Reference to local variable returned - c++

Question

I am trying to implement a character selection method, for a text based game and i know that it will not work the way i did it, because i am returning a refernce to an object with a lifetime limited to the scope of the method call. I have also tried to implement the method without referencing the Fighter parent class and returning a child class (Samus and Ryu) based on the players character choice, but then i would get this error: invalid abstract return type 'Fighter'.
Fighter characterSelection(int player,bool checkForBot, Fighter &fig)
{
int input;
string newName;
if(checkForBot)
{
input = (rand()%2)+1;
}
else{
cout << "Please choose your Fighter player"<<player<<": \n1. Samus\n2. Ryu\n";
input = readInput<int>();
}
if(input == 1)
{
Samus sam;
if(checkForBot)
{
cout << "Bot selected Samus!";
}
else{
cout << "Player"<<player<<" selected Samus!\nDo you wish to change your fighters name?\n1.Yes\n2.No\n";
input = readInput<int>();
if(input == 1)
{
cout << "new Name: ";
newName = readInput<string>();
changeName(newName,sam);
}
}
return sam;
}
else if(input == 2)
{
Ryu ry;
if(checkForBot)
{
cout << "Bot selected Ryu!";
}
else {
cout << "Player"<<player<<" selected Ryu!\nDo you wish to change your fighters name?\n1.Yes\n2.No\n";
input = readInput<int>();
if(input == 1)
{
cout << "new Name: ";
newName = readInput<string>();
changeName(newName,ry);
}
}
return ry;
}
}
After having chosen one character and the end of the function call, the destructor of the object is called upon, thus making the reference linked to a non-existing object.
int main()
{
int input;
string newName;
bool checkForBot;
int player=1;
while(true)
{
cout << "1. PVP \n2. PVE\n";
input = readInput<int>();
if(input == 1)
{
checkForBot = false;
//Character Selection
Fighter &fig1 = characterSelection(player,checkForBot,fig1);
player++;
Fighter &fig2 = characterSelection(player,checkForBot,fig2);
//Battle
fightPVP(fig1, fig2);
}
else if(input ==2)
{
//Character Selection
Fighter &fig1 = characterSelection(player,checkForBot,fig1);
checkForBot = true;
Fighter &bot = characterSelection(player,checkForBot,bot);
//Battle
fightPVE(fig1, bot);
}
}
return 0;
}
Is there any other way i could solve this problem, instead of referencing the parent class and then creating the child in the function call?

Answer 1

In this:
Fighter &fig1 = characterSelection(player,checkForBot,fig1);
you are making a reference to a local object copy returned by the function.
Returning a parent object has also the issue of object slicing; you should have that in account.
If that's not a problem, however, you can add a copy constructor to the class and just remove the reference when declaring the receiving variable:
Fighter fig1 = characterSelection(player,checkForBot,fig1);
If your object is not suited to copy, other way is to new the object in characterSelection and return a pointer (or better, shared pointer) of the base class.

Answer 2

In the case of your code, the compiler is right. From the function 'characterSelection' you return an instance of the abstract class Fighter which is wrong as abstract classes cannot be instantiated. You can only return a pointer or a reference (Fighter* or Figther&) to an abstract class (the pointer is generally the preferred way). From my knowledge the single way to achieve what you want using inheritance form an abstract class and a choice of a kind of Fighter at runtime is that:
#include <iostream>
class Fighter
{
public:
virtual void f() = 0;
virtual ~Fighter() {};
};
class Samus : public Fighter
{
public:
void f() { std:: cout << "Samus\n"; }
};
class Ryu : public Fighter
{
public:
void f() { std:: cout << "Ryu\n"; }
};
Fighter* getFigther()
{
int fighterCode;
std::cin >> fighterCode;
switch (fighterCode)
{
case 0: return new Samus();
case 1: return new Ryu();
default: return nullptr;
}
}
int main()
{
Fighter* myF = getFigther();
// Do what you want with the fighter
myF->f();
// Release the resources
delete myF;
}
If you want to avoid heap allocation from whatever reason, you can do that by using placement new syntax:
#include <iostream>
class Fighter
{
public:
virtual void f() = 0;
virtual ~Fighter() {};
};
class Samus : public Fighter
{
public:
void f() { std:: cout << "Samus\n"; }
};
class Ryu : public Fighter
{
public:
void f() { std:: cout << "Ryu\n"; }
};
constexpr std::size_t size()
{
constexpr std::size_t samusS = sizeof(Samus);
constexpr std::size_t ryuS = sizeof(Ryu);
return samusS < ryuS ? ryuS : samusS;
}
void getFigther(Fighter*& f)
{
int fighterCode;
std::cin >> fighterCode;
// If figtherCode is invaild you can print a message or throw an exception
switch (fighterCode)
{
case 0: new (f) Samus(); break;
case 1: new (f) Ryu(); break;
}
}
int main()
{
char space[size()];
Fighter* myF = (Fighter*) (space);
getFigther(myF);
// Do what you want with the fighter
// No delete requied
myF->f();
}
However, if you have multiple fighter classes with different sizes you will need a compile-time function to give you the maximum size of any class (representing a fighter). I don't think that approach worth the effort in your case.