Answer 1

class methods in C++ are private by default.you have to declare them public.

#include <iostream>
using namespace std;

class BankAccount {
public:
void sayHi() {
cout << "Hi" << endl;
}
};

int main()
{
BankAccount test;
test.sayHi();
}

Answer 2

or maybe you are asking about scope resolution operators?

https://www.tutorialspoint.com/Scope-resolution-operator-in-Cplusplus#:~:text=The%20scope%20resolution%20operator%20can,during%20a%20block%20or%20class.

#include <iostream>
using namespace std;
class X {
public:
static int count;
};
int X::count = 10; // define static data member

int main () {
int X = 0; // hides class type X
cout << X::count << endl; // use static member of class X
}