prohibit copy constructor and assignment operator singleton class

Question

I am implementing Singleton class in c++ and I am wondering if it is necessary to declare copy constructor and assignment operator as private, in case I have the following implementation

class Singleton{ static Singleton* instance; Singleton(){} public: static Singleton* getInstance(); }; Singleton* Singleton::instance = 0; Singleton* Singleton::getInstance(){ if(instance == 0){ instance = new Singleton(); } return instance; }

It seems that I can have only pointer to Singleton and in that case copy constructor is useless, also operator= . So, I can skip declaring these as private, am I wrong ?

Possible duplicate of why explicitly delete the constructor? — mch
– mch, Commented Jul 17, 2018 at 7:12
If you don't want those operations or they are not valid/sane for the class, then you should =delete; the functions. — Jesper Juhl
– Jesper Juhl, Commented Jul 17, 2018 at 7:12

BoBTFish · Accepted Answer · 2018-07-17 08:25:24Z

There's nothing to stop someone writing

Singleton hahaNotReallyASingleton = *Singleton::getInstance();

You can specifically mark these functions as deleted:

class Singleton { // ... other bits ... Singleton(Singleton const&) = delete; // copy ctor Singleton(Singleton &&) = delete; // move ctor Singleton& operator=(Singleton const&) = delete; // copy assignment Singleton& operator=(Singleton &&) = delete; // move assignment };

Note that using delete in this way is C++11 and onwards - if you are stuck with an older codebase, you could make the functions private (copy only, not move of course), or inherit from boost:noncopyable (thanks badola).

thank you, I haven't considered that case, I thought I cannot have object of that class
Kindly note that deleted functions are only allowed C++11 and above. For pre-C++11, a construct similar to boost::noncopyable would be required.

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