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#include <iostream>
using namespace std;
 
template <typename T>
class Singleton
{
public:
	static T& Get() noexcept
	{
		static T instance;
		return instance;
	};
 
private:
	Singleton() = default;
};
 
class Foo : public Singleton<Foo>
{
public:
	int t = 0;
};
 
int main() {
 
	//Foo foo;
	Foo::Get().t += 10;
 
	return 0;
}

prog.cpp: In instantiation of ‘static T& Singleton<T>::Get() [with T = Foo]’:
prog.cpp:27:7:   required from here
prog.cpp:10:12: error: use of deleted function ‘Foo::Foo()’
   static T instance;
            ^~~~~~~~
prog.cpp:18:7: note: ‘Foo::Foo()’ is implicitly deleted because the default definition would be ill-formed:
 class Foo : public Singleton<Foo>
       ^~~
prog.cpp:18:7: error: ‘constexpr Singleton<T>::Singleton() [with T = Foo]’ is private within this context
prog.cpp:15:2: note: declared private here
  Singleton() = default;
  ^~~~~~~~~

Standard output is empty