Why memory_order_release support until C++20?

Question

https://en.cppreference.com/w/cpp/atomic/memory_order

From cppreference, memory_order_release can use until C++20? Can anyone explain why C++ Standard will delete this and which one memory_order we should use after that.

Answer 1

This is just a misunderstanding with the formatting of cppreference. It's not that one line that is "until C++20", it's the whole block.

memory_order_release isn't removed in C++20. It's just that memory_order itself was respecified from being a weak enum:

typedef enum memory_order {
    memory_order_relaxed,
    memory_order_consume,
    memory_order_acquire,
    memory_order_release,
    memory_order_acq_rel,
    memory_order_seq_cst
} memory_order;

to an enum class (as a result of P0439):

enum class memory_order : /*unspecified*/ {
    relaxed, consume, acquire, release, acq_rel, seq_cst
};
inline constexpr memory_order memory_order_relaxed = memory_order::relaxed;
inline constexpr memory_order memory_order_consume = memory_order::consume;
inline constexpr memory_order memory_order_acquire = memory_order::acquire;
inline constexpr memory_order memory_order_release = memory_order::release;
inline constexpr memory_order memory_order_acq_rel = memory_order::acq_rel;
inline constexpr memory_order memory_order_seq_cst = memory_order::seq_cst;

All six memory operations are still there, and are still accessible with the same spelling (although consume is discouraged).

Answer 2

You're misreading the table. It's saying that whole block applies to 11 <= C++ < 20.
The block below that defines all the same constants a different way applies to C++ >= 20.

It's just by chance of formatting that the (until C++20) is on the same line as memory_order_release. Note that (since C++11) is on the same line as acquire but clearly applies to all the constants because std::atomic was new in C++11.

The change is that there's now an enum class with short names for the constants:

enum class memory_order : /*unspecified*/ {
    relaxed, consume, acquire, release, acq_rel, seq_cst
};

And the full global-scope names are defined in terms of that, e.g. as

inline constexpr memory_order memory_order_release = memory_order::release;