C++ simple class declaration program

Question

I got a program to create in C++ in our introduction to C++ class in school. I am doing everything as I got in examples, but still getting errors.

w4x.cpp was given and I have to create Molecule.h and Molecule.cpp. I did that, but I am getting errors because my variables were not declared in scope, but I can't understand why.

// w4x.cpp

#include <iostream>
using namespace std;
#include "w4x.h"
#include "Molecule.h"

int main() {
    int n = MAX_MOLECULES;
    Molecule molecule[MAX_MOLECULES];

    cout << "Molecular Information\n";
    cout << "=====================" << endl;

    for (int i = 0; i < MAX_MOLECULES; i++) {
        if (!molecule[i].read()) {
            n = i;
            i = MAX_MOLECULES;
        }
        cout << endl;
    }

    cout << "Structure            Name                     Mass\n";
    cout << "==================================================" << endl; 

    for (int i = 0; i < n; i++)
        molecule[i].display();
}


//Molecule.h
const int MAX_STRUCT = 10;
const int MAX_NAME = 20;

class Molecule {
    char name[MAX_STRUCT];
    char rate[MAX_NAME];
    double weight;
public:
    Molecule();
    void read(const char*, const char*, double);
    void display() const;
    ~Molecule();
};


//Molecule.cpp     
#include <iostream>
#include <cstring>
using namespace std;
#include "Molecule.h"

Molecule::Molecule(){
    name[0]= '\0';
    rate[0]= '\0';
    weight = 0;
}

void::read(const char* n, const char* r, double w) {
    weight = w;
    strncpy (name, n, MAX_STRUCT);
    name[MAX_STRUCT]='\0';
    strncpy (rate, r, MAX_NAME);
    rate[MAX_NAME]='\0';
    cout << "Enter structure : ";
    cin.getline (n, MAX_CHARS);
    cout << "Enter full name : ";
    cin.getline (r, MAX_NAME);
    cout << "Enter weight : ";
    cin >> w;
}

void::display() const
{
    int x;
    for ( x=0; x<i; x++)
        cout << n << "      " << r << "        " << w << endl;
}

My first question is, how can I pass char name[MAX_STRUCT]; char rate[MAX_NAME]; double weight; from Molecule.h to Molecule.cpp

Answer 1

The problem with your definitions is here:

void::read(const char* n, const char* r, double w)

and here

void::display() const

What :: says here, is that you are implementing a function within a class. So you need to specify which class and which function! What you are telling it now, is that you are implementing a function inside class void, which is nonexistent.

You should convert them to:

void Molecule::read(const char* n, const char* r, double w)
void Molecule::display() const

---

Your other question regarding passing class members:

The functions of a class have access to its variables, therefore, you don't need to concern yourself with that. Just use the variables.

Also, if you notice in your w4x.cpp, the function Molecule::read() is called without parameters, so your TAs ask you to implement it without parameters. Indeed, since you have access to Molecule::name, Molecule::rate and Molecule::weight directly, you should read data and write to those variables instead of asking for parameters. Therefore, your read function would look like this:

void Molecule::read()
{
    // read into name, rate and weight
}

Furthermore, w4x.cpp expects read to report whether it has been successful or not. This means that you should do error checking in Molecule::read and return 0 if no errors or -1 (for example) in case of errors.