C++ Game AI only works in main - c++

Question

I have a problem getting the AI to work if I do it in a subclass.
Here is my main loop in which I access the player and enemy classes for their move, logic, and show functions.
//Logic
myPlayer.player_move();
myEnemy.enemy_logic();
//Rendering
myPlayer.player_show();
myEnemy.enemy_show();
So I access the Player class's function that moves the player then I access the logic(AI) function of the Enemy Class.
void Enemy::enemy_move(){
eX -= 2;
}
void Enemy::enemy_logic(){
Player myPlayer;
if(myPlayer.x > SCREEN_WIDTH / 2){
enemy_move();
}
}
So if the Players x value is greater than half the screen, I want the enemy to start moving left
I can compile the program but when I move the player past the halfway point of the screen, the enemy does nothing. If I were to put this if statement in the main.cpp then it works fine. Any advice?

Answer 1

It's because you're re-initializing your instance of the class Player. Instead, pass it as a parameter:
void Enemy::enemy_logic(Player myPlayer){
if(myPlayer.x > SCREEN_WIDTH / 2){
enemy_move();
}
}
That way the function uses your instance, and not a new instance in a different scope.

Answer 2

Because you are initializing an instance of Player as myPlayer that is not connected to anything.
You should define a private variable for myPlayer inside your Enemy Class or pass a pointer to the Player class instance as an argument, in this way you will be able to maintain the state.

Answer 3

If your enemies depend on a single player you can "bind" each enmy to a player like this:
class Player
{
Position m_x, m_y, m_z;
public:
Position const & x (void) { return m_x; }
};
class Enemy
{
public:
Enemy (Player &p) : m_player(p) { }
Player & m_player;
void logic (void)
{
if (m_player.x() > SCREEN_WIDTH/2)
{
move_x();
}
}
void move_x (void) { /*...*/ }
};
int main (void)
{
Player the_player;
Enemy the_enemy(the_player);
// move the player
the_enemy.logic(); // the_enemy knows the_player and acts accordingly
}