not good, define N, M before x, y:

const int N = 30;
const int M = 2;
extern double x[M][N]; // x position of the particles
extern double y[M][N]; // y position of the particles

And any file using N, M should include variables.h as calc.h.

And your header should have include guards

so

#ifndef VARIABLE_H
#define VARIABLE_H

// your code

#endif

or you may use #pragma once (but it is not standard :-( ).