Setting a value of class variable from another function [closed] - c++

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I'm working on a simple arcade style space shooter and I'm having some problems with my player class.
This is the code:
#include<iostream>
class player {
public:
int xPos, yPos;
void setPlayerPos(int x, int y);
int getX();
int getY();
};
void player::setPlayerPos(int x, int y) {
xPos = x;
yPos = y;
return;
}
int player::getX() {
return xPos;
}
int player::getY() {
return yPos;
}
void onGameStart() {
player Player;
Player.setPlayerPos(3,10);
}
void main() {
onGameStart();
while (gameIsRunning) {
onGameDraw();
}
}
Of course, everything starts in main() on the bottom of the code. It runs a simple onGameStart() function which is supposed to set the player's beginning coordinates.
Here is the thing. When I set the values of xPos and yPos when initializing those variables, Player.getX() and Player.getY() will always return the correct value. However, when set them using Player.setPlayerPos(x,y), then Player.getX() and Player.getY() returns junk. (Or original initialized values, if set).
I would like to know what I'm doing wrong and why Player.setPlayerPos(x,y) does not update xPos and yPos.

Answer 1

Try using a variable in main like this:
class player {
public:
int xPos, yPos;
void setPlayerPos(int x, int y);
int getX();
int getY();
};
void player::setPlayerPos(int x, int y) {
xPos = x;
yPos = y;
return;
}
int player::getX() {
return xPos;
}
int player::getY() {
return yPos;
}
void onGameStart(player & Player) {
Player.setPlayerPos(3,10);
}
int main() {
player Player;
onGameStart(Player);
while (gameIsRunning) {
onGameDraw(Player); // Need to know more about the implementation but it should be similiar to onGameStart.
}
}

Answer 2

your code seems not to be complete ...
this code seems to be not usable:
void onGameStart() {
player Player;
Player.setPlayerPos(3,10);
}
because you are changing the local variable - those player is destroyed outside of function scope. The player you are using in your code is probably some other instance - so it's x and y are not those set in your function, but some junk x and y value - because you are not initializing them in the constructor.
If you declare those player Player globaly (which is not a good practice) then your code should work (but global variables are always bad idea). Maybe try passing those player via pointer or reference to the function onGameStart

Answer 3

In the function onGameStart you are creating a local variable player and you set its position but then after the function ends you can no longer access it, so it is useless. Maybe you want to have a global variable player and set its position on game start? I suspect you are confusing the local variable and another one which you aren't modifying from this function and you aren't getting the results you want.

Answer 4

The following object: player Player; is local to your freestanding void onGameStart() function. Once that function goes out of scope the object gets destroyed. Your other object's data members remain uninitialized hence the garbage values. Your local object has nothing to do with other (main?) objects you have in a program:
int main() {
player p;
onGameStart(); // only modifies its internal object, not the p
std::cout << p.getX(); // prints garbage because xPos is uninitialized
std::cout << p.getY(); // prints garbage because yPos is uninitialized
}
And if you want the onGameStart() function to be able to modify the object one way is to pass the object by reference:
void onGameStart(player& p) {
p.setPlayerPos(3, 10);
}
Now this code prints 3 and 10:
int main() {
player p;
onGameStart(p); // now modifies the p
std::cout << p.getX(); // prints 3
std::cout << p.getY(); // prints 10
}
Maybe that's the confusion. Initialize your data members in a constructor:
class player {
public:
int xPos, yPos;
player() : xPos(0), yPos(0) {}
};