unresolved external symbol "private: static class variable" c++

I'm getting an error while creating a container in c++ with static method and locks.I'm not an expert in c++.Could anyone please help to find the real issue here? below is the code. I'm using v14

#include <iostream>
#include <map>
#include <string>
#include <mutex>
#include <shared_mutex>

class obj {};

class pricercache
{
    static  std::shared_mutex entry_mutex;
private:
    static std::map<std::string,obj>& getInstance()
    {
        static std::map<std::string, obj>    engines;
        return engines;
    }
public:

    pricercache(pricercache const&) = delete;
    void operator=(pricercache const&) = delete;

    static obj get(std::string const& key)
    {
        std::shared_lock<std::shared_mutex> lk(pricercache::entry_mutex);
        std::map<std::string, obj>::const_iterator const it =
            getInstance().find(key);        
        return it->second;
    }
    static void add(std::string const& key,
        obj engine)
    {
        std::lock_guard<std::shared_mutex> lk(pricercache::entry_mutex);
        auto& engines = getInstance();
        if (engines.find(key) == engines.end())
            engines.insert(std::make_pair(key, engine));
    }
};

int main()
{
    std::cout << "Hello World!\n";
    obj v1; 
    obj v2;
    pricercache::add("1", v1); 
    pricercache::add("2", v2);

    auto A = pricercache::get("1");
    auto b = pricercache::get("2");

    std::cout << "Done!\n";

    
}

The error I'm getting is :

error LNK2001: unresolved external symbol "private: static class std::shared_mutex pricercache::entry_mutex" (?entry_mutex@pricercache@@0Vshared_mutex@std@@A)

I would also welcome and really appreciate if you have any other suggestion about the container class. thanks in advance.

The compiler complains because you have declared the static member, but not defined. Somewhere in your code, you should define it like this

std::shared_mutex pricercache::entry_mutex; // definition of the entry_mutex