sqaure.cpp
double square(double n){
    return n * n;
}

main.cpp
#include <iostream>
#include <cstdlib>

using namespace std;

double square(double);

int main(int argc, char *argv[]){

    // Make sure the argument count is 2, otherwise display the help
    if (argc != 2){
        cerr << "Usage: square <number>" << endl;
        return 1;
    }

    // String to double of Argument 1.
    double n = strtod(argv[1], 0);

    // Call the square method in square.cpp and display it on the screen.
    cout << "The square of " << argv[1] << " is " << square(n) << endl;
    return 0;
}