What does Visual Studio do with a deleted pointer and why?

Solution 1:

I noticed that the address stored in ptr was always being overwritten with 00008123...

This seemed odd, so I did a little digging and found this Microsoft blog post containing a section discussing "Automated pointer sanitization when deleting C++ objects".

...checks for NULL are a common code construct meaning that an existing check for NULL combined with using NULL as a sanitization value could fortuitously hide a genuine memory safety issue whose root cause really does needs addressing.

For this reason we have chosen 0x8123 as a sanitization value – from an operating system perspective this is in the same memory page as the zero address (NULL), but an access violation at 0x8123 will better stand out to the developer as needing more detailed attention.

Not only does it explain what Visual Studio does with the pointer after it is deleted, it also answers why they chose NOT to set it to NULL automatically!

This "feature" is enabled as part of the "SDL checks" setting. To enable/disable it go to: PROJECT -> Properties -> Configuration Properties -> C/C++ -> General -> SDL checks

To confirm this:

Changing this setting and rerunning the same code produces the following output:

ptr = 007CBC10
ptr = 007CBC10

"feature" is in quotes because in a case where you have two pointers to the same location, calling delete will only sanitize ONE of them. The other one will be left pointing to the invalid location...

UPDATE:

After 5 more years of C++ programming experience I realize this entire issue is basically a moot point. If you are a C++ programmer and are still using new and delete to manage raw pointers instead of using smart pointers (which circumvent this entire issue) you may want to consider a change in career path to become a C programmer. ;)

Solution 2:

You see the side-effects of the /sdl compile option. Turned on by default for VS2015 projects, it enables additional security checks beyond those provided by /gs. Use Project > Properties > C/C++ > General > SDL checks setting to alter it.

Quoting from the MSDN article:

• Performs limited pointer sanitization. In expressions that do not involve dereferences and in types that have no user-defined destructor, pointer references are set to a non-valid address after a call to delete. This helps to prevent the reuse of stale pointer references.

Do keep in mind that setting deleted pointers to NULL is a bad practice when you use MSVC. It defeats the help you get from both the Debug Heap and this /sdl option, you can no longer detect invalid free/delete calls in your program.