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

3 days ago

C++ is not a superset of C, and compatibility between them requires specifying the language mode (e.g., C17, C23, C++17, C++20, C++23).
Designated initializers in C++20 are available but more constrained than C's versions (e.g., they require direct members and declaration order, no mixing with positional clauses).
C23 changed void f() to behave as void f(void), closing a gap with C++ where it means different things (no parameters in C++).
Implicit conversion from void* to object pointer works in C (e.g., from malloc) but not in C++, requiring explicit casting.
C++20 repaired some object lifetime issues with malloc for implicit-lifetime types, but malloc is still not idiomatic C++ due to lack of constructors and ownership management.
Discarding const requires explicit casting in C++ (const_cast), but writing to actually-const objects remains undefined behavior.
Enums in C23 have more explicit typing, but C++ enums are distinct types and require casting from integers (prefer enum class for C++ APIs).
The restrict keyword is standard in C but not in C++, with compiler-specific extensions available.
Flexible array members are not standard in C++ (use vector or span instead, but keep C layouts at ABI boundaries).
Migration advice includes treating malloc as storage only, preserving C layouts at edges, using C++ constructs for clear ownership/lifetime, and isolating compiler extensions.

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