How to pass argument in a singleton

Question

I've been wondering how to pass argument to a singleton contructor. I already know how to do a singleton, but I've been unlucky to find a way to do it.

Here is my code (part of it).

Questionnary* Questionnary::getInstance(){

    static Questionnary *questionnary = NULL;

    if(questionnary == NULL){
        cout << "Object created";
        questionnary = new Questionnary();

    }
    else if(questionnary != NULL){
        cout << "Object exist";
    }

    return questionnary;
}

Questionnary::Questionnary(){
    cout << "I am an object";
}

//This is want i want to acheive
Questionnary::Questionnary(string name){
    cout << "My name is << name;
}

Many thanks in advance

(BTW i know how and why a singleton is bad)

Answer 1

You don't need to allocate the instance of singleton dynamically. It could look the following way (this is sometimes called "lazy loading singleton" ~ the instance is created late & kinda "automatically"):

#include <iostream>
#include <string>

class Questionnary
{
private:
    // constructor taking string:
    Questionnary(const std::string& name) : name_(name) { }
public:
    static Questionnary& getInstance(const std::string& name)
    {
        static Questionnary q(name);
        std::cout << "My name is: " << q.name_ << std::endl;
        return q;
    }
private:
    std::string name_;
};

int main() {
    Questionnary::getInstance("Josh");
    Questionnary::getInstance("Harry");
}

output:

My name is: Josh
My name is: Josh

Note that constructor will be called only once right when the getInstance is called for the first time.

Answer 2

Let me extend Martin York's answer for your use case. I recommend using pointer for argument(s) in this particular situation, as we make use of its inherent property, that it can be "empty".

class Questionnary
{
  std::string _str;

  static Questionnary& getInstanceImpl(std::string* const s = nullptr)
  {
    static Questionnary instance{ s };
    return instance;
  }

  Questionnary(std::string* const s)
    : _str{ s ? move(*s) : std::string{} } // employ move ctor
  {
    if (nullptr == s)
      throw std::runtime_error{ "Questionnary not initialized" };
  }

public:
  static Questionnary& getInstance()
  {
    return getInstanceImpl();
  }
  static void init(std::string s) // enable moving in
  {
    getInstanceImpl(&s);
  }

  Questionnary(Questionnary const&) = delete;
  void operator=(Questionnary const&) = delete;
};

I find this approach less confusing, as it let's you get the instance after first initialization without (anyway discarded) arguments:

// first init
Questionnary::init("my single Questionnary");

// later on ...
Questionnary& q = Questionnary::getInstance();

Edit: Removed argument from getInstance function and a few optimizations.