Error Singleton in c++

Question

What is the problem?

#include <iostream>
using namespace std;

class Singleton
{
    public:
        void HelloWorld();

        static Singleton* Instance(){
        if (instanza == 0)
            instanza = new Singleton ;
        return instanza;
        }

    protected:
        Singleton();

    private:
        static Singleton* instanza;
};


Singleton* Singleton:: instanza = 0;

void Singleton::HelloWorld()
{
    cout << "Hello World!";
}
int main()
{
    Singleton *p = Singleton ::Instance();
    p->HelloWorld();
    delete p;
}

g++ -Wall -o "singleton" "singleton.cpp" (nella cartella: /home/tarek/Scrivania/Nuovi codici) /tmp/ccL8BxOT.o: nella funzione "Singleton::Instance()": singleton.cpp:(.text._ZN9Singleton8InstanceEv[_ZN9Singleton8InstanceEv]+0x24): riferimento non definito a "Singleton::Singleton()" collect2: error: ld returned 1 exit status Compilazione fallita.

Show us english errors. Just state `LANG=C` in front of the g++ command. — Nidhoegger, Sep 01 '16 at 08:36
The message is pretty clear: `Singleton::Singleton()` is not defined. — molbdnilo, Sep 01 '16 at 08:39
*What is the problem?* is what you should report. Stackoverflow isn't remote debugging service. And your singleton idiom is broken - what if `Instance` is called before `instanza = 0`? Use local static variables for singletons. — Tomáš Zato, Sep 01 '16 at 08:39
Off topic suggestion: Consider replacing what you have with a Meyers Singleton. It could be as simple as `static YourType& instance() { static YourType s; return s; }`. More complete discussion in the top answer here: http://stackoverflow.com/questions/1008019/c-singleton-design-pattern?noredirect=1&lq=1 — user4581301, Sep 01 '16 at 08:46

score 4 · Answer 1 · answered Sep 01 '16 at 08:38

4

Your constructor is just declared not defined.

change it to

...
 protected:
        Singleton() {};//add further implementation here
        //Singleton() = default; for c++11
...

and it should work

answered Sep 01 '16 at 08:38

Hayt

5,210
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37

score 0 · Answer 2 · answered Sep 01 '16 at 08:45

0

You would need define constructor as

Singleton::Singleton() 
{
}

answered Sep 01 '16 at 08:45

Softec

1,087
11
14

Error Singleton in c++

2 Answers2