Singleton CRTP with meyers singleton

Question

i am tring to implement a Singleton template that using meyers singleton inside:

#include <bits/stdc++.h>

template <typename T>
class Singleton {
public:
    static T& instance() {
       static T _instance; 
       return _instance;
    }
protected:
    Singleton() = default;
    ~Singleton() = default;
    Singleton(const Singleton & s) = delete;
    Singleton& operator=(const Singleton & s) = delete;
};


class Foo : Singleton<Foo> {
public:
    void print() {
       std::cout<<"from the foo singleton count : " <<count++<<std::endl; 
    }
private:
    int count = 0;

};

int main () {
   Singleton<Foo>::instance().print();
   Singleton<Foo>::instance().print();
   Singleton<Foo>::instance().print();
   return 0;

and it seems to work : link

but now i want to be able to using it like this :

Foo::instance().print();

is there any way to do it ?

#include never use this. https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h — Alessandro Teruzzi, Aug 19 '21 at 13:04
As an aside, `Foo` is not a true singleton. `Foo` is default constructible in any scope. — StoryTeller - Unslander Monica, Aug 19 '21 at 13:11

score 3 · Accepted Answer · answered Aug 19 '21 at 13:06

You are using private inheritance with class Foo : Singleton<Foo> which means to the outside world, Foo is not a Singleton<Foo> and it doesn't have a instance function. You can add

using Singleton<Foo>::instance;

To the public section of Foo. That will import the instance function into the public space of Foo and allow

Foo::instance().print();

to compile.

score 2 · Answer 2 · answered Aug 19 '21 at 13:05

You should read the error message that results from trying to call Foo::instance().print() because it tells you whats wrong:

<source>:30:17: error: 'static T& Singleton<T>::instance() [with T = Foo]' is inaccessible within this context
   30 |    Foo::instance().print();
      |    ~~~~~~~~~~~~~^~

Singleton<T>::instance() is inaccessible via Foo::instance() because Foo inherits privately. Make Foo inherit publicly and it works.

Singleton CRTP with meyers singleton

2 Answers2