c++ - cpp中如何根据数据类型将数据推送到不同的队列？

Question

我在根据数据类型将数据推送到不同队列时遇到问题。

具体来说，我有多个请求类。例如:

class RequestA {
};
class RequestB {
};
class RequestC {
};
...

这些请求可能会被某些类继承(例如，class Request)。

这些数据类型的每个对象必须放入不同的队列中。例如:

std::vector<RequestA> queueA;
std::vector<RequestB> queueB;
std::vector<RequestC> queueC;
...

我需要为每个类使用不同队列的原因是，队列顶部(前面) 的每个请求都需要不同的(后)处理步骤后面。

例如，

class RequestHandler {
public:
  void tick() {
     // Multiple requests might be accumulated in the queue before calling tick()
     if (!queueA.empty()) {
        processA(queueA.front())
        queueA.pop_front(); // assume std::vector has pop_front()
     }
     // queueB can be empty although other queues are full of elements
     if (!queueB.empty()) {
       ...
     }
     ...
     // Polymorphism might be used.
     // if RequestA, RequestB, ... are derived by Request:
     if (!poly_queue.empty()) {
        Request* req = poly_queue.top();
        req->process(/* status of handler */);
        poly_queue.pop_front(); // assume std::vector has pop_front() 
     // However, the result is not same as one using separated queue,
     // because the order of requests is not preserved.
     // For example, in the separated queue example, the processing order might be, 
     // RequestA -> Request C or only RequestA. 
     // However, if I use the single queue, we cannot achieve same result.
     
   }
private:
 std::vector<RequestA> queueA;
 std::vector<RequestB> queueB;
 std::vector<RequestC> queueC;
...
 std::vector<Request*> poly_queue;
};

但是，问题是，要将数据推送到队列中，我可能会使用以下方法:

if (typeid()) {
   queueA.push_back();
} else if (typeid()) {
   queueB.push_back();
} ...

或者如果 RequestA, RequestB, ... 被 Request 类继承:

std::map<std::type_index, std::vector<Request*>> queue;

queue[typeid()].push_back(...);

可能会用到。

但是，最好避免使用 typeid 或 dynamic_cast。

如何有效处理这种情况？

Answer 1

一种可能的方法是使用模板和 std::map .特别是，您可以创建一个名为 addRequest 的成员函数模板对于 RequestHandler类来处理不同的请求。

方法一

第一步

创建一个 std::map .

std::map<std::type_index, void*> myMap{{typeid(RequestA), &queueA}, 
                                       {typeid(RequestB), &queueB}, 
                                       {typeid(RequestC), &queueC}};

第二步

为成员函数模板添加声明add<>内部类RequestHandler .

template<typename T> void addRequest(T Arg);

第三步

实现 addRequest成员函数模板。

template<typename T> void RequestHandler::addRequest(T Arg) // - STEP 3
{
    
   
    (*static_cast<std::vector<decltype(Arg)>*>(myMap.at(typeid(Arg)))).push_back(Arg);
    
}

工作示例

#include <iostream>
#include <map>
#include <vector>
#include <typeindex>


struct RequestA{};
struct RequestB{};
struct RequestC{};


class RequestHandler {
    public:
        void tick() 
        {
            std::cout<<"tick called"<<std::endl;
        }
    private:
        std::vector<RequestA> queueA;
        std::vector<RequestB> queueB;
        std::vector<RequestC> queueC;
 
        //create std::map -                             STEP 1
        std::map<std::type_index, void*> myMap{{typeid(RequestA), &queueA}, 
                                               {typeid(RequestB), &queueB}, 
                                               {typeid(RequestC), &queueC}};
    public:
        //create member function template -             STEP 2
        template<typename T> void addRequest(T Arg);

};
template<typename T> void RequestHandler::addRequest(T Arg) // - STEP 3
{
    std::cout  << "add called on " <<typeid(Arg).name() << std::endl;//just for debugging purposes
    std::cout << "size before push_back "<< (*static_cast<std::vector<decltype(Arg)>*>(myMap.at(typeid(Arg)))).size()<<std::endl;//just for debugging
    (*static_cast<std::vector<decltype(Arg)>*>(myMap.at(typeid(Arg)))).push_back(Arg);
    std::cout << "size after push_back "<< (*static_cast<std::vector<decltype(Arg)>*>(myMap.at(typeid(Arg)))).size()<<std::endl;

    std::cout<<"--------------------------------------"<<std::endl;
}


int main()
{
    
    RequestA A;
    RequestB B;
    RequestC C;
    
    RequestHandler rq;
    
    //call RequestHandler's add method simulating the requests
    rq.addRequest(A);
    
    rq.addRequest(B);
    rq.addRequest(B);
    
    rq.addRequest(C);
    
}

Working demo

---

方法二

对于 C++17，我们可以使用 std::any而不是 void* .基本步骤与之前的方法相同。

#include <iostream>
#include <map>
#include <vector>
#include <typeindex>
#include <any>
#include <functional>
struct RequestA{};
struct RequestB{};
struct RequestC{};


class RequestHandler {
    public:
        void tick() 
        {
            std::cout<<"tick called"<<std::endl;
        }
    private:
        std::vector<RequestA> queueA;
        std::vector<RequestB> queueB;
        std::vector<RequestC> queueC;
 
        //create std::map -                             STEP 1
        std::map<std::type_index, std::any> myMap{{typeid(RequestA), std::ref(queueA)}, 
                                                  {typeid(RequestB), std::ref(queueB)}, 
                                                  {typeid(RequestC), std::ref(queueC)}};
    public:
        //create member function template -             STEP 2
        template<typename T> void addRequest(T Arg);

};
template<typename T> void RequestHandler::addRequest(T Arg) // - STEP 3
{
    std::cout  << "add called on " <<typeid(Arg).name() << std::endl;//just for debugging purposes
    std::cout << "size before push_back "<< std::any_cast<std::reference_wrapper<std::vector<T>>>(myMap.at(typeid(T))).get().size()<<std::endl;//just for debugging
   
    std::any_cast<std::reference_wrapper<std::vector<T>>>(myMap.at(typeid(T))).get().push_back(Arg);
    std::cout << "size after push_back "<< std::any_cast<std::reference_wrapper<std::vector<T>>>(myMap.at(typeid(T))).get().size()<<std::endl;

    std::cout<<"--------------------------------------"<<std::endl;
}


int main()
{
    
    RequestA A;
    RequestB B;
    RequestC C;
    
    RequestHandler rq;
    
    //call RequestHandler's add method simulating the requests
    rq.addRequest(A);
    
    rq.addRequest(B);
    rq.addRequest(B);
    
    rq.addRequest(C);
    
}

Working demo

以上只是根据我目前对您的需求的了解进行的演示。我花了一段时间(大约 40 分钟)来理解需求并相应地编写代码。我在理解您的要求方面可能仍然是错误的。所以让我知道这是否是您想要的。您也可以根据需要修改代码。