================ @@ -4016,6 +4016,34 @@ Note that the `size` argument must be a compile time constant. Note that this intrinsic cannot yet be called in a ``constexpr`` context. +``__is_bitwise_cloneable`` +-------------------------- + +A type trait is used to check whether a type can be safely copied by memcpy. + +**Syntax**: + +.. code-block:: c++ + + bool __is_bitwise_cloneable(Type) + +**Description**: + +This trait is similar to `std::is_trivially_copyable`, but additionally allows +to have user-defined constructors, virtual functions and virtual bases. It is up +to the user code to guarantee that a bitwise copy results in non-broken object +and that the lifetime of an object is properly started. + +Objects of bitwise cloneable types can be bitwise copied by memcpy/memmove. The +Clang compiler warrants that this behavior is well defined, and won't be +broken by compiler optimizations. + +After the copy, the lifetime of the new object isn't started yet (unless the +type is trivially copyable). Users must ensure its lifetime is started to avoid +undefined behavior. ---------------- ilya-biryukov wrote:
> Users must ensure its lifetime is started What should the intended action taken by the user be? Is this advice actionable? I guess that if the type is implicit-lifetime-type, users could use `start_lifetime_as` (although it's not implemented in Clang). AFAIK, there is no way to start a lifetime of other objects (e.g. containing virtual members). I suggest to either mention the `start_lifetime_as` or simply say that it's still undefined behavior to use this object, i.e. ``` Note that after the copy the lifetime of the new object isn't started and using it is undefined behavior from the C++ Standard's perspective. In C++23 `std::start_lifetime_as` can start the lifetime of implicit-lifetime types. There is no standard way to start a lifetime of other objects, e.g. that have a vtable. ``` https://github.com/llvm/llvm-project/pull/86512 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
