C constructs that still don't work in C++

C constructs that still don't work in C++(lospino.so)

46 points by jalospinoso 3 days ago | 30 comments

mjs01 3 hours ago |

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

jalospinoso 3 days ago |

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.

uecker 57 minutes ago | |

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.

pjmlp 3 hours ago | |

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.

hgs3 3 hours ago | |

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

pjmlp 3 hours ago | | |

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.

Retr0id 1 hour ago | |

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.

cmovq 30 minutes ago |

Designated initializers is one area where C feels much more expressive than C++. And that feature has been standard since C99.

mike_hock 2 hours ago |

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."

Panzerschrek 1 hour ago |

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.

arcadialeak 1 hour ago | |

> 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.

p0w3n3d 2 hours ago |

  Address white_house{
    .street = "1600 Pennsylvania Avenue NW",
    .city = "Washington",
    .state = "District of Columbia",
    .zip = 20500,
  };

For me this is the most important initialization in C that helps with clarity so much, I used mostly structs to have function parameters intialized like this

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++

AdieuToLogic 3 hours ago |

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.

C++ was a superset of C 30-ish years ago. Now, as the author correctly identifies, it is not as both have taken different evolutionary paths.

electroly 2 hours ago | |

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.

pjmlp 3 hours ago | |

Already in C++98 there were differences.

?: has another execution priority.

Implicit cast scenarios are reduced in C++.

grougnax 2 hours ago |

At this point, just give up c++ and use Rust

dhruv3006 3 hours ago |

> restrict: a C promise, not a C++ contract

This takes the cake.

Pay08 1 hour ago |

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++.

uecker 59 minutes ago | |

You can just run it through a compiler in c++ language mode.

grougnax 2 hours ago |

Give up C++ and use Rust

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.