. for accessing object member variables and methods via object instance
Foo foo;
foo.member_var=10;
foo.member_func();
:: for accessing static variables and methods of a class/struct or namespace. It can also be used to access variables and functions from another scope (actually class, struct, namespace are scopes in that case)
int some_val = Foo::static_var;
Foo::static_method();int max_int = std::numeric_limits<int>::max();