我的游戏引擎总共有三个类，一个是从中继承所有组件的抽象IComponent类，一个组件类（本例中我将使用RenderComponent）和一个ComponentManager。我希望ComponentManager类能够使用RenderComponent的构造函数，但我不希望任何其他类创建RenderComponent的实例，但我不想使用'friend'，因为我希望客户用户组件继承来自IComponent并自动在ComponentManager中使用，而不允许实例化它们自己。代码示例模糊地显示了我想要发生的行为：
class GameObject;
class IComponent
{
private:
IComponent() { }
~IComponent() { }
public:
GameObject* Parent;
}
class RenderComponent : IComponent
{
public:
RenderComponent() { }
~RenderComponent() { }
}
class ComponentManager
{
public:
ComponentManager() { }
~ComponentManager() { }
// normally this would be a template function, but for the sake of this example I will directly use RenderComponent
RenderComponent* CreateComponent()
{
// this would not throw a compiler error
return new RenderComponent();
}
}
int main()
{
ComponentManager manager;
// even though the constructor of RenderComponent is public, this would throw an error
RenderComponent* render = new RenderComponent();
// this however would work perfectly fine
RenderComponent* render = manager.CreateComponent();
}
重申一下，我想要它，以便创建组件的用户工作量最小。另一个选择当然是让两者都是公共构造函数，但是这样做，虽然你可以在任何你想要的地方创建一个组件，但它将毫无用处。
答案 0 :(得分：1)
如果您使用工厂设计模式，ComponentManager不需要了解IComponent的具体子类型。不需要将其声明为子类型的朋友。它可以简单地使用 factory 来构造对象。
IComponent子类型的创建者需要为要构造的子类型的实例注册一种方式。他们在工厂中注册一个函数或类，可以构造一个类的实例。
#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;
}
Successfully constructed a Component of type RenderComponent.
Successfully constructed a Component of type AnotherTypeOfComponent.
Unable to construct a Component of type FooComponent.
答案 1 :(得分：0)
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首先做我想要避免的事情，随机能够创建组件的实例：
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，也没有限制对ComponentManager的访问。所以要么我的实施是错误的还是不完整的（这是不可能的，但可能的）或者我要求的东西是不合逻辑的（最可能的答案）。
所以我只是选择公开所有内容，只是让随机创建的组件无用，在我的引擎文档中，我将描述你必须使用GameObject :: AddComponent（）才能创建和使用组件。
感谢。