为什么 std::aligned_storage 在 C++23 中被弃用，用什么代替？

Question

我刚刚看到 C++23 计划弃用这两个 std::aligned_storage and std::aligned_storage_t as well as std::aligned_union and std::aligned_union_t。

据我所知，在对齐存储中放置新对象并不是特别 constexpr 友好，但这似乎不是完全放弃该类型的好理由。这让我假设使用 std::aligned_storage 和我不知道的朋友还有其他一些基本问题。那会是什么？

是否有替代这些类型的提议？

Answer 1

以下是 P1413R3 的三段摘录：

Background

aligned_* are harmful to codebases and should not be used. At a high level:

• Using aligned_* invokes undefined behavior (The types cannot provide storage.)
• The guarantees are incorrect (The standard only requires that the type be at least as large as requested but does not put an upper bound on the size.)
• The API is wrong for a plethora of reasons (See "On the API".)
• Because the API is wrong, almost all usage involves the same repeated pre-work (See "Existing usage".)

---

On the API

std::aligned_* suffer from many poor API design decisions. Some of these are shared, and some are specific to each. As for what is shared, there are three main problems [only one is included here for brevity]:

• Using reinterpret_cast is required to access the value

There is no .data() or even .data on std::aligned_* instances. Instead, the API requires you to take the address of the object, call reinterpret_cast<T*>(...) with it, and then finally indirect the resulting pointer giving you a T&. Not only does this mean that it cannot be used in constexpr, but at runtime it's much easier to accidentally invoke undefined behavior. reinterpret_cast being a requirement for use of an API is unacceptable.

---

Suggested replacement

The easiest replacement for aligned_* is actually not a library feature. Instead, users should use a properly-aligned array of std::byte, potentially with a call to std::max(std::initializer_list<T>) . These can be found in the <cstddef> and <algorithm> headers, respectively (with examples at the end of this section). Unfortunately, this replacement is not ideal. To access the value of aligned_*, users must call reinterpret_cast on the address to read the bytes as T instances. Using a byte array as a replacement does not avoid this problem. That said, it's important to recognize that continuing to use reinterpret_cast where it already exists is not nearly as bad as newly introducing it where it was previously not present. ...

上一节来自已接受的退休提议 aligned_*，然后是一些示例，例如这两个替换建议：

// To replace std::aligned_storage
template <typename T>
class MyContainer {
private:
    //std::aligned_storage_t<sizeof(T), alignof(T)> t_buff;
    alignas(T) std::byte t_buff[sizeof(T)];
};

// To replace std::aligned_union
template <typename... Ts>
class MyContainer {
private:
    //std::aligned_union_t<0, Ts...> t_buff;
    alignas(Ts...) std::byte t_buff[std::max({sizeof(Ts)...})];
};