How to determine a template parameter at runtime in c++ - c++

Question

I define a ThreadPoolclass, and it has a memeber: std::array<Worker, ThreadNum> Workerlist.
The code is as follows:
#ifndef THREADPOOL_H
#define THREADPOOL_H
#include <pthread.h>
#include <memory>
#include "Worker.h"
#include <vector>
#include <array>
const int MAX_THREAD_NUM = 16;
class ThreadPool
{
private:
const unsigned int ThreadNum;
std::shared_ptr<EventLoop> MainLoop;
std::array<std::shared_ptr<Worker>, ThreadNum> WorkerList;
std::array<std::shared_ptr<EventLoop>, ThreadNum> EventLoopList;
unsigned int NextLoopIndex;
public:
ThreadPool(std::shared_ptr<EventLoop> loop, int threadNum = 12);
~ThreadPool();
void RunThreadPool();
std::shared_ptr<EventLoop> GetNextEventLoop();
}
#endif
ThreadPool.cpp
ThreadPool::ThreadPool(std::shared_ptr<EventLoop> loop, int threadNum): MainLoop(loop), ThreadNum(threadNum), NextLoopIndex(0)
{
if (ThreadNum<=0 || ThreadNum> MAX_THREADS)
{
LOG << "The num of threads is out of range.\n";
}
}
ThreadPool::~ThreadPool() {}
void ThreadPool::RunThreadPool()
{
WorkerList.fill(std::make_shared<Worker>());
for (auto i = 0; i < ThreadNum; i++)
{
EventLoopList.at(i) = WorkerList.at(i)->ReturnEventLoopPtr();
}
}
std::shared_ptr<EventLoop> ThreadPool::GetNextEventLoop()
{
if (!EventLoopList.empty())
{
std::shared_ptr<EventLoop> nextLoop = EventLoopList[NextLoopIndex];
NextLoopIndex = (NextLoopIndex + 1) % ThreadNum;
return nextLoop;
}
return;
}
The error message is:
invalid use of data member ThreadPool::ThreadNum
In my opinion, the template parameter ThreadNum should be a constant, but now I need to infer its value when the class is constructed. Any solutions? Thank you very much.