simple singleton example in c++

Question

I am trying to implement singleton that I have used before in PHP and Java 8, to C++. But I do face certain restrictions from the syntax and how C++ works (specifically pointers).

This is what I have tried so far:

#include "iostream"

using namespace std;

class System{
protected:
    static System *obj;

public:
    static System *getInstance(){
        return obj;
    }

    void prn(){
        cout<<"this works!";
    }
};

int main(void){
    System &sys = System::getInstance();
    sys.prn();
}

while executing, I get the following error:

 sameer.cpp:20:10: error: non-const lvalue reference to type 'System'
 cannot bind
       to a temporary of type 'System *'
         System &sys = System::getInstance();
                 ^     ~~~~~~~~~~~~~~~~~~~~~

Please help me solve this error.. as I have no idea what it means. I have checked the forum before posting, and it can be a possible duplicate of previously asked question (which I caould not find).. But I posted this because I wanted to understand the meaning of error my code generated.

Thanks for the help

System::getInstance returns a pointer, but System &sys is a reference. — ForceBru
– ForceBru, Commented Apr 20, 2017 at 15:16
Do note that your class isn't actually a singleton. It has a default constructor and copy constructor so you can create instances and copy them. To see an actual singleton see: stackoverflow.com/questions/1008019/c-singleton-design-pattern — NathanOliver
– NathanOliver, Commented Apr 20, 2017 at 15:16
try to avoid singletons as much as possible, even good explanations like this one stackoverflow.com/questions/137975/… doesn't show all problems that become obvious only after long maintenance — Andriy Tylychko
– Andriy Tylychko, Commented Apr 20, 2017 at 15:16
You should learn C++ basics before trying to implement the Singleton pattern. — Vittorio Romeo
– Vittorio Romeo, Commented Apr 20, 2017 at 15:17
I know, and I am trying to learn C++, but this experiment id just out of curiosity! — sameer
– sameer, Commented Apr 20, 2017 at 16:07

Tristan Brindle · Accepted Answer · 2017-04-20 15:53:22Z

17

In C++, references and pointers are different things. A reference behaves exactly like the original variable, whereas a pointer represents the memory address of that variable. You're getting the error because you're trying to assign a pointer-to-System to a variable of type reference-to-System, which isn't the same thing. If you really wanted to you could dereference the pointer by using the System& sys = *ptr; syntax, but in this case that's the wrong thing to do; the correct fix is to return a reference from your getInstance() function, rather than a pointer.

What's more, in C++ you can actually store the static instance variable within the getInstance() function. This is a so-called "magic static", otherwise known as a "Meyers Singleton", and since C++11 it guarantees you get thread-safe construction of the singleton object. So the final solution would be:

class System
{
private:
    System() {}

public:
    static System& getInstance(){
        static System theInstance;
        return theInstance;
    }

    void prn(){
        cout<<"this works!";
    }
};

int main()
{
    System& sys = System::getInstance();
    sys.prn();
}

Also, as an aside, you should use

#include <iostream>

not

#include "iostream"

to include standard library headers. And you don't need to say main(void) in C++; empty brackets signify that a function takes no arguments, so main() will do.

answered Apr 20, 2017 at 15:53

Tristan Brindle

17k5 gold badges43 silver badges86 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Michal Over a year ago

As someone already noted above, to avoid creating more than one instance (by accidentally missing & or intentionally) you also need to delete copy constructor System(const System&) = delete;.

Ravi · Accepted Answer · 2019-07-28 09:08:29Z

3

#include <iostream>
using namespace std;

class System{
private:
    static System *obj;
    System() {}

public:
    static System *getInstance()
    {
        if(obj == nullptr)
            obj = new System();
        return obj;
    }

    void prn(){
        cout<<"this works!"<< endl;
    }
};

System *System::obj;

int main(void){
    System *sys = System::getInstance();
    sys->prn();
}

answered Jul 28, 2019 at 9:08

Ravi

594 bronze badges

2 Comments

AdrianHHH Over a year ago

This code appears almost identical to the accepted answer written by @TristanBrindle. To make this into a good and useful answer please edit it to explain how it solves the problem, why it is so similar to the other answer and why that difference is significant.

Vic Fanberg Over a year ago

AdrianHHH, I will add one comment on why this difference is significant. If you are working in a C++ Builder VCL environment, the @TristanBrindle version will give an error ("[bcc32c Error]: Delphi-style classes must be allocated with 'new'") and this pattern does not give the error. C++ Builder does not let you implicitly allocate the object without the keyword "new".

aschepler · Accepted Answer · 2017-04-20 15:16:34Z

2

getInstance() returns a pointer, but you are trying to call it and bind it to a reference. If you want it to return a reference, make it return a reference.

static System &getInstance(){
    return *obj;
}

answered Apr 20, 2017 at 15:16

aschepler

72.5k11 gold badges114 silver badges174 bronze badges

Comments

Burak · Accepted Answer · 2022-10-23 06:28:34Z

-1

It might be good to lazy-initialize your singleton. Also, consider making the constructor and the singleton private so you can't create an instance anywhere else.

#include <iostream>

class System
{
private:
    static System *obj;
    System() {}
    
public:
    static System& getInstance(){
        if (nullptr == obj)
            obj = new System();

        return obj;
    }
    
    void prn(){
        std::cout << "this works!";
    }
};

int main(void){
    System& sys = System::getInstance();
    sys.prn();
}

edited Oct 23, 2022 at 6:28

Burak

2,5911 gold badge23 silver badges37 bronze badges

answered Apr 20, 2017 at 15:24

AlexG

1,0937 silver badges16 bronze badges

Collectives™ on Stack Overflow

simple singleton example in c++

4 Answers 4

1 Comment

2 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related