Meyers Singleton Scope

Question

In the following program, it seems like the Registry Singleton isn't being persisted across calls to the static functions. What is the problem with this approach?

#include <iostream>
#include <string>
#include <unordered_map>

using namespace std;

class Test {
typedef unordered_map<string,string> Registry;
public:
    static Registry &registry() {
        static Registry reg;
        return reg;
    }

    static void put(string key, string val) {
        Registry reg = Test::registry();
        reg[key] = val;
    }

    static string get(string key) {
        Registry reg = Test::registry();
        return reg[key];
    }
};

int main() {
    Test::put("a", "apple");
    Test::put("b", "banana");
    cout << Test::get("a") << endl;
    cout << Test::get("b") << endl;
    return 0;
}

Answer 1

You are correctly returning a reference to your singleton, but when you use it you are taking a copy. Offending line follows:

Registry reg = Test::registry();

To fix the problem, modify this to:

Registry & reg = Test::registry();

To prevent this from ever happening, you can prevent the compiler from allowing copies by deleting the copy constructor and assignment operators:

class Registry : public unordered_map<string,string>
{
    public:
        Registry() {}
        Registry( const Registry & ) = delete;
        Registry & operator=( const Registry & ) = delete;
};

Answer 2

Your code makes a copy of the registry in each function call and then throws the copy away.

Instead, you want to make a reference to the one and only registry:

Registry & reg = Test::registry();
//      ^^^