mjhjmtu

In standard jargon of C and C++, the phrase "atomic object" means "object of atomic type," does it not?

No standard will explicitly define every two-word phrase, so one does not fault the C and C++ standards for omitting explicit definition of this one. Nevertheless, when I read in the C++17 standard (draft here), sect. 4.7.1(4), that "all modifications to a particular atomic object M occur in some particular total order, called the modification order of M"—and when the standard repeatedly employs similar language to delimit ever more precise logic for concurrency—I would like to be sure that I am not inadvertently misunderstanding.

Do I assume correctly that the phrase "atomic object" means

• 
  object of atomic type?

The only plausible alternative I can imagine would be that the phrase instead meant

• properly aligned object small enough that hardware could handle it atomically.

Which is it, please?

(Note: I tag this question both C and C++ because, when it comes to atomics, the two standards use almost identical language. For this reason, an expert in either language can answer as far as I know. If for some reason I am mistaken, then please remove the C tag and retain the C++.)

Reference: see also this question, for which my question is preliminary.

The C++ standard imposes a set of rules on operations and effects of operations on atomic objects ([intro.races]). If all operations on an object satisfy those rules, then that object is atomic.

the phrase "atomic object" means "object of atomic type," does it not?

It is not worded so in the standard. But since the effect of operations is determined by the type of the object, this is not an unreasonable conclusion. Also correspondingly: Atomic type is a type whose instances are atomic objects.

The C++ standard library provides a set of types which are guaranteed to be atomic, as well as functions for those types which are guaranteed to be atomic operations ([atomics]).

properly aligned object small enough that hardware could handle it atomically.

C++ standard specifies nothing about alignment or size of atomic objects.

If an object/type is guaranteed to be atomic (see [atomics]), and if the hardware has such requirements for atomicity, then either the implementation of the language must guarantee that those requirements are met, or the implementation must employ locks to enforce atomicity.

In my view atomicity - strictly speaking - does not apply to types or objects, it applies to operations, i.e. you can say an operation is atomic or not.

By an "atomic object" we understand an object whose public interface exposes only atomic operations, i.e. all operations you can do with that object are atomic.

In C and C++ it may be that the concepts are defined the other way around: first define atomic objects and then define atomic operations in terms of atomic objects. It probably made sense for C and C++ to define it this way because the wording of the standard is primarily concerned with defining the language. However from a theoretical and abstract functionality perspective atomic operations are the main concern.

The C++ has the standard std::atomic<T> class template which fits the above descriptions.

I can answer for C, but C++ is indeed intended to be in sync with C on these points.

Yes, when the C standard speaks of "atomic object" it means an object with an effective type that is atomic-qualified. But it also seems that this is not written down explicitly, so it would probably be a good idea to add that. I'll see to that.

Also, other than some people stated, there are no atomic operations in C without atomic objects. This is volontarily fixed like that, such that the atomicity of access to these objects can never be compromized.