Converting from Java to C++. Why can't I use methods of a different class the same way in C++?

Question

I'm trying to convert a Java program to C++. This is the Java code (which IS working perfectly):

//class MyProgram:

public class MyProgram {
    private static ArrayList mylist;
    //no constructor
    public static int numberOfItems(){
        return mylist.size();
    }
}

//second Java program, MyList

public class MyList {
    public MyList(){
        for (int i = 0; i < MyProgram.numberOfItems; i++)
            System.out.println("Test");
    }
}

This works exactly as I'd hope. However, when converting to C++, the for loop header is giving me errors because it doesn't recognize numberOfItems. The implementation is exactly the same, except a vector instead of an arraylist. My question is: why does not NOT work in C++? Why DOES it work in Java? I've never written an object before where I didn't have to call the constructor to use the methods of that class. How can I achieve this same thing in c++? I can post the c++ code but it looks extremely similar and it seems pointless.

EDIT: C++ CODE:

//MyProgram.h

#ifndef MYPROGRAM_H_
#define MYPROGRAM_H_

 #include <vector>

namespace std;

class MyProgram {
public:

    int numberOfItems();

private:
    vector<Checkpoint> mylist;


};

#endif /* MYPROGRAM_H_ */

//MyProgram.cpp

#include "MyProgram.h"

int MyProgram::numberOfItems()
{
    return mylist.size();
}

//MyList.h:

#ifndef MYLIST_H_
#define MYLIST_H_

#include <vector>

using namespace std;

class MyList {
public:
    MyList();
    virtual ~MyList();
private:
    vector<Checkpoint> path;

};

#endif /* MYLIST_H_ */


//mylist.cpp

#include "MyList.h"

MyList::MyList()
{
    int i;
    for(i = 0; i < MyProgram.numberOfItems(); i++)
    {
        path.push_back(0);
    }
}

Could you please add what you did in C++? It is impossible for us to see what is wrong without. — Jérôme, Feb 27 '15 at 22:29
@AnonymousPerson I am adding it in now. It will take a second with the .h files and all that — user2130057, Feb 27 '15 at 22:33
@user2130057 Please do *not* use Java as a model in writing C++ code. They are *not* the same language. — PaulMcKenzie, Feb 27 '15 at 22:41
Absolutely agree with @PaulMcKenzie. Write what you need directly in C++. — fps, Feb 27 '15 at 23:02
@user2130057 You also need to initialize those `static` members in one and only one module otherwise you will get linker errors when building the program. — PaulMcKenzie, Feb 27 '15 at 23:14
In your Java code your members are `static` but they are not `static` in your C++ code. Beyond that, please just forget about Java when learning C++, it will just hold you back and encourage you to do things the wrong way because C++ works very differently to Java. — Galik, Feb 28 '15 at 00:09

franji1 · Answer 1 · 2015-02-28T20:39:16.727

4

change MyProgram.numberOfItems to MyProgram::numberOfItems()

edited Feb 28 '15 at 20:39

answered Feb 27 '15 at 22:39

franji1

3,088
2
23
43

2

And you should put () – amchacon Feb 27 '15 at 22:43
Thank you! This is exactly what I am looking for. Question though, it didn't work with (), only when I removed that did it work. That being the case, what would I do if the method I wanted to call had parameters? In this case it was void so all was well. – user2130057 Feb 27 '15 at 23:18

score 1 · Answer 2 · edited May 23 '17 at 11:56

1

So, you have a misunderstanding of is a class member vs object member. This post will provide you the information that you really need: When do I use a dot, arrow, or double colon to refer to members of a class in C++?.

Also, like frinji1 said, MyProgram::numberOfItems() is what you want.

edited May 23 '17 at 11:56

Community

1
1

answered Feb 27 '15 at 22:55

v4n3ck

51
3

Thank you very much! You are right, I think my core understanding is off. I appreciate the answer. – user2130057 Feb 27 '15 at 23:19

Converting from Java to C++. Why can't I use methods of a different class the same way in C++?

2 Answers2