如何允许一个类由另一个类创建而不使用'friend'但允许继承？

如果使用工厂设计模式，ComponentManager不需要了解IComponent的具体子类型。没有必要将其声明为子类型的朋友。它可以简单地使用工厂来构造对象。

IComponent子类型的创建者需要注册一种方法来构造子类型的实例。它们向可以构造类实例的工厂注册函数或类。

示例程序

#include <iostream>
#include <map>
#include <string>
class GameObject;
class IComponent
{
   // Make sure that sub-classes of IComponent can use the constructor
   // and the destructor.
   protected:
      IComponent() { }
      ~IComponent() { }
   public:
      GameObject* Parent;
};
// Define a function type that can construct a Component.
using ComponentConstructor = IComponent*  (*)();
// Define the interface for the factory.
class ComponentFactory
{
   public:
      // A type alias for simpler coding.
      using ConstructorMap = std::map<std::string, ComponentConstructor>;
      // Allow creators of sub-classes of IComponent to register a
      // function that can be used to construct the sub-type.
      static void registerComponentConstructor(std::string const& componentType,
                                               ComponentConstructor constructor);
      // Construct a Component by providing a name corresponding
      // to the derived sub-type of IComponent.
      static IComponent* constructComponent(std::string const& componentType);
   private:
      // Private function that maintains a map of
      // constructors.
      static ConstructorMap& getConstructrMap();
};
// -----------------------------------------------------
// BEGIN implementation of ComponentFactory.
// It can, obviously, be in a .cpp file of its own.
void ComponentFactory::registerComponentConstructor(std::string const& componentType,
                                                    ComponentConstructor constructor)
{
   getConstructrMap()[componentType] = constructor;
}
IComponent* ComponentFactory::constructComponent(std::string const& componentType)
{
   ConstructorMap& constructorMap = getConstructrMap();
   ConstructorMap::iterator iter = constructorMap.find(componentType);
   if ( iter != constructorMap.end() )
   {
      return iter->second();
   }
   else
   {
      return nullptr;
   }
}
ComponentFactory::ConstructorMap& ComponentFactory::getConstructrMap()
{
   static ConstructorMap theMap;
   return theMap;
}
// END implementation of ComponentFactory.
// -----------------------------------------------------
// ComponentManager can use ComponentFactory to 
// construct Components.
class ComponentManager
{
   public:
      ComponentManager() { }
      ~ComponentManager() { }
      IComponent* CreateComponent(std::string const& componentType)
      {
         return ComponentFactory::constructComponent(componentType);
      }
};
// Test code.
// Construct IComponents by using appropriate names.
int main()
{
   ComponentManager m;
   IComponent* ic1 = m.CreateComponent("RenderComponent");
   if ( ic1 == nullptr )
   {
      std::cout << "Unable to construct a Component of type RenderComponent.n";
   }
   else
   {
      std::cout << "Successfully constructed a Component of type RenderComponent.n";
   }
   IComponent* ic2 = m.CreateComponent("AnotherTypeOfComponent");
   if ( ic2 == nullptr )
   {
      std::cout << "Unable to construct a Component of type AnotherTypeOfComponent.n";
   }
   else
   {
      std::cout << "Successfully constructed a Component of type AnotherTypeOfComponent.n";
   }
   IComponent* ic3 = m.CreateComponent("FooComponent");
   if ( ic3 == nullptr )
   {
      std::cout << "Unable to construct a Component of type FooComponent.n";
   }
   else
   {
      std::cout << "Successfully constructed a Component of type FooComponent.n";
   }
}
// Client components.
// Without these, no Component can be constructed.
namespace Module1
{
   class RenderComponent : IComponent
   {
      public:
         RenderComponent() { }
         ~RenderComponent() { }
         static IComponent* constructComponent()
         {
            return new RenderComponent();
         }
         struct Initer
         {
            Initer()
            {
               ComponentFactory::registerComponentConstructor("RenderComponent",
                                                              RenderComponent::constructComponent);
            }
         };
   };
   // The constructor makes sure that
   // RenderComponent::constructComponent() is
   // registered as the function to be called to
   // construct objects of type RenderComponent when
   // the name "RenderComponent" is used.
   // 
   // A different method may be used for the purpose but
   // this seems like a straight forward method to do that.
   static RenderComponent::Initer initer;
}
namespace Module2
{
   class AnotherTypeOfComponent : IComponent
   {
      public:
         AnotherTypeOfComponent() { }
         ~AnotherTypeOfComponent() { }
         static IComponent* constructComponent()
         {
            return new AnotherTypeOfComponent();
         }
         struct Initer
         {
            Initer()
            {
               ComponentFactory::registerComponentConstructor("AnotherTypeOfComponent",
                                                              AnotherTypeOfComponent::constructComponent);
            }
         };
   };
   // The constructor makes sure that
   // AnotherTypeOfComponent::constructComponent() is
   // registered as the function to be called to
   // construct objects of type AnotherTypeOfComponent when
   // the name "AnotherTypeOfComponent" is used.
   static AnotherTypeOfComponent::Initer initer;
}

输出

Successfully constructed a Component of type RenderComponent.
Successfully constructed a Component of type AnotherTypeOfComponent.
Unable to construct a Component of type FooComponent.

首先，我要感谢 R Sahu 提供他的答案，但在他的示例中，仍然允许用户做我首先想要避免的事情，随机能够创建一个组件的实例：

IComponent* component = new RenderComponent();

现在我意识到我想要的是不可能的，没有办法限制构造函数/析构函数对用户的访问，而不会限制对组件管理器的访问。因此，要么我的实现是错误的或不完整的(这不太可能，但有可能(，要么我要求的东西不合逻辑(最有可能的答案(。

因此，我只是选择将所有内容公开，只是使随机创建的组件无用，在我的引擎文档中，我将描述您必须使用GameObject：：AddComponent((才能创建和使用组件。

谢谢。