Singletons in C++

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Example

class Singleton {
 public:
  inline static Singleton& GetInstance() {
    static Singleton instance;
    return static_cast<Shell&>(*instance);
  }
 
  Singleton(const Singleton&) = delete;
  void operator =(const Singleton&) = delete;
 
 private:
  Singleton() = default;
};

How does it work

We delegate the external construction of the class to the GetInstance() function. This way we ensure that there’s only one instance of the Singleton class.

public:
  inline static Singleton& GetInstance() {
    static Singleton* instance;
    return static_cast<Shell&>(*instance);
  }

For this to be enforced is also required to delete the Copy constructor and the Move operator.

Singleton(const Singleton&) = delete;
void operator =(const Singleton&) = delete;

Additionaly the default constructor has to be hidden from external access.

private:
  Shell() = default;