c++ - std::atomic<X>::value_type 发生了什么？

Question

根据 this reference manual

For every std::atomic<X> (whether or not specialized), std::atomic<X>::value_type is X.

但是如果我尝试使用这种类型，我会得到一个编译错误。

我用 g++ 8.2.1 试过了:

$ g++ -std=c++11 test.cc 
test.cc: In function ‘int main()’:
test.cc:6:23: error: ‘value_type’ is not a member of ‘std::atomic<int>’
     std::atomic<int>::value_type x = 0;

还有 clang 6.0.1

$ clang -std=c++11 test.cc 
test.cc:6:23: error: no type named 'value_type' in 'std::atomic<int>'
    std::atomic<int>::value_type x = 0;
~~~~~~~~~~~~~~~~~~^

前面提到的引用手册还明确指出...

specification was substantially rewritten to resolve numerous issues in particular, member typedefs value_type and difference_type are added

P0558R1然而，规范似乎并不禁止 value_type 的存在。 .

知道发生了什么吗？

编辑

我的一位同事让我意识到 P0558R1 只是一个提案。

Answer 1

您正在明确使用 C++11。如果我们查看 last draft 的第 1119 页在 C++11 标准中，没有提及 std::atomic 的 value_type:

template <class T> struct atomic {
    bool is_lock_free() const volatile;
    bool is_lock_free() const;
    void store(T, memory_order = memory_order_seq_cst) volatile;
    void store(T, memory_order = memory_order_seq_cst);
    T load(memory_order = memory_order_seq_cst) const volatile;
    T load(memory_order = memory_order_seq_cst) const;
    operator T() const volatile;
    operator T() const;
    T exchange(T, memory_order = memory_order_seq_cst) volatile;
    T exchange(T, memory_order = memory_order_seq_cst);
    bool compare_exchange_weak(T&, T, memory_order, memory_order) volatile;
    bool compare_exchange_weak(T&, T, memory_order, memory_order);
    bool compare_exchange_strong(T&, T, memory_order, memory_order) volatile;
    bool compare_exchange_strong(T&, T, memory_order, memory_order);
    bool compare_exchange_weak(T&, T, memory_order = memory_order_seq_cst) volatile;
    bool compare_exchange_weak(T&, T, memory_order = memory_order_seq_cst);
    bool compare_exchange_strong(T&, T, memory_order = memory_order_seq_cst) volatile;
    bool compare_exchange_strong(T&, T, memory_order = memory_order_seq_cst);

    atomic() = default;
    constexpr atomic(T);
    atomic(const atomic&) = delete;
    atomic& operator=(const atomic&) = delete;
    atomic& operator=(const atomic&) volatile = delete;
    T operator=(T) volatile;
    T operator=(T);
};

它在 C++14 草案中同样不存在。

cppreference 未能为 value_type 提及“自 C++17 起”。

编辑:有人指出，value_type 的添加是以缺陷报告的形式出现的，应该追溯应用于 C++11 的实现。因此，cppreference 实际上并没有错，DR 只是尚未在给定的编译器版本中实现。