c++ std::atomic<bool>::fetch_or 没有实现？

Question

使用这段代码摘录：

class myclass {
    volatile std::atomic<bool> flag;
    public:
    myclass(): flag(false) {}
    bool get_flag() { return flag; }
    bool try_set() {
        return !flag.fetch_or(flag, true);
    }
    void reset() {
        flag = false;
    }
};

我有这个编译错误：

error: ‘volatile struct std::atomic<bool>’ has no member named ‘fetch_or’   
   return !flag.fetch_or(flag, true);

但是，如果我将模板参数更改为 int:

，它会编译

class myclass {
    volatile std::atomic<int> flag;
    public:
    myclass(): flag(0) {}
    bool get_flag() { return flag; }
    bool try_set() {
        return !flag.fetch_or(flag, true);
    }
    void reset() {
        flag = 0;
    }
};

atomic 参考资料说 "the full specialization atomic<bool>" 被视为 "non-specialized"，我认为这是问题的根源。所以我的疑惑：

"full specialization" 怎么会是 "treated as non-specialized"？
在调用 flag.fetch_or() 时，使用 as flag 模板参数 int 而不是 bool 是否会遇到任何棘手的陷阱？

我正在使用 gcc 5.1.0，并使用 -std=c++14 进行编译。

Answer 1

C++11 N3337 draft 不需要 bool 的方法。

29.5 "Atomic types"

template <class T> struct atomic {
  [...]
}

template <> struct atomic<integral> {
  [...]
  integral fetch_or(integral , memory_order = memory_order_seq_cst) noexcept;
  [...]
}

29.5/1:

The semantics of the operations on specializations of atomic are defined in 29.6.

29.6.3/2 "Arithmetic operations on atomic types":

In the declarations of these functions and function template specializations, the name integral refers to an integral type and the name atomic-integral refers to either atomic or to a named base class for integral from Table 145 or inferred from Table 146.

和Table145不包含bool.

所以只有 struct 的积分（没有 bool）特化会有那个方法。

这有点令人困惑，因为在标准的其余部分，"integral types" 包括 bool、3.9.1/7 "Fundamental types":

Types bool, char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types. A synonym for integral type is integer type.

c++ std::atomic<bool>::fetch_or 没有实现？

c++ std::atomic<bool>::fetch_or not implemented?

c++

embedded

atomic