undefined | Better HN

0 pointshermitdev6y ago0 comments

The null checks are only optimized away if you've already derefenced the pointer before the null check within a scope. Optimizer rationale being youve already derefenced it, so it must not be null, therefore the null check is unnecessary.

Also, you can "safely" dereference nullptr, just so long as you dont attempt to actually access the memory. C++ references are nothing more than a fancy pointer with syntactic sugar.

For example: int* foo = nullptr; int& bar = *foo; // no blow up std::cout << bar << std::end; // blowup here

My personal $0.02 is that the C++ standard falls short with language like "undefined/unspecified behavior, no diagnostic required." A lot of problems could be prevented if diagnostics (read: warnings) were required, assuming devs pay attention to the warnings, which doesnt always happen. For example: Google ProtoBuf has chosen to ignore at their own and clients' peril potential over/underflow errors and vulnerabilities by ignoring signed/unsigned comparison warnings.

0 comments

tomjakubowski6y ago

Dereferencing a null pointer to convert it to a reference causes undefined behavior, there's nothing safe about it!

"Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” obtained by dereferencing a null pointer, which causes undefined behavior."

MiroF6y ago

UB isn't "safe" so I'm unsure what your comment is getting at

hermitdevOP6y ago

I guess the point I was trying to make is that what is referred to colloquially as dereferencing is different than how the compiler sees it. We see "foo" (can't get HN to emit the asterisk for pointer dereference here, no matter what I try ), and we know that to be UB, but the compiler doesnt really see it until the load. Until its actually used, its effectively a dead store and will be eliminated, anyway.

    int& bar = *foo;

Doesnt actually deference foo. No load is issued from the address stored by foo. Until you either load or store using bar, no null dereference has occurred.

Further if bar is never used, no actual dereference has occurred. In fact, there will be no assembly instructions emitted for the above statement because it is pure syntactic sugar. Pointers and references in C++ are the same, except with different syntax and the secret club handshake that references are presumed to never be null (but there are ways they can become null and thus the UB).

Edit: formatting, at least attempted

MiroF6y ago

The problem is that we don't know what the compiler might think..

If I write something along the lines of

  int& bar = *foo

  if(!foo) {
    // do something
  }

The compiler very well might (and would be perfectly within its rights to) completely eliminate everything inside of the if(!foo) since it can assume the pointer is non-null because it is being dereferenced.

ncmncm6y ago

This is very definitely false. That is totally UB, launch-the-missiles stuff. Check your references before you repeat this silliness.

j / k navigate · click thread line to collapse

0 pointshermitdev6y ago0 comments

Also, you can "safely" dereference nullptr, just so long as you dont attempt to actually access the memory. C++ references are nothing more than a fancy pointer with syntactic sugar.

For example: int* foo = nullptr; int& bar = *foo; // no blow up std::cout << bar << std::end; // blowup here

0 comments

tomjakubowski6y ago

Dereferencing a null pointer to convert it to a reference causes undefined behavior, there's nothing safe about it!

MiroF6y ago

UB isn't "safe" so I'm unsure what your comment is getting at

hermitdevOP6y ago

    int& bar = *foo;

Doesnt actually deference foo. No load is issued from the address stored by foo. Until you either load or store using bar, no null dereference has occurred.

Edit: formatting, at least attempted

MiroF6y ago

The problem is that we don't know what the compiler might think..

If I write something along the lines of

  int& bar = *foo

  if(!foo) {
    // do something
  }

ncmncm6y ago

This is very definitely false. That is totally UB, launch-the-missiles stuff. Check your references before you repeat this silliness.

j / k navigate · click thread line to collapse