what does double semicolon operator (::) do?

Answer 1

this is scope resolution operator.
this is used to show the scope of a variable or a function...
for example.
class A
{
public:
void display();
};
void A::display ()
{
cout<<"hello world";
}

Answer 2

its scope resolution operator and used to define constructors

Answer 3

not semicolon
double colon used for constructor

Answer 4

It's called scope resolution operator