void DoSomething(const String& paramOne) {}

void do_something(const string& param_one) {}

class X
{
  class Y { public: void operator() {} };
public:
  Y variableName;
  void CallMe() {}
};

class x
{
  class y { public: void operator() {} };
public:
  y variable_name;
  void call_me() {}
};

int main()
{
  X camelCaseNotation;
  x underscore_notation;
  camelCaseNotation.variableName(); // means calling operator () from variableName
  camelCaseNotation.CallMe(); // means calling a member function

  underscore_notation.variable_name(); // looks like we are calling a member function
  underscore_notation.call_me(); // ok, we are calling a member function
  return 0;
}