Getter return null C++

Question

I have class car and I use car as an attribute in class App .

class Car{
private:
  string color;
public:
  //setter getter
  void setColor(string c){
     color = c;
  }
  string getColor(){
     return color;
  }
}

and

class App(){
private:
   Car car;
public:
   App():car(){
   }
   Car getCar(){
      return car;
   }
}

this is my main app

int main(){
  App app[2];

  app[0].getCar().setColor("red")
  //WHY RETURN NULL?
  cout << app[0]/getCar().getColor();
  return 0;
}

the setter is working well but why does the getter return null as if the object app[0] is reinstance? Thank You

Answer 1

the setter is working well but, why the getter is return null like the object (app[0]) is reinstance?

You are having a problem because you are returning a temporary copy of your Car member variable rather than a reference to the member variable itself. More specifically, your member function getCar() returns Car rather than Car& . Your current version will copy the value of the member variable car into a temporary and return the temporary as its return value, not the member variable itself. So then in this code:

app[0].getCar().setColor("red");

You are actually modifying the contents of the temporary copy that getCar() returned. Since this is a temporary copy, the temporary will be destroyed at the end of the statement, and all your modifications will be lost. Moreover, the car member variable of app[0] will continue to have its default value.

What you want is to return a reference to the member variable itself that will allow you to edit its contents directly. To do this, you need to modify your App class in the following way:

class App(){
private:
   Car car;
public:
   App():car(){
   }
   Car& getCar(){
      return car;
   }
}

Now getCar() returns a reference to the member variable car , and modifying this reference will directly modify the car member variable of app[0] , giving you your expected behavior.

Answer 2

In this specific case, return reference could solve your problem.

However, in general, return reference of a member variable is a bad practise.( see detail in this post ).

In your case, I think you want the shared ownership when get car. So may be use shared_ptr for car is better than returning reference directly.

class App
{
public:
    App() : car(std::make_shared<Car>()){};
    std::shared_ptr<Car> getCar() {return car;}
private:
    std::shared_ptr<Car> car;
};