C++ When do I use -> or ::

I know that in C++, you use either -> or :: instead of . in language such as C# to access object's values, e.g. button->Text or System::String^, but I don't know when I should use -> or ::, and it is very frustrating as it causes me many compiler errors. I would be very grateful if you could help. Thanks :)

I try to show an examples of some usages of ::, . and ->. I hope it helps:

int g;

namespace test
{

  struct Test
  {
     int x;
     static void func();
  };

  void Test:: func() {
     int g = ::g;
  }

}

int main() {

  test::Test v;
  test::Test *p = &v;

  v.x = 1;
  v.func();
  p->x = 2;
  p->func();

  test::Test::func();

}