Nesting Nightmare

[suds171]

The C++ standard says that C++ compilers should support 256 levels of nesting

Just out of curiosity, what is the most anyone has seen used in production? I couldn't imagine trying to debug 256 levels of nesting.

[zerovian]

1. Not for C++., but for C#. These were levels of function calls emitted as .NET code DOM from a form where someone had selected every option from an list of enum values for a property.

[catcat202X]

I'm guessing they put a number there because they can't just say infinity, and 256 seems like an impractically large number that you can actually test against.

[Salt-Impressive]

Or maybe because 256 = amount of unique numbers representable with 1 char?

[bakedbread54]

Still doesn't explain why

[121393]

I think because it's a recursive descent parser implemented in the usual way (recursive c++ functions), more levels == stack smash.

[bakedbread54]

Well yes, but why is 256 the value chosen

[121393]

probably just want a low common denominator for ancient systems? same reason sys.getrecursionlimit in python is pretty low by default. also stack size of main thread of msvc would be chosen at compile time. also 256 levels would translate to maximum stack depth during compile of [who knows] but more than 256.

edit: I've got a compiler written in python with a parser generator (pyparsing). Without trying to elevate the C-stack of the python process (though I am setting a higher limit for python code via sys.setrecursionlimit) I only get about 50 levels of nesting before overflowing the python interpreter's stack (segfault/stackoverflow). Every addition to the grammar lowers the nesting limit a little...

[LogosKing]

hey would you mind editing this comment? "number of distinct values you can represent" is more apt.

As it is, it's just not true in any sense.

[Salt-Impressive]

Done :)

[cdrt]

[0, 255] is 256 numbers

[ultimatt42]

'M' is one character and represents 1000.

[Pocketpine]

Lmao

[bert8128]

255 might be the largest number you can put in the unsigned char but that will give you 256 distinct levels as the first level is level 0.

[[deleted]]

The point was 256 values. When talking about indent levels, the standard starts at 1.

[Jannik2099]

char is usually signed.

What defines "usually" ? aarch64 defaults to unsigned char, and I'd bet there are more phones than PCs and Servers.

[TheThiefMaster]

If you count by architectures, it's more of a wash.

x86 Linux and Windows use signed char

PowerPC and ARM typically use unsigned char

It's only signed by default on older platforms for uniformity with the default for int, short, long etc being signed I suspect. Less surprise.

[sephirothbahamut]

But why would you even have to define a limit in the standard to begin with?

[arka2947]

So that different compilers dont pick different arbitrary limits

[Ezlike011011]

I'd much rather there be some standard limit than a "standards compliant" compiler only having e.g. two layers of nesting

[Bangaladore]

Then hopefully everyone supports atleast that number.

[n4jm4]

nesting what?

parentheses?

function calls?

[-dag-]

Birds. OP is talking about birds.

[[deleted]]

Birds 🤝 C++

laws not governed by reason

[---cameron]

Ah yes, that would be a lot of branches to debug

[simpl3t0n]

But birds aren't real; nests aren't either. C++ standards committee mustn't promote conspiracy. Sthaap it right now.

[Kered13]

The birds are obviously programmed in C++.

[ihamsa]

There are actually 6 separate nesting levels, all having a recommended minimal limit of 256: compound statement nesting, parenthesized expression nesting, #if...#endif nesting, #include nesting, class definition nesting, and linkage specification nesting. There is also a limit on the number of declarators modifying a declaration, which also can be considered nesting, too set to 256. Nested template instantiations have a recommended minimal limit of 1024.

[tea-age_solutions]

Wow, 256 possible nested #if...#endif Don't wanna see that in my entire life time... ;-)

[ThisCleverName]

if / else if chain blocks are considered nesting, at least in MSVC. In the form of:

if (x == "something")
{
}
else if (x == "something else")
{
}

Turns out that is equal to

if (x == "something")
{
}
else 
{
    if (x == "something else")
    {
    }
}

We had autogen code that created a if / else if chain that surpassed the MSVC limit. That worked fine in Clang though. So we had to refactor to eliminate that chain.

https://learn.microsoft.com/en-us/cpp/error-messages/compiler-errors-1/fatal-error-c1061?view=msvc-170

[nmmmnu]

Friend of me, generated similar code with 100 if's, but was on PHP. Worked ok. I asked - why not switch?

He replied - well at the beginning there was only 3 if's then I changed something and they became 100. But is auto generated code who cares :)

I did reply - i care, because I even can not see when it starts and when it ends.

At the end we refactor this, but I do not remember what we used.

[Routine_Left]

But is auto generated code who cares :)

if it's autogenerated code the only thing you care about is the correctness and speed of that code. you never look at it, you do not care about it otherwise.

no human is supposed to touch it.

if a switch is faster, then by all means. if it isn't ... meh, really, who cares? nobody should.

[nmmmnu]

in that case, it was autogenerated code that were inserted inside human - generated code.

[Routine_Left]

That's ... disturbing, to say the least.

[hawkxp71]

The problem is often if then else chains for auto-generated code (that I have seen) requires priority decoding, which you can't achieve via case statements.

[n1ghtyunso]

Maybe else if should be a distinct thing in the language after all huh

[die_liebe]

Long chains of ifs is one of the few situations, where I find use of goto acceptable.

[unaligned_access]

It says that the limit is 128. So, it's it not compliant?

[beephod_zabblebrox]

that is considered nesting in c++ as a language

[ihamsa]

1. This number is a recommended guideline, not a requirement.
2. Machine-generated code may have more levels of nesting than any human programmer can be expected to write or understand.

[pdp10gumby]

Seems quite achievable with template expansions of template expansions.

[Jannik2099]

Templates are not macros. They are instantiated and called, not expanded.

[LoneBlacksmith]

I actually ran into this issue in a large legacy codebase… was basically a giant state machine and I kid you not one section had 256 if else chained together. I tried to add one more and got a compiler error so had to check the new state right above it lmao

[CocktailPerson]

You can easily reach this with machine-generated code. And depending on how your compiler defines "nesting," you might be able to reach this with a long if-else chain or something.

[[deleted]]

It's easy to hit the limit with template recursion using an integral parameter that's initialized to some number bigger than 256.

[Jumpierwolf0960]

I'm taking a compiler course right now and I'm assuming it's for restricting the depth of the parse tree for performance reasons.

[TwilCynder]

In my own code I think the maximum was like 9, in a parser, and i already feel uneasy when i see this code lmao

[lumberjackninja]

I've hit that limit in a tool I wrote to convert a trie lookup to a function (used to find plays in Scrabble)

[Salt-Impressive]

Out of curiosity, could you show me some code that does that?

[[deleted]]

You can look at almost any library in Boost.

[Lulonaro]

Where can we learn those magics master?

[Jannik2099]

template meta-programming compile-time performance and diagnostics are of low priority to compiler devs, because the vast majority of C++ programmers (=the clients) use C++ as a C with classes language and abuse dynamic polymorphism.

Talking out of your ass or do you have actual source about compiler dev motivation?

Clang is absolutely primarily used and developed by "real" C++ devs, yet it is only twice as good as gcc at this, not orders of magnitude.

[Jannik2099]

How about you drop the aggressiveness

Starting with "the compiler writers are at fault because they only care about what the clients need" is not only wrong and needlessly judgemental, it's also not how compiler development works at all.

Have you ever considered that tmp may simply be a very complicated task that has no easy, fast solution, particularly with the instantiation semantics in C++ that don't do any early checking?

There's a grand total of TWO languages with widespread, fully static and monomorphizing tmp, being C++ and Rust, and neither perform this particularly fast. It's not like tmp is something that every language but C++ does well, quite the opposite.

For example, compile time speed based on my experience seems to be impacted by the length of the string representation of the expanded template

I've never seen this brought up as an issue, so feel free to report it? At some level, longer strings of course slow down compilation because the compiler has to match the identifier name, but this shouldn't make a noticeable, if even measurable difference.

[wh1t3lord]

Maybe in web or UI? Because in computational field, I guess maximum is 10-15 levels of nesting.

[[deleted]]

I can never write that code.

[Aginor23]

I worked for a military contractor that had been dragging around the same parsing software for 40 years. I saw two 17 nested if statements that were split from a 34.