如何在C++类的外部调用类的私有方法

class Widget {
   private:
    void forbidden();
  };
void hijack(Widget& w) {
    w.forbidden();  // ERROR!
  }

class Calculator {
  float current_val = 0.f;
 public:
   void clear_value() { current_val = 0.f; };
   float value() const {
     return current_val;
   };

   void add(float x) { current_val += x; };
   void multiply(float x) { current_val *= x; };
};

  using Operation = void (Calculator::*)(float);
 
  Operation op1 = &Calculator::add;
  Operation op2 = &Calculator::multiply;
  
  using Getter = float (Calculator::*)() const;
  Getter get = &Calculator::value;
  
  Calculator calc{};
  (calc.*op1)(123.0f); // Calls add
  (calc.*op2)(10.0f);  // Calls multiply
  // Prints 1230.0
  std::cout << (calc.*get)() << '\n';

class Widget {
 public:
  static auto forbidden_fun() {
    return &Widget::forbidden;
  }
 private:
  void forbidden();
};

void hijack(Widget& w) {
  using ForbiddenFun = void (Widget::*)();
  ForbiddenFun const forbidden_fun =
    Widget::forbidden_fun();

  // Calls a private member function on the Widget
  // instance passed in to the function.
  (w.*forbidden_fun)();
}

class Foo
{
private:
  struct Bar;

  template<typename T> class Baz { };

public:
  void f();  // does things with Baz<Bar>
};

// explicit instantiation declaration
extern template class Foo::Baz<Foo::Bar>;

template <class T, int size> // size is the non-type parameter
class StaticArray
{
private:
    // The non-type parameter controls the size of the array
    T m_array[size];
 
public:
    T* getArray();
	
    T& operator[](int index)
    {
        return m_array[index];
    }
};

class SpaceShip {
 public:
  void dock();
  // ...
};

// Member function alias that matches the
// signature of SpaceShip::dock()
using SpaceShipFun = void (SpaceShip::*)();

// spaceship_fun is a pointer-to-member-function
// value which is baked-in to the type of the
// SpaceStation template at compile time.
template <SpaceShipFun spaceship_fun>
class SpaceStation {
  // ...
};

// Instantiate a SpaceStation and pass in a
// pointer to member function statically as a
// template argument.
SpaceStation<&SpaceShip::dock> space_station{};

template <
  typename SpaceShipFun,
  SpaceShipFun spaceship_fun
>
class SpaceStation {
  // ...
};

// Now we must also pass the type of the pointer to
// member function when we instantiate the
// SpaceStation template.
SpaceStation<
  void (SpaceShip::*)(),
  &SpaceShip::dock
> space_station{};

 SpaceStation<
    decltype(&SpaceShip::dock),
    &SpaceShip::dock
  > space_station{};

// The first template parameter is the type
// signature of the pointer-to-member-function.
// The second template parameter is the pointer
// itself.
template <
  typename ForbiddenFun,
  ForbiddenFun forbidden_fun
> struct HijackImpl {
  static void apply(Widget& w) {
    // Calls a private method of Widget
    (w.*forbidden_fun)();
  }
};

// Explicit instantiation is allowed to refer to
// `Widget::forbidden` in a scope where it's not
// normally permissible.
template struct HijackImpl<
  decltype(&Widget::forbidden),
  &Widget::forbidden
>;

void hijack(Widget& w) {
    HijackImpl<
      decltype(&Widget::forbidden),
      &Widget::forbidden
    >::apply(w);
  }

error: 'forbidden' is a private member of 'Widget'
   HijackImpl<decltype(&Widget::forbidden),
     &Widget::forbidden>::hijack(w);

class Gadget {
  // Friend declaration gives `frobnicate` access
  // to Gadget's private members.
  friend void frobnicate();

 private:
  void internal() {
    // ...
  }
};

// Definition as a normal free function
void frobnicate() {
  Gadget g;
  // OK because `frobnicate()` is a friend of
  // `Gadget`.
  g.internal();
}

class Gadget {
  // Free function declared as a friend of Gadget
  friend void frobnicate() {
    Gadget g;
    g.internal(); // Still OK
  }

 private:
   void internal();
};

void do_something() {
  // NOT OK: Compiler can't find frobnicate()
  // during name lookup
  frobnicate();
}

class Gadget {
  friend void frobnicate(Gadget& gadget) {
    gadget.internal();
  }

 private:
   void internal();
};

void do_something(Gadget& gadget) {
  // OK: Compiler is now able to find the
  // definition of `frobnicate` inside Gadget
  // because ADL adds it to the candidate set for
  // name lookup.
  frobnicate(gadget);
}

class Gadget {
  // Definition stays inside the Gadget class
  friend void frobnicate() {
    Gadget g;
    g.internal();
  }

 private:
   void internal();
};

// An additional namespace-scope declaration makes
// the function available for normal name lookup.
void frobnicate();

void do_something() {
  // The compiler can now find the function
  frobnicate();
}

#include <iostream>

template <int N>
class SpookyAction {
  friend int observe() {
    return N;
  }
};

int observe();

int main() {
  SpookyAction<42>{};
  std::cout << observe() << '\n';  // Prints 42
}

namespace {
// This is a *different* type in every translation
// unit because of the anonymous namespace.
struct TranslationUnitTag {};
}
void hijack(Widget& w);
template <
  typename Tag,
  typename ForbiddenFun,
  ForbiddenFun forbidden_fun
> class HijackImpl {
  friend void hijack(Widget& w) {
    (w.*forbidden_fun)();
  }
};

// Every translation unit gets its own unique
// explicit instantiation because of the
// guaranteed-unique tag parameter.
template class HijackImpl<
  TranslationUnitTag,
  decltype(&Widget::forbidden),
  &Widget::forbidden
>;