can't get virtual function of parent class to work [duplicate] - c++

Question

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C++ overridden virtual function not getting called [duplicate]
(3 answers)
Closed 4 years ago.
I have a parent class with a virtual function in it, I then make a child class and define the function. I then make a vector of vectors and insert one of the child classes into it. I then try to call the virtual function and nothing outputs to the screen. I do not know why this is happening, does anyone know?
Parent Class
class insect{
public:
string type;
int food_cost;
int armor;
int damage;
insect();
void set_food_cost(int x);
void set_armor(int x);
void set_damage(int x);
virtual void attack(){} // this is the problematic function
};
Child Class
class bee: public insect{
public:
bee();
int armor;
int damage;
void set_armor(int x);
void attack();
};
void bee::attack(){
cout << "im a bee, stab stab!\n";
}
Creating Vector of Vectors
vector< vector<insect> > insects_on_board(10);
Adding a bee to the vector of vectors
void add_bee(vector< vector<insect> > &insects_on_board, int &bees){
bees++;
insects_on_board[9].push_back(bee());
}
Function Call
cout << "testing " << insects_on_board.at(9).at(0).type << endl;
insects_on_board.at(9).at(0).attack();
Output
testing B
My Question Again
so in the output im expecting to see "testing B" and then "im a bee, stab stab!"
but only the "testing B" is outputted to the screen, any ideas why the other part is not?

Answer 1

That's because you are storing actual insects and not any bees. Polymorphism (in its basic C++ way) works when you are doing 3 things:
Have a type hierarchy with properly defined and overridden virtual methods (like you are doing here,)
Create child instances (or a variety of parent and child instances,)
Access them through pointers (or references.)
You are missing points 2 and 3.
So, one way to fix it would be to store pointers to insect, and initialize them as either bees or plain insects, like so:
vector<vector<insect *>> insects_on_board (10, vector<insect *>(2)); // note the type
insects_on_board[9][0] = new bee;
insects_on_board[9][1] = new insect;
// Note the use of "->" instead of "."
cout << "testing " << insects_on_board[9][0]->type << endl;
insects_on_board[9][0]->attack();
// Contrast the output with the above's
cout << "testing " << insects_on_board[9][1]->type << endl;
insects_on_board[9][1]->attack();
Update:
Note that (at the basic level) any container that stores insects by value cannot contain anything else; not even classes derived from insect. All the polymorphism and stuff you read and hear about is only applicable for pointers to parent and child types (or references to them.)
So, your add_bee function should look like this:
void add_bee (vector<vector<insect *>> & insects_on_board, int & bees) {
bees++;
insects_on_board[9].push_back(new bee());
}
I've made only two changes in there: the vectors now contain pointers to insect and I'm newing the bees.