我有使用此设计的代码,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类成为朋友会允许模板化的基类访问派生的CRTP类的私有成员,包括其私有构造函数。
但显然它没有。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()
}
并获得此编译器错误(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。所以我希望派生类声明它们的构造函数是私有的,但可以访问父类的静态factory方法。此示例显示在堆栈上创建的派生类实例,但如果在堆中创建(并返回),则会发生相同的错误。)
你在错误的班级里得到了friend
声明。 Derived
需要宣布Base<Derived>
为朋友,因为朋友可以访问私人会员。 (一个类声明另一个类是朋友。一个类不会将自己声明为另一个类的朋友。)
你想要添加
friend Base<Derived>;
进入Derived
类声明。
你提到的具体问题是你把friend
声明放在Base
而不是Derived
。
接下来的问题是Derived::Hello()
没有实施。如果您不需要(如上例所示)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()