如何禁止公共继承但允许私有(和受保护)继承
C++11引入了关键字final来禁止将来的覆盖或禁止继承。可以使用它的最常见的例子是不打算用作基类的类的情况(例如，具有非虚拟析构函数)。然而，有时我们可能希望两个类之间存在is-implemented-in-terms-of关系(即private继承)，而不是is-a关系(public继承)。但是，final禁止这两种类型的继承。
我的问题如下:有没有办法既允许private继承又禁止public继承(可能不是直接的，但至少我们可以“模拟”它)？在这种情况下，即使我们使用带有非虚析构函数的类也不会有任何问题，因为我们不能通过指向基类的指针直接使用派生类，所以我们应该没问题。
我想的代码是这样的：
class Base /*final*/ {}; // making it final prohibits both private and public inheritance
class PrivateDerived: private Base{}; // this should work
class PublicDerived: public Base{}; // this shouldn't
int main()
{
PrivateDerived prvd;
PublicDerived pubd; // this should not compile
// Base* pBase = new PrivateDerived; // doesn't work, so we are ok
}回答 2
Stack Overflow用户
发布于 2015-05-05 18:17:44
有趣的问题！如果你不介意放弃析构函数的琐碎之处，我认为下面的方法就可以完成任务：
#include <type_traits>
template <typename T>
class Base {
protected:
~Base() {
static_assert(!std::is_convertible<T*,Base*>::value, "Invalid use of public inheritance.");
}
};
class Derived : public Base<Derived> {
};
int main() {
Derived d;
}上面的代码无法编译:因为Derived*可以转换为Base<Derived>*，所以会触发static_assert。但是，如果将继承更改为protected或private，则代码将进行编译。
不幸的是，用户仍然可以搬起石头砸自己的脚：
class Bad : public Base<Derived> {
};Stack Overflow用户
发布于 2015-05-05 10:43:32
我不确定这是你想要的，还是对你的情况有帮助。但是，我将演示多态行为。
受保护的构造函数抽象类
class BaseProtected {
// ----- Member Variable Section -----
public:
// There Shouldn't Be Public Variables In A Base Class Unless
// That Is The Behavior You Are Looking For - Keep In Mind
// Every Inherited Class & Outside Class Can Change Them.
protected:
// Member Variables Here To Be Shared With Each Derived Class
private:
// Member Variables Here To Be Used By Base Class Only
// ----- Member Function Section -----
public:
virtual ~BaseProtected(); // Virtual Destructor
void somefunc() const; // Common Function Between All Derived Class
virtual void allDerivedMustImplement() const; = 0 // Purely Virtual
protected:
// Default Constructor - Can Not Declare An Instance Of Base Class
BaseProtected(); // Abstract Class
// Protected Functions Shared Between Classes
// Protected Functions That Are Purely Virtual If Needed
private:
// Private Functions Used By Base Class Only
}; // BaseProtected具有可能继承的派生类
class DerivedWithPossibleInheritance : public BaseProtected {
// ----- Member Variable Section -----
public:
// Public Member If Giving Free Access
protected:
// Protected Members If Being Inherited From
private:
// Private Members Unique To This Derived Class
// ----- Member Function Section -----
public:
DerivedWithPossibleInheritance(); // Default Constructor
virtual ~DerivedWithPossibleInheritance(); // Virtual Destructor
void uniqueFunctionForThisClass() const;
void allDerivedMustImplement() const override;
private:
// Private Functions Unique To This Class
}; // DerivedWithPossibleInheritance 无法继承的派生类
class DerivedClassCanNotBeInheritedFrom sealed : public BaseProtected {
// ----- Member Variable Section -----
public:
// Public Members Variables
protected:
// Should Not Have Member Variables Here For This Class Can Not Be Inherited from
private:
// Private Members Variables
// ----- Member Function Section ------
public:
DerivedClassCanNotBeInheritedFrom(); // Default Constructor
virtual ~DerivedClassCanNotBeInheritedFrom(); // Default Virtual Destructor
void anotherUniqueFunctionForThisClass() const;
void allDerivedMustImplement() const override;
protected:
// There Should Not Be Any Functions Here This Can Not Be Inherited From
private:
// Private Member Functions Here
}; // DerivedClassCanNotBeInheritedFrom我在这里演示的是使用继承和多态时的关键字sealed。如果您不想让您的类派生自，则使用sealed关键字。
对于声明任何基类、独立类或具有私有构造函数的单例类，都需要使用friend关键字。这里涉及到许多类型的实现来展示它们，但是防止类被继承的概念是相同的。就我个人而言，我没有使用关键字final，但我使用了关键字sealed，它工作得非常好。
我没有使用过除public类以外的其他类的继承:因此，从受保护或私有继承的角度回答您的问题并不是我真正熟悉的事情。但是也许使用关键字sealed可能会对您有所帮助。
https://stackoverflow.com/questions/30038266
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