boost shared_ptr operator =

Question

Here is code example ``` class A{ int i; public: A(int i) : i(i) {} void f() { prn(i); } }; int main() { A* pi = new A(9); A* pi2= new A(87); boost::shared_ptr spi(pi); boost::shared_ptr spi2(pi2); spi=spi2; spi->f(); spi2->f(); pi->f(); pi2->f(); } ``` output: ``` 87 87 0 87 ``` The question is why is 0 in the output? Note from documentation: Effects: Equivalent to shared_ptr(r).swap(*this). But if `shared_ptr` objects just swapped, the result should be 9. And if the first object is deleted, there should be Segmentation fault. So why 0?

Accepted Answer

Pay careful attention to what is being swapped:

```
shared_ptr(r).swap(*this)
// ^^^^^^^^^^
```

That's a temporary object constructed from `r`. The temporary goes out of scope immediately and dies, with whichever effects this has on the owned resource. Since in your code `spi` was the only owner of `*spi`, the object is deleted, and your subsequent access of `pi->f()` is simply undefined behaviour.

boost shared_ptr operator =

Solution

Problem