C++: no member named 'xPos' in 'Star' - c++

Question

Trying to compile my code in Xcode, but I am obviously running into some problems as I get the error in the title. Here is the code for my header file called "myClasses.h":
#ifndef myClasses_h
#define myClasses_h
class Star
{
public:
Star(int x,int y)
{
int xPos = x;
int yPos = y;
}
};
#endif
So I obviously want a constructor for Star so I can declare a Star object like this:
Star sol(10,30);
Then the code in my "main.cpp":
#include <iostream>
#include <math.h>
#include <stdlib.h>
#include <vector>
#include "myClasses.h" //includes the Star class
bool checkOverlap(Star check, int a, int b)
{
double diffX = a - check.xPos;
double diffY = b - check.yPos;
double checkDist = sqrt( pow( diffX,2.0 ) + pow( diffY,2.0) );
if( checkDist > 20 )
return true;
else
return false;
};
int main()
{
//some other code here with no errors
};
Here in the main file I get the error, so I am lost as to what is going wrong? Am I using incorrect syntax to access the object's member variables? Shouldn't the code below print 10:
Star sol(10,30);
cout << sol.xPos
If not, how do I rewrite my class so it behaves like that or how do I properly access the member variables in the constructor?

Answer 1

You need to declare the variables in your class, you can't do that in the body of the constructor as those will be temporary variables not associated with the class instance. This will leave the xPos and yPos as their default values (which is 0) which is probably not what you want. Try something like this instead:
class Star
{
public:
int xPos;
int yPos;
Star(int x,int y):
xPos(x), yPos(y) //initializing the variables here
{
}
};
I've used a member initializer list here to initialize the members.
It's worth noting that this is likely not the best design for a class, you probably want to make xPos and yPos private along with some functions to change those values. You probably want to read up about encapsulation. Essentially you want to hide away information so that people don't need to know the internals of how your classes work in order to use them. This big benefit is that this lets people use your code without needing to worry about the internals of how your code works and lets them keep using your code without having to change their code even if some of those internal details happen to change over time. Imagine the hassle if you had to know exactly how your network card driver was programmed in order to write an application that used the network. It would be a big pain, you might have to change your code whenever you updated the other code, however because this driver code has (hopefully) been encapsulated you don't need to worry about these details in order to use that code. The code and classes you write are no different, think about the people who will use them, try to make it easy for them to use your code.
A possibly better design would be to do something like this:
class Star
{
private:
int xPos;
int yPos;
public:
Star(int x,int y):
xPos(x), yPos(y) //initializing the variables here
{
}
int get_xPos(){
return xPos;
}
int get_yPos(){
return yPos;
}
};
Now in your main code you change:
Star sol(10,30);
cout << sol.xPos;
To:
Star sol(10,30);
cout << sol.get_xPos();
The benefits of doing it this way really start to become more obvious when you get larger software or you have to deal with changes. For example later on the code changes and we decide to store the coordinates in a coordinates struct:
struct coords{
int xPos;
int yPos;
}
class Star
{
private:
coords Pos;
public:
Star(int x,int y):
Pos{x,y} //initializing the variables here
{
}
int get_xPos(){
return Pos.xPos;
}
int get_yPos(){
return Pos.yPos;
}
};
The original code would break:
Star sol(10,30);
cout << sol.xPos; //There's no xPos anymore
but with our new design this:
Star sol(10,30);
cout << sol.get_xPos();
Works just like before! We only needed to change the code in one place in the getter function get_xPos() and everthing will keep working just like it did before we made the changes.

Answer 2

Declare member variables:
class Star
{
public:
int xPos;
int yPos;
Star(int x,int y)
{
xPos = x;
yPos = y;
}
};