What does the “::” mean in C++?

:: is the scope resolution operator – used to qualify names. In this case it is used to separate the class AirlineTicket from the constructor AirlineTicket(), forming the qualified name AirlineTicket::AirlineTicket()

You use this whenever you need to be explicit with regards to what you’re referring to. Some samples:

namespace foo {
  class bar;
}
class bar;
using namespace foo;

Now you have to use the scope resolution operator to refer to a specific bar.

::foo::bar is a fully qualified name.

::bar is another fully qualified name. (:: first means “global namespace”)

struct Base {
    void foo();
};
struct Derived : Base {
    void foo();
    void bar() {
       Derived::foo();
       Base::foo();
    }
};

This uses scope resolution to select specific versions of foo.