C++ Linker Error Undefined reference in Singleton class

Question

I'm trying to port this java singleton class to c++:

public class Singleton {
private static Singleton uniqueInstance;

private Singleton() {} 
public static Singleton getInstance() {
    if (uniqueInstance == null) {
        uniqueInstance = new Singleton();
    }
        return uniqueInstance;
    }
}

I ported to this C++ code:

class Singleton {
private:
  Singleton() {
    cout << "new Singleton is called" << endl;
  }
  static Singleton* uniqueInstance;
public:
   static Singleton* getInstance() {
    if (!uniqueInstance) {
      uniqueInstance = new Singleton();
    }
    return uniqueInstance;
  }
};

But I can't compile this! and gcc linker error occurred.

Please show exactly what the linker error was. – seand Mar 24 '13 at 08:04 — seand, Mar 24 '13 at 08:04

Tushar · Accepted Answer · 2013-03-25T22:39:48.293

7

Make sure you define the static member outside the declaration:

Singleton* Singleton::uniqueInstance = nullptr;

edited Mar 25 '13 at 22:39

answered Mar 24 '13 at 08:06

Tushar

8,019
31
38

3

You should initialise to nullptr too. – Alex Chamberlain Mar 24 '13 at 08:59
@AlexChamberlain Why? My program was complied without error without using nullptr! – Milad Khajavi Mar 24 '13 at 09:59
1

@Khajavi Also without high optimisation? Pointers are not guarenteed to be zero on initialisation. – Alex Chamberlain Mar 24 '13 at 18:10
In C++11 you should not use pointers. New standard ensures initialization of static member variables is thread safe. – Anupam Srivastava Jun 30 '17 at 10:07

score 2 · Answer 2 · answered Mar 24 '13 at 08:07

2

Your cpp file probably does not reserve space for the static instance, you need to add Singelton* Singleton::uniqueInstance = NULL; to your cpp file and then allocate it in the first call

answered Mar 24 '13 at 08:07

Alon

1,776
13
31

C++ Linker Error Undefined reference in Singleton class

2 Answers2