C++ Singleton usage: compiler complains about private constructor

Question

I know that there is a million questions and answers about Singletons out there but I just can't seem to find the solution to this. So running the risk of negative votes, here's my problem:

I want to use this singleton implementation from Andrei Alexandrescu' Modern C++ Design:

header:

class Singleton
{
    static Singleton& Instance();
  private:
    Singleton(){};
    Singleton(const Singleton&){};
    Singleton& operator=(const Singleton&){};
    ~Singleton(){};
};

implementation:

#include "s.hh"

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

test:

#include "s.hh"

int main(void)
{
    Singleton& single = Singleton::Instance();
    return 0;
}

Now,

$g++ A.cc s.cc  && ./a.out 
In file included from A.cc:1:0:
s.hh: In function ‘int main()’:
s.hh:3:19: error: ‘static Singleton& Singleton::Instance()’ is private
 static Singleton& Instance();
               ^
A.cc:6:42: error: within this context
  Singleton& single = Singleton::Instance();
                                      ^

What is wrong with that? I am stuck...

Argh, that's what happens if you post a question before breakfast... In the original code I had the public specifier. I will post the "real" question in a minute. — steffen, Apr 06 '13 at 08:03

score 4 · Accepted Answer · answered Apr 06 '13 at 07:54

4

By default, a class' members are private. To access your singleton, you need to make Singleton::Instance public:

class Singleton
{
  // here!
  public:
    static Singleton& Instance();

  private:
    Singleton(){};
    Singleton(const Singleton&){};
    Singleton& operator=(const Singleton&){};
    ~Singleton(){};
};

Note that this is not the constructor (as you said in your title), it's the static member function that is supposed to return a reference to the singleton.

answered Apr 06 '13 at 07:54

Daniel Frey

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Where would be the implementation of Instance() function? Implementing in .cpp it complains about private ctor, while implementing in .h - works. – didinino Aug 13 '20 at 14:30
It should be possible in the .cpp. Did you forget the Singleton:: ? – Daniel Frey Aug 13 '20 at 18:34

score 3 · Answer 2 · answered Apr 06 '13 at 07:53

Default access for a class is private, so you need to make the Instance() method explicitly public:

class Singleton
{
 public:
    static Singleton& Instance();
 private:
   // as before
....
};

Alternatively, you could use struct, whose default access specifiers are public:

struct Singleton
{
    static Singleton& Instance(); // public
  private:
    Singleton(){};
    Singleton(const Singleton&){};
    Singleton& operator=(const Singleton&){};
    ~Singleton(){};
};

score 3 · Answer 3 · edited May 23 '17 at 12:13

3

The default access specifier for a class is private. Add the method under public access specifier.

public:
    static Singleton& Instance();

Good Read:
What are access specifiers? Should I inherit with private, protected or public?

edited May 23 '17 at 12:13

Community

1
1

answered Apr 06 '13 at 07:54

Alok Save

196,531
48
417
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score 2 · Answer 4 · answered Apr 06 '13 at 07:55

2

class S
{
    public:
        static S& getInstance()
        {
            static S    instance; 

            return instance;
        }
    private:
            // other stuff here
};

answered Apr 06 '13 at 07:55

Saqlain

16,750
4
26
33

score 0 · Answer 5 · answered Apr 06 '13 at 11:57

Also don't make your other destructor private.

class Singleton
{
  // here!
  public:
    static Singleton& Instance();
    ~Singleton(){};

  private:
    Singleton(){};
    Singleton(const Singleton&){};
    Singleton& operator=(const Singleton&){};

};

C++ Singleton usage: compiler complains about private constructor

5 Answers5