alignas 用于内联静态成员时是否合法？

Question

当 alignas 与 inline static 成员变量一起使用时，

MSVC 会抛出错误，而 Clang 不会。哪一个是正确的？ clang 的标志：-O2 -std=c++2a。 MSVC 的标志：/std:c++latest.

错误：

error C2024: 'alignas' attribute applies to variables, data members and tag types only

#include <iostream> //https://godbolt.org/z/q4ScM7
#include <atomic>
#include <random>
class PRNG {
public:
    alignas(128) inline static std::atomic<uint64_t> state{ [] {
        auto rd{ std::random_device{} };
        return static_cast<uint64_t>(rd()) << 32
            | static_cast<uint64_t>(rd());
    }() };
};

int main()
{
    std::cout << PRNG::state;
}

以下变体，其中初始化与定义分开并且 inline 未使用，两者都成功：

#include <iostream> //https://godbolt.org/z/rvq6AP
#include <atomic>
#include <random>
class PRNG {
public:
    alignas(128) static std::atomic<uint64_t> state;
};
alignas(128) std::atomic<uint64_t> PRNG::state{ [] {
    auto rd{ std::random_device{} };
    return static_cast<uint64_t>(rd()) << 32
        | static_cast<uint64_t>(rd());
}() };

int main()
{
    std::cout << PRNG::state;
}

当 inline static 不是成员变量时，它们都接受 alignas：

#include <iostream> //https://godbolt.org/z/Kd9NEV
#include <atomic>
#include <random>

alignas(128) inline static std::atomic<uint64_t> state{ [] {
    auto rd{ std::random_device{} };
    return static_cast<uint64_t>(rd()) << 32
        | static_cast<uint64_t>(rd());
}() };

int main()
{
    std::cout << state;
}

Answer 1

在 C++17 [dcl.align]/1 中它说：

An alignment-specifier may be applied to a variable or to a class data member, but it shall not be applied to a bit-field, a function parameter, or an exception-declaration.

在您的代码中，state 是数据成员（静态数据成员是数据成员），因此允许使用对齐说明符。

所以我建议报告编译器错误。

Answer 2

在 MSVC 2019 std:c++latest 中指的是最新工作草案中的预览功能，我认为它仍然存在问题。所以你应该使用 std:c++17 并且它会编译没有错误。 /std

The /std:c++latest option enables the post-C++17 language and library features currently implemented in the compiler and libraries. These features may include changes from the C++20 Working Draft, defect updates that aren't included in C++17, and experimental proposals for the draft standard. For a list of supported language and library features, see What's New for Visual C++. The /std:c++latest option doesn't enable features guarded by the /experimental switch, but may be required to enable them.

alignas 用于内联静态成员时是否合法？

Is alignas legal when used for an inline static member?

c++

clang

visual-c++

language-lawyer

c++17