My own Smart pointer Implementation is creating issue with move operation

Question

In the below code, the output is turning out to be blank instead of constructor and destructor printouts. Without move operation, the code is working well. I am somehow messing up the move operation, please go through and give your inputs to guide further.

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
using namespace std;

template <class T>
class smart{
    public:
    smart():pt{nullptr}{
        
    }
    smart(T *ptr):pt{ptr}{
        
    } 
    
    T *pt;
    smart(const smart& ob)=delete;
    smart operator=(const smart& ob)=delete;

    smart (smart&& dyingob){
        this->pt=dyingob.pt;
        dyingob.pt=nullptr;
    }
    void operator=(smart&& dyingob){
        pt=dyingob.pt;
        dyingob.pt=nullptr;
    }
    T* operator ->(){return this->pt;};
    ~smart(){
        if(pt!=nullptr)
        delete pt;
    }
};
class A{
    public:
    A(int n){
        data=n;
        cout<<"\nconstructed";
    }
    ~A(){
        cout<<"\ndestructed";
    }
    int data;
};
int main()
{
    smart<A> ob(new A(5));
   smart<A> ob1;
   ob1=(std::move(ob));
    cout<<ob->data;
    return 1;
}

What are you expecting for `cout<data;` after `ob` has been moved? — songyuanyao, Aug 16 '23 at 02:14
Is `ob->data` supposed to be `ob1->data`? You're accessing `ob` after it's been gutted in `ob1=(std::move(ob));`. — Brian61354270, Aug 16 '23 at 02:14
got it thank you..when I am printing ob1->data it is working good. but why the constructor call is not getting printed when I use ob->data, I don't understand as I am creating object before the print statement, — Tej Ravi, Aug 16 '23 at 02:20
@TejRavi You can't expect anything from undefined behavior.. — songyuanyao, Aug 16 '23 at 02:29
You did not miss `#include `, you used the code formatter improperly. — 273K, Aug 16 '23 at 03:17
`std::cout << ob->data` is probably crashing and exiting without flushing any buffers. Try adding a `<< std::flush` to the end of your printing commands — Chris Dodd, Aug 16 '23 at 05:27
@songyuanyao: Indeed, and - rather than ask us on SO, OP could have had that said to him by a static analyzer. — einpoklum, Aug 17 '23 at 22:23

score 0 · Answer 1 · answered Aug 17 '23 at 22:21

When noticing unexpected behavior, the first thing to check for is compiler warnings. But - without very specific flags perhaps - you are not likely to get those here.

However, a second thing to check for is what a static analysis tool tells you about your program. Let's try clang-tidy (version 14) for example:

1 warning generated.
/tmp/a.cpp:51:11: warning: Method called on moved-from object 'ob' [clang-analyzer-cplusplus.Move]
    cout<<ob->data;
          ^~~~
/tmp/a.cpp:50:4: note: Object 'ob' is moved
   ob1=(std::move(ob));
   ^~~~~~~~~~~~~~~~~~~
/tmp/a.cpp:51:11: note: Method called on moved-from object 'ob'
    cout<<ob->data;
          ^~~~

So, you're acting on a moved-from object (like commenters also told you... e.g. @songyuangyo). ob->data is a nullptr after the move, is it not?

My own Smart pointer Implementation is creating issue with move operation

1 Answers1