I wrote in ”[[trivial_abi]] 101” (May 2018),
during the active development of the [[clang::trivial_abi]] attribute:


  Relation to trivial relocatability: None… well, some?

  As you can see, there is no requirement that a [[trivial_abi]] class type
should have any particular semantics for its move constructor, its destructor,
or its default constructor. Any given class type will likely be trivially
relocatable, simply because most class types are trivially relocatable by accident…


A correspondent writes in with a query for Socrates
(previously seen on this blog in July 2023).

Epistolographos writes: Actually, I think trivial-abi-ness and trivial relocatability are
almost the same thing.

Authored by Arthur O'Dwyer, Quuxplusone offers  explorations of C++ programming language features, standard library design, and modern C++ development practices. Readers can delve into advanced topics such as template metaprogramming, design patterns, and effective use of language features. Additionally, they can learn about best practices for writing efficient and maintainable C++ code, optimizing performance, and leveraging the latest language standards.

Arthur O'Dwyer

A Socratic dialogue exploring the relationship between Clang's `[[clang::trivial_abi]]` attribute and trivial relocatability in C++. The discussion establishes that trivial-abi types are effectively trivially relocated when passed in registers (bits copied in and out), which is why Clang's `__is_trivially_relocatable` returns true for trivial-abi types. However, the implication is one-directional: many trivially relocatable types are not trivial-abi. The dialogue warns against using `[[clang::trivial_abi]]` as a substitute for P1144's `[[trivially_relocatable]]` because: (1) trivial-abi changes the calling convention ABI, causing linker errors or UB if TUs disagree, and (2) it has 'dull-knife' semantics that silently drop the attribute when data members are non-trivial-abi, unlike a true trivially-relocatable warrant.

A dialogue on trivial-abi and trivially relocatable