Returning a reference from a pointer

Question

I have this class in singleton.h:

class SingletonSample {
public:
     static SingletonSample& Instance();
     int DoSomething() const;
private:
     SingletonSample();
     ~SingletonSample();
     SingletonSample(const SingletonSample&) = delete;
     SingletonSample& operator=(const SingletonSample&) = delete;
     static SingletonSample* _instance;
}

And I have this implementation in singleton.cpp:

SingletonSample::SingletonSample() {}
SingletonSample::~SingletonSample() {}
SingletonSample& SingletonSample::Instance() {
    if(!_instance) _instance = new SingletonSample();
    return _instance;
}
int SingletonSample::DoSomething() const {
    return 20;
}

I have a compiler error when trying to return _instance.

I want to return a SingletonSample&, but my _instance is a pointer SingletonSample*.

I'm a beginner in C++ but as far as I understand, a reference to something is just the address to that something, while a pointer to something is just a pointer to that address.

I would like to return that address being pointed by the pointer, as a reference SingletonSample&.

However, I cannot manage to achieve that. How can I do this?

even if you already got an answer, you might consider to post the compiler error. — 463035818_is_not_a_number, May 13 '15 at 12:08
Note that "reference" in Java has a different meaning than "reference" in C++. — molbdnilo, May 13 '15 at 12:22

score 6 · Accepted Answer · edited May 23 '17 at 11:58

6

A reference is not an address of an object. It is just an alias for one. You simply have to de-reference the pointer:

return *_instance;

This lets the reference refer to the object pointed at by _instance.

Note that the implementation can be greatly simplified:

SingletonSample& SingletonSample::Instance() {
  static SingletonSample instance;
  return instance;
}

Also note that singletons are generally considered a bad idea.

edited May 23 '17 at 11:58

Community

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answered May 13 '15 at 12:06

juanchopanza

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Are you creating a new static member inside the `Instance` method? I didn't know that was possible! – Matias Cicero May 13 '15 at 12:10
@MatiCicero Yes, it is, and it is even thread-safe since C++11 (the creation if the instance, that is.) – juanchopanza May 13 '15 at 12:16

Returning a reference from a pointer

1 Answers1