Deleting a pointer to an automatic variable

Question

Please look at this code

int i = 10;                                     //line 1 
int *p = &i;                                    //line 2  
delete p;                                       //line 3 
cout << "*p = " << *p << ", i = " << i << endl; //line 4  
i = 20;                                         //line 5  
cout << "*p = " << *p << ", i = " << i << endl; //line 6  
*p = 30;                                        //line 7
cout << "*p = " << *p << ", i = " << i << endl; //line 8

What is the result of this code? Especially of line 3, 5 and 7? Do they invoke undefined behavior? What would be the output?

EDIT : I tried running it using g++, and it's compiling and running fine! I'm using MinGW on Windows 7.

What does Standard say in this context?

have you tried running it? may be you can add the result of your run and we can discuss it then? — Nim, Dec 03 '10 at 08:59
`*** error for object 0x7fff5fbff5ec: pointer being freed was not allocated *** set a breakpoint in malloc_error_break to debug` while running it... compiled with g++ 4.1 in MacOS X. By my definition of "running" it is not running fine. — David Rodríguez - dribeas, Dec 03 '10 at 09:06

score 5 · Accepted Answer · edited May 23 '17 at 11:43

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You can delete only a pointer if you have ever allocated it dynamically using new. In this case you have not allocated the pointer using new but simply defined and initialized it to point to a local variable of type int.

Invoking delete on a pointer not allocated dynamically using new is something called Undefined Behavior. In short, it means that anything on the earth can happen when such a code is executed and you can't complaint a bit to anyone on this planet.

edited May 23 '17 at 11:43

Community

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answered Dec 03 '10 at 09:05

Chubsdad

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Not only should memory be allocated dynamically, it should be allocated with `new` only. – sharptooth Dec 03 '10 at 09:23

score 1 · Answer 2 · answered Dec 03 '10 at 09:01

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delete p; is UB and so any further behavior can't be predicted or relied upon. You program might crash immediately or spend all your money or just exit from main() and pretend nothing happened.

answered Dec 03 '10 at 09:01

sharptooth

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score 0 · Answer 3 · answered Dec 03 '10 at 09:02

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Line 3 is definitely undefined behaviour, since you're trying to deleting memory at an address that is not on the heap.

answered Dec 03 '10 at 09:02

Simon Courtenage

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why has this been downvoted, since it says the same as the one that has been upvoted? – Simon Courtenage Dec 05 '10 at 10:43

Deleting a pointer to an automatic variable

3 Answers3