不能使用友好的 CRTP 类的私有构造函数

Question

我有使用这种设计的代码，经过简化以获得这个 MCVE - 代码和编译器错误随之而来。

基本问题是我认为与 CRTP 类交朋友将允许模板化基类访问派生 CRTP 类的私有成员，包括其私有构造函数。

但显然不是。为什么？

this code on wandbox:

#include <iostream>
using namespace std;

template <class CRTP>
class Base
{
  friend CRTP;
public:
  static void Factory()
  {
      cout << "Base::Factory()" << endl;
      CRTP x;
      x.Hello();
  }
  virtual void Hello() { cout << "Base::Hello()" << endl; }
protected:
  Base() { cout << "Base::Base()" << endl; }
  virtual ~Base() { cout << "Base::~Base()" << endl; }
};


class Derived final : public Base<Derived>
{
public:
  void Hello() override;
private:
  Derived() { cout << "Derived::Derived()" << endl; }
  ~Derived() override { cout << "Derived::~Derived()" << endl; }
};

int main()
{
    Derived::Factory();
    // Expected output:
    //   Base::Factory()
    //   Base::Base()
    //   Derived::Derived()
    //   Derived::Hello()
    //   Derived::~Derived()
    //   Base::~Base()
}

并得到这个编译器错误（来自 clang 9.0.0，但 gcc 以同样的方式抱怨）：

prog.cc:12:12: error: calling a private constructor of class 'Derived'
      CRTP x;
           ^
prog.cc:33:14: note: in instantiation of member function 'Base<Derived>::Factory' requested here
    Derived::Factory();
             ^
prog.cc:27:3: note: declared private here
  Derived() { cout << "Derived::Derived()" << endl; }
  ^
prog.cc:12:12: error: variable of type 'Derived' has private destructor
      CRTP x;
           ^
prog.cc:28:11: note: declared private here
  virtual ~Derived() { cout << "Derived::~Derived()" << endl; }
          ^
2 errors generated.

（仅供参考：用例是我希望模板基类通过静态工厂控制（CRTP）派生类实例的生命周期——包括构造。所以我希望派生类将其构造函数声明为私有，但可访问父类的静态工厂方法。此示例显示了在堆栈上创建的派生类实例，但如果在堆中创建（并返回）也会发生相同的错误。）

Answer 1

你在错误的类中得到了friend 声明。 Derived 需要将Base<Derived> 声明为好友，因为好友可以访问私有成员。 （一个类将另一个类声明为友元。一个类不将自己声明为另一个类的友元。）

你想添加

friend Base<Derived>;

进入Derived 类声明。

Answer 2

您提到的具体问题是您将friend 声明放在Base 而不是Derived。

下一个问题是Derived::Hello() 没有实现。如果您不需要（如上面的示例）仅在Base 中实现它并删除virtual 声明。 Derived 无论如何都会继承它。

以下作品：

#include <iostream>

using namespace std;

template <class CRTP>
class Base
{
 public:
  static void Factory()
  {
    cout << "Base::Factory()" << endl;
    CRTP x;
    x.Hello();
  }
  void Hello() { cout << "Base::Hello()" << endl; }

 protected:
  Base() { cout << "Base::Base()" << endl; }
  virtual ~Base() { cout << "Base::~Base()" << endl; }
};

class Derived final : public Base<Derived>
{
  friend Base<Derived>;
/*^^^^^^ friend declaration goes here, not in Base */

  Derived() { cout << "Derived::Derived()" << endl; }
  ~Derived() override { cout << "Derived::~Derived()" << endl; }
}; 

输出：

Base::Factory()
Base::Base()
Derived::Derived()
Base::Hello()
Derived::~Derived()
Base::~Base()