Auto Pointer in C++ (auto_ptr)

Question

I am trying to figure out what this piece of code prints but I couldn't output it for some reason, it gave me an error: "1 [main] Q1c 5752 open_stackdumpfile: Dumping stack trace to Q1c.exe.stackdump".

double *dp=new double(1.2);
auto_ptr <double> autodp1(dp);
auto_ptr <double> autodp2=autodp1;
cout<<*autodp1<<endl;

I just want to know what it will print, if it even prints.

Notice: this question was in past exam paper, just for revision.

`auto_ptr` is very difficult to use, let alone to use correctly. Unless you're a C++ expert, just don't use it. — Kerrek SB, May 05 '14 at 21:01
@KerrekSB I am not using it, it is just a question that I want to know the answer for. — Khalifa Bader, May 05 '14 at 21:04
Just for reference, `auto_ptr` has officially been deprecated in C++11 and beyond instead use `unique_ptr` — YoungJohn, May 05 '14 at 21:34

score 5 · Accepted Answer · answered May 05 '14 at 21:04

5

The code *autodp1 is effectively a dereferencing of a null pointer. Therefore the code exhibits undefined behavior.

You first construct autodp1 to point to the newly allocated double. But then the constructor of autodp2 takes the owned memory for itself and sets autodp1 to null.

answered May 05 '14 at 21:04

Juraj Blaho

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Alright then, I guess that's the answer for this question. Thank you @JurajBlaho – Khalifa Bader May 05 '14 at 21:05
2

Then please mark it as such so Juraj gets proper credit for it. – Remy Lebeau May 05 '14 at 21:26
Sorry I forgot, Done. – Khalifa Bader May 05 '14 at 22:21

score 1 · Answer 2 · answered May 05 '14 at 21:24

That's becouse the operator assignement of auto_ptr takes's the ownership (move) the pointer

Take a read on Wiki, it's quite good general explanation:

http://en.wikipedia.org/wiki/Smart_pointer

"The copy constructor and assignment operators of std::auto_ptr do not actually copy the stored pointer. Instead, they transfer it, leaving the previous std::auto_ptr object empty. This was one way to implement strict ownership, so that only one auto_ptr object could own the pointer at any given time. This means that auto_ptr should not be used where copy semantics are needed."

Auto Pointer in C++ (auto_ptr)

2 Answers2