Class properties not holding values

Question

im working in a text-based RPG game, but when I'm setting the values to X variable, when I access that propertie again, it is in its default value, am I doing something wrong?

class Game
{
private:
    bool podeAndar;
    bool estaBatalhando;
    Jogador _jogador;
    Mapa _mapa;
public:
    Game() { }

    Game(Jogador _j){
        _jogador = Jogador(_j.getNome());
        _mapa.LoadMapa();
        podeAndar = true;
        estaBatalhando = false;
    }

    ~Game(void)
    {
    }

    Jogador getJogador() {
        return _jogador;
    }

    void setJogador(Jogador v) {
        _jogador = v;
    }
}

My "Player" class

#pragma once
#include "Criatura.h"

#include <string>

class Jogador :
    public Criatura
{
private:
    int _cap;
public:

    Jogador(std::string nome)
    {
        setNome(nome);
        setCap(150);
    }

    Jogador() { }

    ~Jogador(void)
    {
    }

    int getCap(){
        return _cap;
    }

    void setCap(int v){
        _cap = v;
    }
}

Them my "Main" - when I set the value, when I'm following it in the debugger, it sets the value correctly, but when I access the game.getJogador().getCap() again, it has the default value 150.

int _tmain(int argc, _TCHAR* argv[])
{
     Jogador _player = Jogador("Kyore");
     Game game = Game(_player);

     while(true){
          std::cout << game.getJogador().getCap(); //print 150
          game.getJogador().setCap(100); //set cap to 100
          std::cout << game.getJogador().getCap(); //print 150 again
          break;
     }
}

elimad · Accepted Answer · 2014-10-30T05:18:28.547

In Game class, change this

Jogador getJogador() {
        return _jogador;
    }

to

Jogador& getJogador() {
        return _jogador;
    }

And add one more method only to read:

const Jogador& getJogador()const {
            return _jogador;
        }

Update for the questions asked in the comment

To fix your specific issue of value remaining as 150 inspite of setting a new value, converting the return type to reference is enough.
Why returning reference works? Because, whenever your original version of getJogador() is called, a copy of the object is created. Even though you are changing its value, you are actually changing the value of the temporary object created, not the original one.
So as your intention is to modify the original object, we need to access the original one, not its temporary copy. Reference is the better mechanism in such cases (pointer being the other mechanism, but less safer than reference)
Now about why I suggested the new over load of a const member function, returning a const reference: this is to highlight to you that it is possible to still get the object without changing its internal state unintentionally.
Your sample code does not differentiate between the two getJogador() functions.
So to understand, add these two functions to your Game class:

void DontManipulate()const { std::cout<<getJogador().getCap(); }
void Manipulate() { std::cout<<getJogador().getCap(); }

See the compiler error(s) that you get: - it should throw light on the differences.
Additionally, if you std::cout some message in both the getJogador() functions, you should be able to figure out the differences.

Thank you, it worked perfectly. For learn purpose, without the const method it wont work? And why? Wich one is called when I use the .getJogador()? The one returning the address or the const one? Thanks again. — Kyore, Oct 27 '14 at 14:46
Sorry, I couldn't answer your comment immediately. Edited the answer now - check if it gives some more info so that you can figure out yourself. — elimad, Oct 30 '14 at 05:20

score 1 · Answer 2 · answered Oct 27 '14 at 04:19

The problem is in your getJogador() method.

In C++, objects can be passed "by value" - which is where the program (usually) copies the object's raw data into a new location, whereas in C# and Java objects are always passed by reference (not counting C#'s structs which are passed by-value similar to C++). C++ will use the "copy constructor" to perform this copy. C++ will create the copy constructor if it isn't explicitly defined in your code, the signature has the form ClassName(ClassName& other);, the default (non-explicit) copy-constructor performs a shallow, member-wise copy operation.

In your case, your getJogador method is returning a copy of your Jogador instance field's data.

Change the method to return a reference or a pointer, like so:

Jogador& getJogador() const {
    return _jogador;
}

or

Jogador* getJogador() const {
    return &_jogador;
}

The const modifier informs the compiler that this method is not intended to modify the state of your Game class, so the compiler might perform certain optimizations as well as prevent successful compilation if the method does attempt to modify state.

Class properties not holding values

2 Answers2