How is obj1 is referenced and calling show method if it is NULL

user_11379578 0
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I am trying to create the class that can only allow one object to be created at a time, so i have created private constructor and one public wrapper getInstance() method that will create object for this class, the code goes as follows

#include <iostream>
using namespace std;
class sample
{
    static int cnt;
    int temp;
    private: sample()
    {
        //temp++;
        cnt++;
        cout<<"Created "<<++temp<<endl;
    }
    public:
    void show()
    {

        cout<<"Showing \n";
    }

    static sample* getInstance()
    {
        cout<<"count is "<<cnt<<endl;
        if(cnt<1)
            return (new sample());
        else
            return NULL;
    }

};
int sample::cnt=0;
int main()
{
   // cout<<"Hello World";
    sample *obj = sample::getInstance();
    obj->show();

    sample *obj1 = sample::getInstance();
    if(obj1 == NULL)
        cout<<"Object is NULL\n";
    obj1->show();

    return 0;
}

How is obj1->show() is getting called?
OUTPUT :

count is 0
Created 1
Showing
count is 1
Object is NULL
Showing
class singleton c++
2 Answers

In a vacuum, this is just because your function:

public:
void show()
{

    cout<<"Showing \n";
}

don't actually try to do anything with the object - to get into the correct mindset of why this works just think of a member function as an abstraction over a free function taking the object as it's first argument:

void show(Object* this)
{

    cout<<"Showing \n";
}

Now it is easy to see why this works since you don't use this - the null pointer.

If you change to something ala. this:

public:
void show()
{

    cout<< this->temp << "Showing \n";
}

Your program should almost certainly crash.

user_22615380

What is almost certainly happening here is that the compiler is optimizing the call to show by inlining it. Further, it can also make it a "pseudo-static" function, as the there is no reference inside show to any other class member.

To 'break' the optimisation (and cause a crash) you can do one of these:

  1. Decare the show function virtual
  2. Reference a non-static member (e.g. temp) inside the function
user_108710730
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