Create singleton class in C++

create_singleton.cpp

// Example program
#include <iostream>
#include <string>
#include <memory>
 
class singleton
{
    static std::shared_ptr<singleton> instance(int a)
    {
        if (!instance_)
        {
            instance_.reset(new singleton(a));
        }
        return instance_;
       
    }
   
    private:
    singleton(int a);
    static std::shared_ptr<singleton> instance_;
   
};
 
std::shared_ptr<singleton> singleton::instance_(0);
 
int main()
{
    auto s = singleton::instance();
  std::string name;
  std::cout << "What is your name? ";
  getline (std::cin, name);
  std::cout << "Hello, " << name << "!\n";
}