How to get a reference to a different class in order to get a variable value from inside said class?

Question

I have a class called MapGraph and another class called GameState. I'm calling two functions called GoToLocation(NextLocation); and Game->SidePanel(); from main. GoToLocation() declaration and definition exist in MapGraph.h and MapGraph.cpp respectively and SidePanel() declaration and definition exist in GameState.h and GameState.cpp respectively. In SidePanel() I'm trying to get the value of a variable called CurrentLocation that exists in MapGraph and is public. I've included MapGraph.h in GameState.cpp and declared the class as class MapGraph; but I don't know how to get the value of the variable. If I do MapGraph* Map = new MapGraph; that always gives me the initial value of the variable and not the updated one. Any help would be appreciated. Thank you.

The code in main.cpp:

int main()
{
    MapGraph* Map = new MapGraph;
    GameState* Game = new GameState;
    //Game->MainMenu();

    Map->GoToLocation(MapGraph::LocationNames::CastleSquare);
    Game->SidePanel();

    //system("CLS");
    return 0;
}

MapGraph.h:

#pragma once

#include <iostream>
#include <list>
#include <string.h>

class MapGraph
{
public:
    MapGraph();

    enum class LocationNames
    {
        GraveYard = 0,
        Sewers = 1,
        Outskirts = 2,
        Barracks = 3,
        Town = 4,
        CastleSquare = 5,
        ThroneRoom = 6,
        Forest = 7,
        Gutter = 8,
        HunterShack = 9
    };

    std::string LocNamesString[10] =
    {
        "Grave Yard",
        "Sewers",
        "Outskirts",
        "Barracks",
        "Town",
        "Castle Square",
        "Throne Room",
        "Forest",
        "Gutter",
        "Hunter Shack"
    };

    LocationNames CurrentLocation;
    void GoToLocation(LocationNames NextLocation);
};

and GameState.cpp:

#include <iostream>
#include <stdlib.h>
#include "GameState.h"
#include "MapGraph.h"

class MapGraph;

void GameState::SidePanel()
{
    MapGraph* Map = new MapGraph;

    std::cout << Map->LocNamesString[(int)Map->CurrentLocation];
}

P.S.: I tried changing CurrentLocation in MapGraph.h to static but that always generates a linker error 2001. The error goes aways as soon as i remove the word static.

Thank you.

Answer 1

There are several ways to design this relationship. It's up to you to decide which fits best with your overall design. (One big question would be how many MapGraph objects do you expect to exist at the same time?) The simplest approach might be to pass the needed variable as a parameter.

void GameState::SidePanel(cosnt MapGraph & Map)
{
    std::cout << Map.LocNamesString[(int)Map.CurrentLocation];
}

Still, I would go a bit further and incorporate this line of logic into the MapGraph class. In that case, your function becomes

void GameState::SidePanel(cosnt MapGraph & Map)
{
    std::cout << Map.CurrentLocationName();
}

and the following gets added to the MapGraph definition

class MapGraph {
    // Existing stuff here

public:
    std::string CurrentLocationName()
    {
        return LocNamesString[(int)CurrentLocation];
    }

    // Possibly more definitions here
};

As a general rule, only your MapGraph implementation should need to know how MapGraph data is stored. When other code needs a combination of MapGraph data (such as getting the current location converted to a string), the MapGraph class should provide an interface for that.

By the way, LocNamesString should probably be static and const. The static keyword says that the name strings do not vary from MapGraph object to MapGraph object. The const keyword says that the name strings are not changed after initialization.