CODEWITHSUNDEEP
c++pointersnulldestructornullptr
CS Student
CS Student

Reputation: 33

What is a "known bad pointer value"?

In this answer to this question, it was noted that:

If you're going to 'clear' the pointer in the dtor, a different idiom would be better - set the pointer to a known bad pointer value.

and also that the destructor should be:

~Foo()
{
    delete bar;
    if (DEBUG) bar = (bar_type*)(long_ptr)(0xDEADBEEF);
}

I have two question about these parts of the answer.

Firstly, how can you set the pointer to a known bad pointer value? How can you set a pointer to an address which you can assure won't be allocated?

Secondly, what does: if (DEBUG) bar = (bar_type*)(long_ptr)(0xDEADBEEF); even do? What's DEBUG? I couldn't find a macro named so. Also, what's long_ptr? What does it do?

Upvotes: 1

Views: 277

Answers (3)

Michael Burr
Michael Burr

Reputation: 340436

how can you set the pointer to a known bad pointer value? How can you set a pointer to an address which you can assure won't be allocated?

First, let me be clear that the example code you refer to is a dirty hack and is intended to provide some guidance to assist debugging. It is not intended as a production quality memory management tool; it isn't even intended to be "drop in" code - it's an example of a debugging technique.

Setting a pointer to a hardcoded value isn't guaranteed to be a "bad pointer" unless you know something about the target environment. 0xDEADBEEF is a value that is likely to be an invalid pointer on many environments just out of luck. The value was chosen in because I had seen it used as a marker for "invalid data" in other code and it is easily spotted when viewing memory dumps. I believe it is (or was, maybe not anymore - that answer was from 14 years ago!) commonly used to indicate memory areas that are invalid/unused. Similar to some of the values Microsoft used in their debug library versions of some memory management routines (see https://stackoverflow.com/a/370362)

what does: if (DEBUG) bar = (bar_type*)(long_ptr)(0xDEADBEEF); even do? What's DEBUG? I couldn't find a macro named so.

I might not have said explicitly in the answer you refer to, but the example code might more properly be call a pseudo-code example. if (DEBUG) is used to indicate a bit of code that is conditionally executed "if this is a debug build".

For example, DEBUG could be a macro (or variable) that is defined as non-zero during a debug build of the problem. Possibly on the compiler's command line (maybe something like -DDEBUG=1). A DEBUG macro is something that I found commonly used for code that is enabled in debug builds only.

Also, what's long_ptr? What does it do?

To aid in the transition from 32-bit to 64-bit systems, MS added types that are size of pointers. LONG_PTR is an integer type that has the size of a pointer (32 or 64 bits as appropriate). I probably should have used LONG_PTR instead of long_ptr. I believe the cast is technically unnecessary, but I think it's still useful as a notation that makes clear that an integer is being 'converted' to a pointer - a coding idiom that uses dirty looking casts to call out a dirty hack.

Upvotes: 1

Dúthomhas
Dúthomhas

Reputation: 10103

Unless you have system-level access, pointer values near NULL are all bad. Like 1, 2, 3, etc.

I disagree with the premise of the linked question. If you think that testing should find memory leaks using magic numbers for pointer values, then there is a serious design failure in your application and testing strategies.

Write good code. Don’t write bad code to find bad code.

Upvotes: 3

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 123263

0xDEADBEEF is a pointer value like any other. Strictly speaking, there is no "known bad pointer value".

However, as a very rough simplification we can assume pointer values to be random. The chances to see 0xDEADBEEF as 32-bit value is 2^-32, that is: 2.3283064e-10. It is much more likely to be hit by a lightning or to find 2 four leaved glovers in a row than to see exactly that value on any given pointer. In other words, for all practical purposes we can assume that a 0xDEADBEEF is from our assignment when we see it.

DEBUG is not a standard macro. The author just wanted to illustrate that doing the assignment can be enabled in debug builds and disabled in non-debug builds (by employing a dedicated flag).

Upvotes: 3

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