C++ delete pointer twice [duplicate]

Answer 1

I compiled this program in g++ 4.9.1 and it gave me a runtime error:

*** Error in `./t': free(): invalid pointer: 0xbfa8c9d4 ***

You are trying to free something which is already freed. Hence, the error.

Answer 2

The behaviour of your program in not defined after the second delete command so you do not really know which error will be occured and printed in your terminal.

Answer 3

If you compile your program in Visual Studio Debug mode then the first delete should set the memory to 0xFEEEFEEE (see this question) - however the address is still accessible by your process so is unlikely to cause an exception. Linking in a product like BoundsChecker or Purify I believe checks for these byte patterns so that it can detect accessing of deleted memory.

Answer 4

Deleting the same memory twice is undefined behaviour. Anything may happen, including nothing. It may e.g. cause a crash sometime later.

Answer 5

I tried doing this in Visual Studio. There are two cases:

1)

delete p;
delete p;

This compiles properly but gives debug assertion failure when you run the program because you are trying to delete the memory location which is already deleted and no longer belongs to you.

2)

delete p;
p = NULL;
delete p;

This compiles properly and runs properly. There is no error. Try printing p before and after delete.