与std :: atomic类型的比较语义 [英] Comparison semantics with std::atomic types

问题描述

我试图找到定义T与std::atomic的比较语义的位置.

I'm trying to find where the comparison semantics for the type T with std::atomic is defined.

我知道，除了整数类型的内置专门知识之外，T可以是任何TriviallyCopyable类型.但是，像compare_and_exchange_X这样的操作如何知道如何比较 T的实例?

I know that beside the builtin specializations for integral types, T can be any TriviallyCopyable type. But how do operations like compare_and_exchange_X know how to compare an instance of T?

我想他们必须简单地对用户定义的对象(如memcmp)进行逐字节比较，但是我看不出在标准中明确提到了什么.

I imagine they must simply do a byte by byte comparison of the user defined object (like a memcmp) but I don't see where in the standard this is explicitly mentioned.

所以，假设我有:

struct foo
{
  std::uint64_t x;
  std::uint64_t y;
};

当我调用std::atomic<foo>::compare_and_exchange_weak()时，编译器如何知道如何比较两个std::atomic<foo>实例?

How does the compiler know how to compare two std::atomic<foo> instances when I call std::atomic<foo>::compare_and_exchange_weak()?

推荐答案

在草案n3936中，memcmp语义在第29.6.5节中得到了明确描述.

In draft n3936, memcmp semantics are explicitly described in section 29.6.5.

注意:例如，atomic_compare_exchange_strong的作用是 if(memcmp(对象，预期，sizeof(*对象))== 0) memcpy(object，& desired，sizeof(* object)); 别的 memcpy(expected，object，sizeof(* object));

Note: For example, the effect of atomic_compare_exchange_strong is if (memcmp(object, expected, sizeof(*object)) == 0) memcpy(object, &desired, sizeof(*object)); else memcpy(expected, object, sizeof(*object));

和

注意:如果基础类型具有填充位，陷阱位或备用表示形式，则比较和交换操作的memcpy和memcmp语义可能会导致比较与operator==相等的值的比较失败具有相同的值.

Note: The memcpy and memcmp semantics of the compare-and-exchange operations may result in failed comparisons for values that compare equal with operator== if the underlying type has padding bits, trap bits, or alternate representations of the same value.

至少从n3485起就出现了这种措辞.

That wording has been present at least since n3485.

请注意，只有memcmp(p1, p2, sizeof(T)) != 0对compare_and_exchange_weak有意义(保证失败). memcmp(p1, p2, sizeof(T)) == 0允许但不能保证成功.

Note that only memcmp(p1, p2, sizeof(T)) != 0 is meaningful to compare_and_exchange_weak (failure guaranteed). memcmp(p1, p2, sizeof(T)) == 0 allows but does not guarantee success.

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