C++ 带有模板 class 的模板队列

Question

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, eg LockingQueue<CommonMsgConstPtr > . I know it is wrong. what should I do?

Answer 1

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