Singleton with inheritance, Derived class is not able to get instantiated in parent?

Question

Below code instantiates a derived singleton object based on environment variable. The compiler errors saying error C2512: 'Dotted' : no appropriate default constructor. I don't understand what the compiler is complaining about.

EDIT: Fixed issues with implementing the get instance method which requires definition of both the parent and derived class. By separating the class definitions in separate header files and including the same in Singleton.cpp where the instance function is implemented.

Mainfile – 1
#include <iostream>
#include <string>
#include "Singleton.h"
using namespace std;

int main(){

     Singleton::instant().print();
     cin.get();
}
Singleton.h
#pragma once
#include <iostream>
using std::cout;
class Singleton{
public:
    static Singleton & instant();
    virtual void print(){cout<<"Singleton";}
protected:
    Singleton(){};
private:
    static Singleton * instance_;
    Singleton(const Singleton & );
    void operator=(const Singleton & );

};

Singleton.cpp
#include "Singleton.h"
#include "Dotted.h"
Singleton * Singleton::instance_ = 0;
Singleton & Singleton::instant(){
    if (!instance_)
    {
        char * style = getenv("STYLE");
        if (style){
            if (strcmp(style,"dotted")==0)
            {
                instance_ = new Dotted();
                return *instance_; 
            }           else{
                instance_ = new Singleton();
                return *instance_;
            }       
        }

        else{
            instance_ = new Singleton();
            return *instance_;
        }
    }
    return *instance_;

}
Dotted.h

#pragma once
class Dotted;

class Dotted:public Singleton{
public:
    friend class Singleton;
    void print(){cout<<"Dotted";}
    private:
        Dotted(){};

};

Answer 1

I don't see how this can work. At this point:

instance_ = new Dotted();

Dotted is an incomplete type. With g++ 4.4.1 I get:

error: invalid use of incomplete type 'struct Dotted'

Which compiler are you using?

Answer 2

The first error I'm getting is: strcmp not declared. Hint: it's in the <cstring> (≈ <string.h>) header.

After that the next error is:

instance_ = new Dotted();

"Invalid use of incomplete type."

If you use forward declarations, you must separate the declarations and implementations, so you can do things that require complete types after they are defined.

Answer 3

It is not a great error message. The problem is that the compiler cannot generate code for the constructor call, it hasn't yet seen the definition of the Dotted class. C++ compilers are still single-pass compilers. You cannot write the method inline, you have to move it.

class Singleton {
public:
    static Singleton & instant();
    // etc..
};

class Dotted : public Singleton {
    // etc..
};

// Now it works:
Singleton & Singleton::instant() {
    // etc..
}