What does Visual Studio do with a deleted pointer and why?
A C++ book I have been reading states that when a pointer is deleted using the delete operator the memory at the location it is pointing to is "freed" and it can be overwritten. It also states that the pointer will continue to point to the same location until it is reassigned or set to NULL .
In Visual Studio 2012 however; this doesn't seem to be the case!
Example:
#include <iostream>
using namespace std;
int main()
{
int* ptr = new int;
cout << "ptr = " << ptr << endl;
delete ptr;
cout << "ptr = " << ptr << endl;
system("pause");
return 0;
}
When I compile and run this program I get the following output:
ptr = 0050BC10
ptr = 00008123
Press any key to continue....
Clearly the address that the pointer is pointing to changes when delete is called!
Why is this happening? Does this have something to do with Visual Studio specifically?
And if delete can change the address it is pointing to anyways, why wouldn't delete automatically set the pointer to NULL instead of some random address?
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.
Visual Studio could set you up for a sticky situation by failing to document this flaw in its design.
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:
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.
It also states that the pointer will continue to point to the same location until it is reassigned or set to NULL.
That is definitely misleading information.
Clearly the address that the pointer is pointing to changes when delete is called!
Why is this happening? Does this have something to do with Visual Studio specifically?
This is clearly within the language specifications. ptr is not valid after the call to delete . Using ptr after it has been delete d is cause for undefined behavior. Don't do it. The run time environment is free to do whatever it wants to with ptr after the call to delete .
And if delete can change the address it is pointing to anyways, why wouldn't delete automatically set the pointer to NULL instead of some random address???
Changing the value of the pointer to any old value is within the language specification. As far as changing it to NULL, I would say, that would be bad. The program would behave in a more sane manner if the value of the pointer were set to NULL. However, that will hide the problem. When the program is compiled with different optimization settings or ported to a different environment, the problem will likely show up in the most inopportune moment.
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