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I am trying to pass objects into functions, but which object to pass in will depend on user input. I'm not sure how to do this.

In int main(){

I've declared four objects all of a child class type:

Human Kite(20, 100, "Kite"); Cyberdemon Ky(20, 100, "Ky Ky"); Balrogs Xor(20, 100, "HaXoR"); Elf Peet(20, 100, "Peet");

Then depending on user input, I pass in one of the four objects into an object of type Creature, the Parent Class.

int player_creature_switch;
Creature player1creature;
Creature player2creature;

cout << "Player 1 choose your class: (1) Human (2) Cyberdemon (3) Balrogs (4) Elf" << endl;
cin >> player_creature_switch;
        switch(player_creature_switch){
        case 1:
            player1creature = Kite;
            break;
        case 2:
            player1creature = Ky;
            break;
        case 3:
            player1creature = Xor;
            break;
        case 4:
            player1creature = Peet;
            break;
        default:

            break;
    }

cout << "Player 2 choose your class: (1) Human (2) Cyberdemon (3) Balrogs (4) Elf" << endl;
cin >> player_creature_switch;
switch(player_creature_switch){
        case 1:
            player2creature = Kite;
            break;
        case 2:
            player2creature = Ky;
            break;
        case 3:
            player2creature = Xor;
            break;
        case 4:
            player2creature = Peet;
            break;
        default:

            break;
    }

execute_battle(player1creature, player2creature);

return 0;

}

Then I wish to pass the two Creature objects holding any one of the four child class objects into a function, execute_battle, which will access the member function of the objects passed in as arguments. 

void execute_battle(Creature& pl_creature, Creature& p2_creature){
        cout << pl_creature.getType() << p1_creature.getName() << "has entered the        fray!";

  .... more code.... 
    }

I have my header files and my base/child classes set up fine. I know this because my member functions are working fine in main if I do not pass an object to a function. I also have my function protocol, etc. I know I am not correctly passing the objects or accessing member functions within the function execute_battle, because currently I am getting varying errors such as In function execute_battle: p1_creature was not declared in this scope. Which I am really confused about, because I clearly passed it in as an argument.

I have also tried creating another Creature instance within the function: 

    void execute_battle(Creature& pl_creature, Creature& p2_creature){
        Creature p1_creat = p1_creature;
        cout << pl_creat.getType() << p1_creat.getName() << "has entered the fray!";
}

But to no avail. How could I do this? Much more importantly, could someone explain what is going on in terms of references/addresses of the objects being passed?

For example, I understand that when, within the switch statement, I put the address of the child object into player1creature of the Parent type Creature, I am not just putting all of its contents into player1creature and overwriting everything in player1creature. Instead I am just passing the address (hopefully?) so it retains all the parent functions as well. Then, I pass the parent object into the function execute_battle, and the function will create a local copy of that object (right)? This is what I think is going on, but I wouldn't be surprised if I have something wrong.

I have also tried using pointers and references in other places, but things just got very messy quickly. Let me know if I could provide more code somehow.

Thank you!

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2  
There is a typo: In your execute_battle() function sometimes you use pl_creature (like your parameter) and sometimes p1_creature (with a 1 instead of l –  JDS May 25 at 23:45
 
Wow, I feel stupid. super facepalm I wonder how I managed to type an L... So my original code worked after changing this. Thanks. –  Gnuey May 26 at 0:00
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2 Answers

up vote 1 down vote accepted

What you try to achieve is polymorphism.

Here are what you should do :

1.Declare your creature variable as pointers :

Creature* player1creature;
Creature* player2creature;

Only this way you'll be able to store derived objects. I don't even know how does your code compile this way (with assignement of derived objects to base objects). This can't be done, because when you declare an object of type Creature this way : Creature myCreature, you tell the compiler "Reserve place for an object of type Creature ", and the compiler can know the size of this object thanks to the class declaration. You can't assign directly objects of a derived type because the memory layout for these isn't the same (usually, it's bigger, as derived class add things).

Once you've got these, polymorphism can work (that is, calls to virtual functions, either directly or indirectly with the non-virtual-interface idiom will be resolved dynamically, by looking up in the vtable for the correct function).

2.Then, in your function execute_battle, you'll have to pass pointers to objects of type Creature for the same reasons (you can pass pointers, or pointers-to-const, depending on what you want to do)

Thus, declare your function with one of these :

void execute_battle(Creature* p1_creature, Creature* p2_creature){}
void execute_battle(const Creature* p1_creature, const Creature* p2_creature){}

(and as noted in the comments, avoid typo, that should prevent not declared in this scope errors).

You can now call member functions of Creature and polymorphism will work.

Then, about argument passing method, I suggest you read about the different possibilities

share|improve this answer
 
Hmm well strangely my code above worked after fixing type. And yes earlier I tried using pointers in my declarations like you have written here. With my typo fixed, I will go back and revisit the pointer method. Thank you, I'll take a look at this method again. –  Gnuey May 26 at 0:04
1  
@Gnuey You're welcome. Do not hesitate to ask if you have other questions about specific details on this topic, I'll edit my answer. Otherwise, you can just accept the answer as it is. –  JBL May 26 at 0:07
 
Hmm, ok so when I try to use the pointer method, I am getting the compiler error: in function "void execute_battle(Creature*, Creature*)", error: request for member "getType" in "p1_creature" which is of non-class type "Creature*". And the same error for getName. In my switch statement, I am assigning Creature* player[1/2]creature like this: player1creature = &Kite. Is this right? Later I am passing it into execute_battle like so: execute_battle(player1creature, player2creature). execute_battle` parameter now looks like your suggestion:(Creature* p1_creature, Creature* p2_creature) –  Gnuey May 26 at 2:00
 
Oh, huh, it now works perfectly when I use the arrow operator pl_creature->getType() within execute_battle instead of the dot operator pl_creature.getType(). Why is this? And it works perfectly no matter if I do player1creature = &Kite or player1creature = new Human(20, 100, "Kite"); , which I guess makes sense because both ways I am returning an address. –  Gnuey May 26 at 2:56
1  
@Gnuey: Yes, when you use pointers, you must use the -> operator, which "dereference" the pointer before accessing the requested member (be it a function or a variable). And indeed, with your first method (passing references) you could only access parent member functions because, as the parameters weren't pointers, the static type was considered when invoking functions, and polymorphism wouldn't work. I suggest you try to find out more about "pointers", "polymorphism" and "static vs dynamic binding" (which is closely related to polymorphism). –  JBL May 26 at 10:45
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up vote 1 down vote

So here is my comment as an answer: Fix the typo in your execute_battle() function. The parameter is declared as p1_creature but sometimes you use pl_creature with l instead of 1

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