C constructs that still don't work in C++
lospino.soI think the problem is that C++ is a poorly designed language with a fundamentally flawed development process.
Instead of letting compiler implementers decide which features to add and how to implement them, C++ employs a standards-first, top-down approach. Features are often defined by committee members who may not use modern C++ in their daily workflows, leaving it entirely up to the individual implementations to catch up.
Some features were standardized back in 2023, yet not a single implementation supports them in 2026.
Rust is impl-first bottom-up and it's stuck with a single implementation and GCC for Rust is still in the works, meanwhile C++26 reflection is already in GCC trunk.
Is that supposed to be a bad thing? I like having only one implementation. Multiple compilers is annoying for users, have to write "portable" code which can only target the lowest common denominator. Only when a feature ships in Clang, GCC and VC++ can you use it. Each compiler needs its own flags/project as well.
Loosely coupling a language to its compiler is 20th century thinking for when programming languages were simple. It works for C because C is simple enough to be implemented over and over again. But for today's hyper complicated languages, multiple implementations is a pain for everyone.
C++ had it's own ling periods of badly lagging/buggy implementations of the standards. It's better today but I'm not sure how much of that I'd credit or discredit to the way the standards process works.
Rust is also the systems programming language of the future. They're pretty much doing everything right.
What IMHO Rust does not get right and why I do not use it: long compilation times, high complexity, its syntax, polymorphism based on monomorphization, the requirement for many dependencies to get anything done, an ecosystem susceptible to supply-chain attacks, no ISO standard.
Out of curiosity, what do you think is wrong with monomorphization-based polymorphism? The other alternatives I'm aware of are 1. type-erasure via v-table based dynamic dispatch (which Rust also has in the form of the `dyn` keyword), which has performance and memory-allocation overhead and 2. macros, which Rust also has and, if used for polymorphism, would essentially be like compile-time monomorphization, but clunkier.
Maybe I'm missing something though and there are other alternatives done differently in other languages?
> committee members who may not use modern C++ in their daily workflows
Are you sure about this one? I don’t know exactly who’s in the committee these days but last I checked they were all hardcore C++ programmers with decades of experience from the trenches.
This is so true and it's a danger for other ecosystems too. Once the big corporations throw their weight in (be it in a committee or only informally) they make their use cases dominant regardless how niche they might be for the rest of the world.
For example in Rust there is one big entity that currently pours a lot of energy into improving C++ interop. Now, this is not exactly a niche topic, but especially in a world where AI makes many rewrites possible that we wouldn't have daunted to think about a couple of years before, we shouldn't waste too much effort to save legacy companies enormous codebases at the detriment of our preferred language.
I think Rust has similar flaws as C++: too much unneeded complexity and abysmal compilation times.
unneeded complexity such as?
> Features are often defined by committee members who may not use modern C++ in their daily workflows, leaving it entirely up to the individual implementations to catch up.
Eh, all of the committee members I've known are obsessed with modern C++, and "can this feature be implemented?" is definitely a blocker; numerous features got kicked down the road from C++0x to later versions because compilers weren't ready for them.
C++ is a language with perfect 30-year backwards compatibility, and which mostly-maintains compatibility with another language it forked from (C), after 40 years of diverging development.
Rust is a language which isn't backwards-compatible, and certainly not compatible with source code in other languages.
Now, sure, Rust has its advantages, but - how can you fault C++ in the context of compatibility?
this compatibility sucks sometimes.
for example you want to add nice feature to c++ with nice syntax, but there is a similar syntax somewhere in C that nobody uses, but you have to support it. you end up with nice feature with horrible syntax.
Some unmentioned incompatibilities I've encountered that makes a C header not directly usable in C++:
- C `_Atomic(T)` and C++ `std::atomic<T>`. C++23 has C compatible header `stdatomic.h` that defines `_Atomic(T)`, but it's still problematic
- C `_Noreturn/noreturn` and C++ `[[noreturn]]`. C23 `[[noreturn]]` makes them compatible
- C inline and C++ inline are different. Good news is their `static inline` are the same
- C has anonymous struct. C++ doesn't. Both have anonymous union though
I wrote this after repeatedly seeing experienced C programmers hit the same sharp edges while moving into modern C++ codebases.
Many of these differences are intentional and defensible from the C++ side. But some are still surprising because they invalidate patterns that were historically common, performant, or idiomatic in C.
The interesting part to me isn’t "C vs C++," but where the languages diverged philosophically: object lifetime vs raw storage, stronger type systems, implicit conversions, ABI and optimization assumptions, and the boundary between "portable" and "works on my compiler."
I’d also be curious which C constructs people still genuinely miss in modern C++. For me, restrict is still near the top of the list.
You might be interested in the scpptool feature to help convert C code to a subset of C that will also compile as C++ (under clang++ at least) [1]. While many of the necessary modifications are fairly trivial, some of them aren't completely so. For example, C++ does not allow `goto`s that would skip over the declaration/initialization of a variable that would be accessible after the jump. So getting the C code to work as C++ can involve some (automatic) code restructuring.
Another annoying detail is that C++ doesn't seem to like forward references of `enum`s. That is, while
is fine in both C and C++ even before `struct A` has been defined, apparentlystruct A* a_ptr;
is not cool in C++ until after `enum A` has been defined.enum A* a_ptr;One arguable benefit of keeping your C code compatible with (or at least convertible to) C++, is that you can theoretically use scpptool's auto-translation feature as build step to produce memory-safe executables from C code via transpilation to a memory-safe subset of C++.
[1] https://github.com/duneroadrunner/SaferCPlusPlus-AutoTransla...
The "stronger type system" is mostly a myth in my opinion. It was true in the past in pre-prototype C. The void pointer rules are better in C IMHO as they avoid unneeded casts (that then remove more type safety) and FAMs and variably-modified types can express things C++ simply can't do well.
I don’t understand your point at all, C++ objectively has a much stronger type system. It’s turing complete!
I’m not arguing that that’s better, or worse, but it’s definitely true and by no means a myth.
I don't think GP meant "it's completely made up", I think he meant the distinction doesn't matter most of the time.
I.e. most of the time the typing in real C++ code isn't meaningfully stronger than that found in C code.
> The void pointer rules are better in C IMHO as they avoid unneeded casts
...so much this! A void pointer is an "any-pointer" by design. It shouldn't require casting from and to specific pointer types, that defeats the whole point of having void pointers in the first place.
> It shouldn't require casting from and to specific pointer types
You don't need to explicitly cast T* to void* (guaranteed to be safe), you only need to cast when converting out of void*.
The rules are basically the same as casting between pointer-to-derived-class and pointer-to-base-class and they make sense.
Not sure if you're aware, but defer is proposed for C2Y [1]. It's already available in Clang behind a compiler flag. It is interesting how the languages continue to diverge.
[1] https://www.open-std.org/JTC1/SC22/WG14/www/docs/n3734.pdf
Because the communities aren't the same.
C++ is 1990's Typescript for C++, while C folks still think is a portable Assembly instead of designed to an abstract machine model.
As such C++ community embraces high level abstractions and type systems improvements, whereas C wants to still code as targeting classical hardware.
Caring for the actual assembler output in selected critical pieces of code is not the same as ignoring the abstract machine model. What you claim is simply not the case if you check actual proficient systems programmers. Of which there are an astonishingly high share C and C++-but-mostly-C programmers.
Any user of compiled languages cares about Assembly, which is why regardless of the compiled language, an Assembler was always shipped alongside.
Also it isn't a C invention to have the compiler dump the Assembly output instead of object code.
Now the culture that C language constructs in 2026 are still 1:1 to Assembly instructions, that pretty much prevails, despite easy proof that isn't the case at various compiler optimization levels.
Proficient devs, well many still don't know to distinguish what is their compiler, and what ISO says.
It is the case that you can more easily know what happens when you don't use the wrong abstractions but stay in control. Highly-abstracted C++ code basically makes allocations and syscalls in the whitespace between the source code tokens. You can't do systems software like that, you have to roll back the abstractions and roll back the use of pre-canned containers and libraries that you don't understand.
So it's all about understanding and control, not about some idea that C was defined in terms of assembly instructions, which it obviously is not. That's a total strawman.
Except modern C also has plenty of abstractions, devs wrongly assume it doesn't.
Then get surprised when it doesn't map to the SIMD/SIMT NUMA machine their code actually executes on.
such a strawman again... You don't want to be writing explicit platform specific SIMD most of the time. You just want to write a dumb function that doesn't do any non-obvious calls, doesn't cause thread contention, doesn't hide complexity, isn't going to be a nightmare to change later, no surprises.
I am talking about self-inflicted complexity that is entirely within the C(++) machine model. Avoid that complexity and you're pretty good already. Only drop down to concrete hardware arch level where it makes sense. But largely, the C machine model is still very much suited as a model for actual hardware. Writing straightforward obvious code allows you to stay in control of memory layout and the data transformation paths. It easily gets you within <<2x of what you could achieve with hand coded assembler for the >90% of the code that are pretty boring and straightforward. And obviously you couldn't get the work done in time when coding everything in assembler.
There is not much real evidence for "devs wrongly assume" and as someone writing numerical code (clusters, NUMA, SIMD, etc.) I think C is still the ideal tool for this.
That is the entire point, yes. Reasoning about layers of completely imaginary entities is what demotivates me about C++ and Rust. Meanwhile, hardware bits are very real (and getting more expensive recently). Having implemented slices and generics in C, now C++ feels like Vietnam flashbacks.
Yet C23 isn't K&R C any longer, nor is the hardware a PDP 11.
Also when we eventually start talking to agents that perform the whole execution steps by themselves, that is kind of irrelevant.
Except for the lucky ones that still code to keep the infrastructure going, which is mostly C++.
The "nor is the hardware a PDP 11". Byte access was the main new feature of the PDP 11 that C adopted. Are you saying being able to access individual bytes is not relevant on modern hardware?
Might more mean that we've standardised on a few things like what a byte even is.
The PDP-11 had both 8 and 9-bit bytes. Thats a complexity that few programmers have to touch on, today.
IIRC PDP-11 was a 16 bit word machine with an 8-bit byte. Maybe you remember PDP-10 with 4x9=36 bit words?
On the 11, the UNIBUS was 18 bit, the program space was 16 bit, and addressing was 22 bit. So it depended if you were using I-space or D-space.
Anyway, I do not see how this affects the design of C in a way that makes no sense anymore today (except that one could require CHAR_BIT to be eight, but there are still DSPs where this is not the case). I think people repeat the "the C design reflects the out-dated PDP-11 hardware" meme because it sounds smart while in reality it is just nonsense.
The PDP-11 myth is getting a bit tired by now ;)
If C would be so hardwired to the PDP-11 architecture it would have died with it. In reality C works just fine on all sorts of hardware (like GPUs) with only minor extensions.
I appreciate that restrict isn't there, because it is yet another UB source, programmer knows not to do errors kind of attitude, and secondly no one seems to care enough to write a language proposal for it.
I take it you probably never tried to use any of these languages for HPC. Without a language standard, you have two options there to compile decently performant executables: (1) compiler pragmas, (2) give up and drop to assembly code.
I should add here that there's also (3): Switch to Fortran, which made fundamentally different choices and is IMO the only fully supported higher-than-C level language that can produce HPC applications without fighting a compiler left and right.
Can you link to some writeup regarding how Fortran is preferential to C++ (or rather C++ plus compiler `__restrict`) in this respect?
I keep looking around and not finding any, so let me just try here before someone just takes it and slopifies it:
* built-in multidimensional arrays with efficient storage.
* related to this: built-in array intrinsics
```
real, dimension(100,100) :: A, B, C
C = A + B
```
this kind of code is already a close-to optimal "naive" implementation (not considering parallelization). so you start already at a solid place. then you can easily run it in parallel without too much specialized knowledge with OpenMP, OpenACC, MPI or even CUDA. the only thing you really need to be aware of when implementing your own loops/kernels: the intrinsic storage order, to optimize for cache hits.
* crucial: all the above amounts to a standard/best practice about how data is structured and formatted. everyone just uses the built-ins. Thus, interoperability between native Fortran numerical libraries is usually a complete non-issue. Meanwhile, Cpp has a fractured ecosystem with different array/vector types for its libraries. Converting between one and the other is usually a no-go.
* next, the intent plus pass-by-reference system. it combines IMO the best of both worlds of a functional vs. procedural approach:
* finally, a clean symbol definition system that decouples types from byte lengths. a `float` in fortran is just `real(4)`, a double is `real(8)`, a long int is `integer(8)` and so on. now, it's trivial to do a bit of preprocessing to switch the precision.- I can predefine if an array / variable is intended as input, output or both, simply with `intent(in)`, `intent(out)` etc. - compiler can thus do checks for me if I'm breaking a contract. - yet, since I pass by ref, I don't have to worry about memory not being used efficiently. This really matters once you deal with GB, TB or even PB worth of data going into a simulation over time - there's just no way to deal with that in a purely functional way. - only where really necessary, you can still drop down to pointer semantics, e.g. for the outer glue code of a simulation that swaps outputs and inputs for the next time step. - having `restrict`-by-default semantics here helps immensely. Imagine you have many arrays with lots of data to deal with, and your kernels access always several at a time to do calculations. In Fortran I can write it intuitively, while in C/C++ I must remember to specify `restrict` or to preload all point-inputs first into separate variables to direct the compiler. Otherwise the memory pressure increases, and by far most such physics simulations are memory bandwidth bound - every access counts.However, the last part is where Cpp has a strong advantage: Well supported meta-programming (generics, templating or even just well supported pre-processors). Fortran's compilers come with a lot of built-ins, so the lack of these is less of an issue than you might think, but it's still a limiting factor. All that being said, a typical scientist doesn't tend to care and just wants to solve a particular problem rather than thinking in generalized frameworks - and that's why I find Fortran still serves them better for numerics than anything that came since.
> I wrote this after repeatedly seeing experienced C programmers hit the same sharp edges while moving into modern C++ codebases.
...I've seen this more often in the opposite direction. Since C++ is stuck with a ca 1995 non-standard subset of C, C++ coders usually have a very outdated view of C.
> I’d also be curious which C constructs people still genuinely miss in modern C++.
Not implementing the full C99 designated init feature set was a huge missed opportunity in C++20. Every single feature of C99 designated init is important and clicks with the other features and the rest of the language, take one or two away and it becomes mostly useless (e.g. the order requirement in C++20 means that designated init is only useful for trvial structs).
It's especially tragic because Clang already had the full C99 designated init feature set in C++ mode implemented long before C++20 and it worked just fine.
> The interesting part to me isn’t "C vs C++," but where the languages diverged philosophically
IMHO this "schism" was completely unnecessary and only happened because of ignorance and hubris by the C++ designers. Objective-C shows that C can be extended with radical new features but without messing up the "C side" (e.g. ObjC features don't overlap with C features, which means that ObjC is automatically compatible with the latest C standards).
In the end it's not a big deal of course, C and C++ are now entirely different languages and longterm that's for the better. Even the C++ peeps seem to have come to that realization and no longer recommend to "compile C in C++ mode" (like Herb Sutter in 2012 when trying to justify why MSVC had no C99 support: https://herbsutter.com/2012/05/03/reader-qa-what-about-vc-an...):
This was bad advice back then and is even worse advice today. At least MSVC got "good enough" C99 support a couple of years later (in VS2015), but after a few hopeful years after 2019 it looks like MSVC development has completely stalled again."We recommend that C developers use the C++ compiler to compile C code (using /TP if the file is named something.c). This is the best choice for using Visual C++ to compile C code."Did you use an LLM to write this comment? (I don't mean this as an accusation, I'm uncertain. I'm just trying to calibrate myself.)
Edit: I should've had more conviction in my instincts, this is slop.
The thing with the flexible trailing array member is a C++ design flaw. Now the fix wouldn't be to allow those "flexible arrays" in C++, at least not the way C has them, but it should have a concept (not that kind of concept) of types that are indeterminately sized at compile time and whose size is determined at construction.
If you're allocating something on the heap anyway, you shouldn't be forced to pay for an indirection in order to have some variable-sized data in the object, you should just be able to put it all in the one allocation. (Sure, you can achieve that with placement new hackery but that certainly isn't "idiomatic" C++.)
Of course that's completely incompatible with the way allocation and construction work (storage has to be allocated before the constructor runs). Hence "design flaw" rather than "missing feature."
There is no problem putting objects past the end of another object in C++.
I use this approach as part my zero-copy serialization library for what I call "out-of-line" sequences.
It does require smart usage of std::launder to be standards-compliant though.
You can pass a flag to clang to allow reordering field initializations in designator initializer thing. It makes the syntax super annoying in case of large structs anyway.
It doesn't matter unless you are using constructors or modifying some variables in the initialization expression anyway.
Address white_house{
.street = "1600 Pennsylvania Avenue NW",
.city = "Washington",
.state = "District of Columbia",
.zip = 20500,
};
However C++ had at time no default initialization for unmentioned fields, so in 2017 I had to remove it when converting the code to C++
Designated initializers were unofficially supported by GCC (and clang IIANM) since around when C++11 was supported. See:
https://godbolt.org/z/3aKaa7dnM
only if you specifically ask to get an error, would you actually get it.
restrict in C++ can't work well. One can mark pointer parameters with this attribute, but in C++ it's not recommended to pass raw pointers, std::string_view or std::span should be used instead, but there is no way to specify restrict for the internal pointer of these containers.
> std::string_view or std::span should be used instead
That is for when the owner is a std::string or an owning range respectively. But a raw pointer does still make sense as a non-owning view over a single element, doesn't it? I'm new to C++ so I might be wrong.
Non-owning view over a single element should simply be a reference, you don’t care where this element is located.
Designated initializers is one area where C feels much more expressive than C++. And that feature has been standard since C99.
From the article:
In 2019 I wrote a short survey of C constructs that do not
work in C++. The point was not that C is sloppy or that C++
is superior. The point was that C++ is not a superset of C,
and that C programmers crossing the border should know
where the checkpoints are.
30 years ago, in C89 and pre-standard C++, it was the case that `int foo()` in C is a function that accepts any parameters, and in C++ it is a function with no parameters. In C89 you have to write `int foo(void)` if you want no parameters. This counterexample to C++ being a superset of C was well-known even back then.
Another well-known counterexample is implicit conversion from void*. In C89 you can do `int* foo = malloc(100);` but in C++ it requires an explicit cast from void* to int*.
I don't believe there was ever a time, even pre-standardization, when C++ was a strict superset of C; it always had little incompatibilities here and there.
Perhaps in c-with-classes(Cpre)? To the extent that its output could be considered C.
It looks like you're right and the answer to when was C++ a superset of C may well be "never".
From the description, Cfront had always been a full-fledged parser that only happened to output C since the very beginning.
> a full-fledged parser
perhaps more accurately a fully fledged compiler (that emitted C)
Already in C++98 there were differences.
?: has another execution priority.
Implicit cast scenarios are reduced in C++.
I find variable-length arrays (i.e. arrays whose length is defined at run-time and typically live on the stack) to be kind of dangerous, and try to avoid them, even in C.
Man this table is disturbing. "Same", same as?
> restrict: a C promise, not a C++ contract
This takes the cake.
Is there some sort of tool that checks headers for this stuff? On the occasion that I write a C library, I prefer it to be directly usable in C++.
You can just run it through a compiler in c++ language mode.