如何将不同的c++类映射到枚举类值

Question

生成消息，每条消息由enum class成员选择的一个对象接收:

enum class ReceiverID
{
R1,
R2,
MAX_NUM_RECEIVERS
};
struct Msg
{
ReceiverID _receiverID;
Data _data;
};

接收类存储在数组中。枚举成员索引数组以访问接收对象:

void receive(const Msg& msg)
{
const size_t arrIndex = static_cast<size_t>(msg._receiverID);

if(nullptr == _array[arrIndex])
{
_array[arrIndex] = ???  // How do I associate the enum to the class?
}

_array[arrIndex].processMsg(msg);
}

接收对象可能丢失。如果发生这种情况，我想使用枚举来实例化丢失的对象。但是，这需要将枚举值映射到接收对象类型。

如何将类映射到每个enum?(适用于所有枚举)。

我想生成一个编译器错误，如果一个新的枚举被添加，但没有相应的接收器类。

更新接收对象是多态的，因此有基类。数组为:

std::array<Base*, MAX_NUM_RECEIVERS> _array;

(去掉unique_ptr以简化问题)

Answer 1

对于动态创建对象，我们可以使用某种工厂方法，例如:

//In the Base class:
static Base* createReceiver(ReceiverID recvID) //static member function
{
switch (recvID)
{
case ReceiverID::R1: return new R1Receiver();
case ReceiverID::R2: return new R2Receiver();
//...
default: throw std::logic_error("Invalid ReceiverID");
}
}
//...
void receive(const Msg& msg) except(true)
{
const size_t arrIndex = static_cast<size_t>(msg._receiverID);
if(nullptr == _array[arrIndex])
{
_array[arrIndex] = Base::createReceiver(msg._receiverID);
}    
_array[arrIndex]->processMsg(msg);
}

Answer 2

而不是有一个全局的std::array<Base*, MAX_NUM_RECEIVERS> _array;，然后懒惰地填写它的需求，我认为正常的事情是让它填写在施工时间:

std::array<Base*, MAX_NUM_RECEIVERS>& _array() {
//use a method to bypass https://stackoverflow.com/questions/1005685/c-static-initialization-order
static std::array<Base*, MAX_NUM_RECEIVERS> array = make_array();
return array;
}
std::array<Base*, MAX_NUM_RECEIVERS> make_array() {
std::array<Base*, MAX_NUM_RECEIVERS> array;
array[static_cast<size_t>(R1)] = &myR1ProcessorObject();
array[static_cast<size_t>(R2)] = &myR2ProcessorObject();
return array;
}

那么你的receive方法很简单:

void receive(const Msg& msg)
{
const size_t arrIndex = static_cast<size_t>(msg._receiverID);
assert(arrIndex< MAX_NUM_RECEIVERS);
_array()[arrIndex].processMsg(msg);
}