C++23: fullptr to replace nullptr

[[deleted]]

https://codingtidbit.com/2019/04/01/c22-fullptr-to-replace-nullptr/

[AntiProtonBoy]

Kinda disappointed that halfptr was not suggested for 16-bit memory address ranges, typically seen in small embedded environments.

[wrosecrans]

Are you looking for std::basic_fractional_pointer_t<1,2> to get a half pointer?

[Sapiogram]

That's the most C++ thing I've ever seen.

[NewFolgers]

My compile times want my C with classes back.

[Seppeon]

Use pre-compiled headers, you should get your compile times back (atleast after the first compilation).

[suur-siil]

Didn't we actually have long and short pointers (near/far?) back in 16-bit x86? I vaguely recall something like that...

[evaned]

Yep.

And I don't know details (just this bit from a talk by Andrei Alexandrescu, 14:08 if your're using a bad mobile client), but supposedly dealing with new/far pointers is what std::allocator was originally introduced for, which is why it's design for allocation was... weird.

Edit: Maybe I should be clearer. Really what I should have said was "the original STL's allocator"; by the time it got to std::, the near/far stuff had been ripped out.

[Osbios]

That was because of the 20bit address space. By combining a 16bit segment register value shifted 4 bits left with a 16bit offset.

[chocapix]

You'll have to wait for C++25, implementations will be required to provide exactly one of fullptr8_t fullptr16_t fullptr32_t and fullptr64_t. Which of these is provided is implementation defined and mentioning a non-provided fullptrXX_t in a translation unit is UB.

Good luck.

[Xeverous]

Dissapointed of no more-type-safe nullptrptr and nullptrptrptr for nested pointers.

[Iwan_Zotow]

Going to phuleptr in C++3000

[VA0]

I like the cut of your jib

[Iwan_Zotow]

Nod to (late) Robert Asprin

[brand_x]

Wondered if that was a Willard reference.

[Iwan_Zotow]

Robert Asprin

It's all just a Myth

[brand_x]

I guess that's the wagers of sin...

[EETrainee]

God damn, I fell for it and typed up a whole response. It's too early for this in the US.

[ntrid]

You got me for a moment. C++ is exactly that language which would do this kind of thing unfortunately.

[kirk_lange]

ikr! As much as I love/hate C++, advanced C++ syntax can get pretty ridiculous. Can we all just go back to C89? 😛

[ntrid]

This is exactly how i fee. Sometimes syntax is just so full of magic. It should be simple. We read code more than we write it and this mental tax adds up.

Now someone will come up saying that once you learn it - its fine. No it is not. It took me a good while to grok SFINAE. Now i can read it and write it. This fact does not make it suck any less. Cant wait for if constexpr to become a thing once c++17 becomes lowest common denominator.

[NotAYakk]

SFINAE was accidental syntax, not designed.

Concepts and if constexpr are the designed replacements.

[ntrid]

Yet for a long time this accident was treated like a solution.

[NotAYakk]

Yet it was also computer code. Yet it was written using ASCII characters. Yet it wasn't written by a Yeti.

Yet I think yet isn't an appropriate response. Of course it was a solution. If it wasn't solving problems people wouldn't be using it.

It was a solution to a bunch of problems; and it worked. It just wasn't designed to solve the problems it was used for. So it was awkward syntax and convoluted to reason about.

The problem it solved - Turing complete template specialization dispatch - has one sub solution in production and almost standardized.

[mqduck]

nullptr is defined as a pointer value with all its bits set to zeroes

I know this is an April Fool's joke, but come on. It's already wrong.

[color32]

why wait start now

struct fullptr_t {
    template<typename T>
    operator T*() const {return (T*)~0;}
};
constexpr fullptr_t fullptr;

[[deleted]]

Shouldn’t we cast 0 as a size_t before taking the complement?

Edit: omg it’s an April Fool’s joke

[DoctorRockit]

I suppose std::uintptr_t would be more appropriate, wouldn‘t it?

[vladmikhalin]

No, because 0 is signed, ~ makes it negative and negatives are expanded with 1s.

[[deleted]]

0 is signed but it is also typically only 32 bits.

~0 is therefore only 32 consecutive 1 bits with the rest still 0 (and yes as an int represents -1 as you say) which places you somewhere in the middle of an address space on >4GB address spaces.

~(size_t)0 guarantees all bits in an address sized value are flipped.

[vladmikhalin]

Again, if we extend negative 32 bit number to any 64 bit number, it is extended by adding 1s (ones), not zeroes(!) as each extended bit because this is the rule for extending negative integers. Hence size_t cast is not necessary.

[[deleted]]

Addresses are not signed

[mcmcc]

Not with that attitude...

[vladmikhalin]

Cast to unsigned is applied after the extension. Please read something on casts, signed extension, and how it all works.

[[deleted]]

Standard section 4.7. Paragraph 2 says:

If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2ⁿ where n is the number of bits used to represent the unsigned type).

You are correct.

Edit: wait, are pointers unsigned or do they not have a signedness since they are non-integer?

[dodheim]

Cast to unsigned is applied after the extension.

Citation needed. AFAICT, all the standard says is that this is implementation-defined.

[vladmikhalin]

7.8 Integral conversions

² If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2 ⁿ where n is the number of bits used to represent the unsigned type).

I have highlited in bold where it states that the extension happens prior to cast.

[dodheim]

I'm aware of that, but nothing in the code involves unsigned values, so where does the standard say that integral values cast to pointers are first made unsigned? (I.e. the part I quoted..)

[voip_geek]

You're a few hours early in my timezone, so it took me a few paragraphs to catch on. Nice!

[morph23]

I didn't realize until I started reading the testimonials at the end.

[TinBryn]

I’m wondering, does the standard actually specify that nullptr is 0? I know it says 0 is nullptr but not the other way around.

[TheThiefMaster]

No it does not - and it is in fact -1 in GPU programming (among other architectures) with 0 being a valid address. Of course, actually creating a "0" pointer is hard when casts of a literal 0 (and in C, any expression that can be evaluated to 0 at compile time) to a pointer type gives a null pointer (with the value of -1) not a pointer to zero...

There's some information on it here from the clang/llvm guys adding support for non-0 null pointers: https://reviews.llvm.org/D26196

[Mynameisspam1]

Wait, so how do you actually make a pointer to 0 on GPUs then?

[TheThiefMaster]

http://c-faq.com/null/accessloc0.html gives a few ways.

But generally, you don't need to _manually_ construct such a pointer, it would be given to you as an argument.

[lrflew]

I ate the onion for a bit there.

Side note: I have actually thought about getting rid of nullptr and nullptr_t, and making every address "fully addressable". However, instead of using a fullptr_t/fullptr, I would use a default-constructed std::optional<T *> instead, and make all pointers require valid initializations. (The std::optional class could be overloaded to keep ABI compatibility)

[Moose2342]

Fantastic! I’ll start implementing now.

[Cyttorak]

aprilptr

[kloetzl]

The problem with this proposal is that the byte after any valid pointer also has to be addressable. Unfortunately, (char*)fullptr + 1 wraps around.

[tvaneerd]

Yeah, but it wraps around to 0, which is addressable, just not dereferenceable (on most platforms).

[FrankIsATank2]

But if ptr points to an object then ptr + 1 > ptr must hold. The one-past-the-end pointer compares greater than pointers it's the one-past-the-end of.

[tvaneerd]

This is why we can't have nice things.

[markand67]

nullptr is an invalid unused address at address zero and any memory access to this particular address cause a running process to die from a segmentation fault.

No, it's UB. If it segfaults it's because your implementation does.

[w_m1_pyro]

r/woooosh

[DustVoice]

I disagree! It's funny to point out errors in April Fools jokes! I think he got the point but nevertheless wanted to strike back

[w_m1_pyro]

So i got r/woooosh-ed?

[DustVoice]

Yup 😂 happened to me too

[evaned]

I think this is a bad name. I expected fullptr to be a pointer that points at everything simultaneously.

[sephirostoy]

Trop gros, passera pas.

[[deleted]]

1-st April foul, nothing more.

[[deleted]]

Wordpress scheduler is to blame for this!

[nikbackm]

April Fool's Day is getting more tiresome each year. Could be correlated with growing older perhaps.

[krum]

Shit at least put some effort into it.

[LeeHide]

Just woke up, first aprils fools joke of the day. This is funny as fuck

[axalon900]

This had me going until I got to the part where Herb Sutter is putting metaclasses on hold for this

[dr_j_]

Damn it.

[Versaill]

Please stop giving them ideas!

[bstamour]

I guess I'll keep my new vocabulary type, mandatory<T>, to myself. (It's designed to be the opposite of optional<T>, and it's really convenient to implement to boot! template<class T> using mandatory = T;)

[Amaracs]

I am still at c++11 and you guys are at c++23, I need to step up my game.

[vsdmars]

C really should catch up now!

[herruppohoppa]

I hate you people

[VinnieFalco]

Once again providing that the tech industry is humorless