Using CRTP as an alternative to abstract static methods in C++11

I'm trying to implement a generic resource manager which would ensure that every resource gets only loaded once with C++11.

My first attempt:

resourcemanager.h

#ifndef RESOURCEMANAGER_H
#define RESOURCEMANAGER_H

#include <map>
#include <memory>


template<typename T>
class ResourceManager {

public:
    static std::shared_ptr<T> load(std::string filePath);

private:
    static map<std::string, std::weak_ptr<T>> resources;
    virtual static std::shared_ptr<T> loadResource(std::string filePath) = 0;
};

#endif // RESOURCEMANAGER_H

#include "resourcemanager.h"

resourcemanager.cpp

using namespace std;

template<typename T>
map<string, weak_ptr<T>>  ResourceManager<T>::resources;

template<typename T>
shared_ptr<T> ResourceManager<T>::load(std::string filePath) {
    auto search = resources.find(filePath);
    if (search != resources.end()) {
        auto ptr = search->second.lock();
        if (ptr) {
            return ptr;
        }
    }
    auto ptr = loadResource(filePath);
    resources[filePath] = ptr;
    return ptr;
}

However since abstract static methods are apparently forbidden black magic I tried to use CRTP:

resourcemanager.h

#ifndef RESOURCEMANAGER_H
#define RESOURCEMANAGER_H

#include <map>
#include <memory>

template<typename T, class Derived>
class ResourceManager {

public:
    static std::shared_ptr<T> load(std::string filePath);

private:
    static std::map<std::string, std::weak_ptr<T>> resources;
    static std::shared_ptr<T> loadResource(std::string filePath);
};

#endif // RESOURCEMANAGER_H

resourcemanager.cpp

#include "resourcemanager.h"

using namespace std;

template<typename T, class Derived>
map<string, weak_ptr<T>>  ResourceManager<T, Derived>::resources;

template<typename T, class Derived>
shared_ptr<T> ResourceManager<T, Derived>::load(string filePath) {
    auto search = resources.find(filePath);
    if (search != resources.end()) {
        auto ptr = search->second.lock();
        if (ptr) {
            return ptr;
        }
    }
    auto ptr = ResourceManager::loadResource(filePath);
    resources[filePath] = ptr;
    return ptr;
}

template<typename T, class Derived>
shared_ptr<T> ResourceManager<T, Derived>::loadResource(string filePath) {
    return Derived::loadResource(filePath);
}

This looks like it should do what I want. However when I try to use it, it fails at the linking stage:

managedstring.h

#ifndef MANAGEDSTRING_H
#define MANAGEDSTRING_H

#include "resourcemanager.h"

class ManagedString {

public:
    ManagedString(std::string filePath);
    std::string get();

private:
    std::shared_ptr<std::string> ptr;

    class StringManager : public ResourceManager<std::string, StringManager> {
    private:
        static std::shared_ptr<std::string> loadResource(std::string filePath);
    };
};

#endif // MANAGEDSTRING_H

managedstring.cpp

#include "managedstring.h"

using namespace std;

ManagedString::ManagedString(string filePath) {
    ptr = StringManager::load(filePath);
}

string ManagedString::get() {
    return *ptr;
}

shared_ptr<string> ManagedString::StringManager::loadResource(string filePath) {
    // dummy implementation
    return make_shared<string>("foo");
}

main.cpp

#include <iostream>
#include "managedstring.h"

using namespace std;

int main() {
    ManagedString string1 = ManagedString("bar");
    ManagedString string2 = ManagedString("foobar");
    cout << string1.get() << endl; 
    cout << string2.get() << endl; 
}

When I try to compile this with g++ -std=c++11 -o bin -Wall main.cpp managedstring.cpp resourcemanager.cpp (using gcc version 5.3.0) I get this error message:

/tmp/ccgqljOQ.o: In function `ManagedString::ManagedString(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)':
managedstring.cpp:(.text+0xdd): undefined reference to `ResourceManager<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >,
 ManagedString::StringManager>::load(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)'

Should this work? Is this a compiler shortcoming? Or am I trying to do something I shouldn't do.

I also thought about altering my design, however I think it's not that bad. Feel free to disagree with me on this.