why does the element before being entered already have a value? - c++

Question

I'm learning about C++ and when I was learning about Class, I ran into a problem. I try cout my element before entering the value, but the element already has a value. I can't understand why?
class cusTomer
{
char abc[30];
public:
void input();
void output();
};
void cusTomer::input(){
cout<<"abc: "; fflush(stdin); gets(abc);
}
void cusTomer::output(){
cout<<"abc: "<<abc<<endl;
}
class proDuct
{
char name[30];
public:
void input();
void output();
};
void proDuct::input()
{
cout<<"Name: "; fflush(stdin); gets(name);
}
void proDuct::output()
{
cout<<"Name: "<<name<<endl;
}
class bill
{
char date[30];
proDuct *y;
int n;
public:
void input();
void output();
};
void bill::input()
{
cout<<"Date: "; fflush(stdin); gets(date);
cout<<n<<endl; //16
cout<<"Input n: "; cin>>n;
cout<<n<<endl;
y=new proDuct[n];
for(int i=0;i<n;i++){
y[i].input();
}
}
void bill::output()
{
cout<<"Date: "<<date<<endl;
for(int i=0;i<n;i++){
y[i].output();
}
}

Answer 1

but the element already has a value.
An integer always has a value. There is no concept of valueless integer in C++.
If you haven't initialised an integer, then that value is indeterminate. If you read an indeterminate value, then the behaviour of the program is undefined. You should never read an indeterminate value.
P.S. gets is no longer part of C++, and it should never have been used.
my class is taught that fflush(stdin) to clear the buffer memory, then gets()/cin.getline() to input a string
Your class has been taught bad things.