Explicit template static member instantiation in a derived class

Question

I am trying to implement a template class with a static member. Classes that are derived from the template class shall be instantiated without the need to write extra code.

Here is my naive (and not successful) approach:

Singleton.h:

template <class T> class Singleton {
protected:
  Singleton();
  static T instance_;
}

// explicit instantiation of 'instance_' ???, 
// where 'instance_' is an instance of the derived class
template <class T> T Singleton<T>::instance_;

ConcreteA.h:

class ConcreteA : public Singleton<ConcreteA> {
public:
  ConcreteA();
  void foo();
}

main.c:

int main() {
  // an instance of ConcreteA should have been created (no extra code)!!!
  return 0;
}

Is there a way to force the instantiation of ConcreteA by just deriving ConcreteA from Singleton, without writing extra instantiation code?

A dirty workaround is to call an method on instance_ in the ConcreteA constructor, for example:

ConcreteA.c

ConcrereA::ConcreteA { instance_.foo(); }

Are there better workarounds?

Answer 1

Building upon your own "dirty trick", this works in all the compilers I tested, and doesn't require any code in the derived class constructor:

#include <iostream>

template <class T> class Singleton {
protected:
  Singleton() { instptr_ = &instance_; }
  static T instance_;
private:
  static T* instptr_;
};

template<class T> T Singleton<T>::instance_;
template<class T> T* Singleton<T>::instptr_;

class ConcreteA : public Singleton<ConcreteA> {
public:
  ConcreteA() { std::cout << "ConcreteA constructed.\n"; }
  void foo();
};

int main() {
  //Prints 'ConcreteA constructed.'.
  return 0;
}

My understanding of it is that taking the address of instance_ odr-uses it, forcing it into existence. I must say I'm not 100% sure this is guaranteed not to be optimized away in future versions of some compiler (I tested it with -O2 everywhere).

EDIT: Looks like even writing the base class constructor like this

Singleton() { (void)&instance_; }

is enough, which gets rid of instptr_ altogether.