c++templatescontainerstemplate-templates

C++ Template queue with template class


My template queue is below

template <typename T>
class LockingQueue
{
private:
    std::queue<T> s_queue; 
public:
    void push(T const& value)
    {}

    T pop()
    {}
};

And my template class is below

template <typename TaskData, typename TaskName>
class CommonMsg
{
public:
    TaskData dataType;
    TaskName taskName;
};
template <typename TaskData, typename TaskName>
using CommonMsgPtr = boost::shared_ptr<CommonMsg<TaskData, TaskName>>;
template <typename TaskData, typename TaskName>
using CommonMsgConstPtr = boost::shared_ptr<const CommonMsg<TaskData, TaskName>>;

I want to put the template class as the parameter of LockingQueue, e.g. LockingQueue<CommonMsgConstPtr >. I know it is wrong. what should I do?


Solution

  • As far as I am aware, you can do this in two ways. In both ways you need to specialize your original class template so that it gets instantiated when you pass a smart pointer. The simplest way is option 1 (see code below), but you must specify the type in the main function. A bit dirtier approach is option 2 (see code below), but the call in the main function is cleaner and closer to your original post.

    Btw, why boost::shared_ptr and not std::shared_ptr?

    #include <iostream>
    #include <queue>
    #include <memory>
    
    template <typename T>
    struct LockingQueue
    {
        std::queue<T> s_queue;
        ...
    };
    
    // Option 1
    template < typename CM, template<typename> class SP>
    struct LockingQueue<SP<CM>>
    {
        std::queue< SP<CM> > s_queue;
        ...
    };
    
    // Option 2
    template < typename T, typename R, template<typename,typename> class CM, template<typename> class SP>
    struct LockingQueue<SP<CM<T,R>>>
    {
        std::queue< SP<CM<T,R>> > s_queue;
        ...
    };
    
    template <typename TaskData, typename TaskName>
    struct CommonMsg
    {
        TaskData dataType;
        TaskName taskName;
    };
    
    template <typename TaskData, typename TaskName>
    using CommonMsgPtr = std::shared_ptr<CommonMsg<TaskData, TaskName>>;
    
    template <typename TaskData, typename TaskName>
    using CommonMsgConstPtr = std::shared_ptr<const CommonMsg<TaskData, TaskName>>;
    
    // Option 1 alias
    template < typename CM, template<typename> class SP = std::shared_ptr>
    using CommonMsgConstSPtr1 = SP< const CM >;
    
    // Option 2 alias
    template < typename TaskData = int, typename TaskName = char, template<typename,typename> class CM = CommonMsg, template<typename> class SP = std::shared_ptr>
    using CommonMsgConstSPtr2 = SP< const CM<TaskData, TaskName> >;
    
    int main()
    {
        LockingQueue< CommonMsgConstSPtr1< CommonMsg<int,char> > > option1;
        LockingQueue< CommonMsgConstSPtr2<> > option2;
    }
    

    Online example: https://ideone.com/h6o02M