不能使用好友 CRTP 的私有构造函数 class

Question

我有使用此设计的代码，经过简化以获得此 MCVE - 代码和编译器错误如下。

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

但显然不是。为什么？

#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：用例是我希望模板库 class 通过静态工厂控制（CRTP）派生 class 实例的生命周期 - 包括构造。所以我想要派生classes 将它们的构造函数声明为私有的，但父类的静态工厂方法可以访问。此示例显示了在堆栈上创建的派生 class 实例，但如果它是在堆中创建的（和返回）。）

Answer 1

您的 friend 声明有误 class。 Derived 需要声明 Base<Derived> 为好友，因为好友可以访问私有成员。（A class 声明另一个 class 为朋友。A class 没有声明自己为另一个 class 的朋友。）

您要添加

friend Base<Derived>;

进入Derived class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。如果您不需要一个（如上面的示例所示），则仅在 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()

不能使用好友 CRTP 的私有构造函数 class

Can't use private constructor of befriended CRTP class

c++

friend

crtp