Dereferencing deleted pointers always result in an Access Violation?

Accessing memory through a deleted pointer is undefined behavior. You can't expect any reliable/repeatable behavior.

Most likely it "works" in the one case because the string is still "sitting there" in the now available memory -= but you cannot rely on that. VS fills memory with debug values to help force crashes to help find these errors.

The difference is that a debugger, and debug libraries, and code built in "debug" mode, likes to break stuff that should break. Your code should break (because it accesses memory it no longer technically owns), so it breaks easier when compiled for debugging and run in the debugger.

In real life, you don't generally get such unsubtle notice. All that stuff that makes things break when they should in the debugger...that stuff's expensive. So it's not checked as strictly in release. You might be able 99 times out of 100 to get away with freeing some memory and accessing it right after, cause the runtime libs don't always hand the memory back to the OS right away. But that 100th time, either the memory's gone, or another thread owns it now and you're getting the length of a string that's no longer a string, but a 252462649-byte array of crap that runs headlong into unallocated (and thus non-existent, as far as you or the runtime should care) memory. And there's next to nothing to tell you what just happened.

So don't do that. Once you've deleted something, consider it dead and gone. Or you'll be wasting half your life tracking down heisenbugs.