Need help using objects and functions -- new to programming - c++

Question

I'm in beginning computer science class and we're currently learning about functions, however I thought that I would take it one step further and start teaching myself about objects early on. I tried to make this Average game score calculator, however upon entering the values 10, 20, 30 and 40 then entering the loop sentinel value of -999. Where I should get an average of 25, as its 100/4, I get 80. I've played around with it quite a bit but just cannot figure out what I'm doing wrong exactly -- I try not to ask for help anymore unless I really can't figure something out after a few hours of reading through google and my books. So any help would be much appreciated :)
EDIT: My question is: Why when I enter all these values do they not go through the proper route and get a correct average? (This is my first time ever even attempting to use objects and don't know very much about how to use them, but I figured trial and error would help me learn.)
Thank you in advanced!
P.S I'm new here, but I hope that I formatted this question correctly as to not get anyone mad at me again. Oh and I know I haven't used the iomanip or string libraries yet, however I plan to in the future so I just left them in there.
//COMSCI 110
//HUNTER DURNFORD
//INPUT
//SPECIAL COMPILER FUNCTIONS
//NONE
//PROCESSING
//
//DATA
//
//LIBRARIES
#include <iomanip>
#include <string>
#include <iostream>
using namespace std;
//PROGRAMMER DEFINED FUNCTIONS
//introduction
//
//Object Classes
//AvgScoreCalculator
//Object class
class AvgScoreCalculator
{
private:
//double total_Scores = game_Scores;// The total scores
//double score_Average = total_Scores / score_Count; // the average equation
public:
double total_Scores; // the total that gets input into the average function
double score_Average; // the average of all the games
double score_Count; // the counter for each individual game entry
double game_Scores; // the scores for the individual games
//char exit_Value;
};//end class
//introduction function
void introduction()
{
cout << "Enter any amount of characters and this program will encrypt them." << endl;
cout << "By Hunter Durnford." << endl;
cout << "Editor(s) used: XCODE TEXT EDITOR and JNotePad 2\n";
cout << "Compiler(s) used: XCODE\n";
cout << "File: " << __FILE__ << endl;
cout << "Complied: " << __DATE__ << " at " << __TIME__ << endl << endl;
}//introduction
//function to get scores from user
double getScoreFunction()
{
double score_Count = 0.0;
double gameScores = 0.0;
double total_Scores = 0.0;
while(true)
{
cout << "What score would you like to input? Or type -999 to exit. ";
cin >> gameScores;
if(gameScores == -999)
{
break;
}
score_Count = score_Count + 1;
gameScores = gameScores + gameScores;
total_Scores = gameScores;
}
return total_Scores;
}//end score function
//Average of all the scores function
double getAverageFunction(double total_Scores, double score_Count)
{
double average_Score;
average_Score = total_Scores / score_Count;
return average_Score;
}
//main program
int main()
{
introduction();
AvgScoreCalculator average_Score; //Declaring the object under the class
//set the integers for the average_score object
average_Score.score_Count = 0.0;
average_Score.game_Scores = 0.0;
average_Score.score_Average = 0.0;
average_Score.total_Scores = 0.0;
average_Score.game_Scores = getScoreFunction();
average_Score.score_Average = getAverageFunction(average_Score.total_Scores, average_Score.score_Count);
cout << "The average of all the game scores is: " << average_Score.game_Scores << endl;
}

Answer 1

(leaving apart problems with the precision of doubles that has been pointed out in the comments)
Problem here:
score_Count = score_Count + 1;
gameScores = gameScores + gameScores;
total_Scores = gameScores;
play it in your head:
gameScores=0.0
total_Scores=0.0
... now you read 10
gameScores=10.0
gameScore=gameScore + gameScore -> 10.0 + 10.0 = 20.0
total_Scores = gameScore = 20.0
... next loop just overwrites.
should be:
score_Count = score_Count + 1;
total_Scores += gameScores;
in addition, score_Count is a local variable of getScoreFunction()... so its changes do not affect the average_Score.score_Count used for the average calculation.