幻像类型与原始类型具有相同的对齐方式吗? [英] Does a phantom type have the same alignment as the original one?

问题描述

考虑以下包含一些环境值的结构:

Consider the following struct that contains some environment values:

struct environment_values {
  uint16_t humidity;
  uint16_t temperature;
  uint16_t charging;
};

我想向那些具有幻像类型*的值添加一些其他信息，并同时使其类型不同:

I would like to add some additional information to those values with a phantom type* and make their types distinct at the same time:

template <typename T, typename P>
struct Tagged {
    T value;
};

// Actual implementation will contain some more features
struct Celsius{};
struct Power{};
struct Percent{};

struct Environment {
  Tagged<uint16_t,Percent> humidity;
  Tagged<uint16_t,Celsius> temperature;
  Tagged<uint16_t,Power>   charging;
};

Environment的内存布局与environment_values相同吗?这是否也适用于混合类型的布局，例如:

Is the memory-layout of Environment the same as environment_values? Does this also hold for mixed type layouts, e.g.:

struct foo {
    uint16_t value1;
    uint8_t  value2;
    uint64_t value3;
}

struct Foo {
    Tagged<uint16_t, Foo>  Value1;
    Tagged<uint8_t , Bar>  Value2;
    Tagged<uint64_t, Quux> Value3;
}

对于到目前为止我尝试过的所有类型，都保留以下断言:

For all types I've tried so far, the following assertions held:

template <typename T, typename P = int>
constexpr void check() {
    static_assert(alignof(T) == alignof(Tagged<T,P>), "alignment differs");
    static_assert(sizeof(T)  == sizeof(Tagged<T,P>),  "size differs");
}

// check<uint16_t>(), check<uint32_t>(), check<char>() …

由于标记和未标记变体的大小也相同，所以我猜是，但是我希望可以肯定.

Since the size of the tagged and untagged variants is also the same, I guess the answer should be yes, but I would like to have some certainty.

^{*我不知道如何在C ++中调用这些标记的值. 强类型typedef"?我是从Haskell取这个名字的.}

推荐答案

该标准在 [basic.align]/1 :

对象类型具有对齐要求(3.9.1、3.9.2)，这些要求放在 对该类型的对象可能位于的地址的限制 已分配. 对齐方式是实现定义的整数值 代表连续地址之间的字节数 可以分配给定的对象.对象类型强制对齐 对该类型的每个物体的要求；可以更严格地对齐 使用对齐说明符(7.6.2)请求.

Object types have alignment requirements (3.9.1, 3.9.2) which place restrictions on the addresses at which an object of that type may be allocated. An alignment is an implementation-defined integer value representing the number of bytes between successive addresses at which a given object can be allocated. An object type imposes an alignment requirement on every object of that type; stricter alignment can be requested using the alignment specifier (7.6.2).

此外， [basic.compound]/3 提到:

指针类型的值表示形式是实现定义的. 指向 layout-compatible类型的指针应具有相同的值 表示和对齐要求(6.11). [注意:指向 过度对齐的类型(6.11)没有特殊的表示形式，但是它们的 有效值范围受扩展对齐方式的限制 要求].

The value representation of pointer types is implementation-defined. Pointers to layout-compatible types shall have the same value representation and alignment requirements (6.11). [Note: Pointers to over-aligned types (6.11) have no special representation, but their range of valid values is restricted by the extended alignment requirement].

因此，可以保证与布局兼容的类型具有相同的对齐方式.

As a result, there is a guarantee that layout-compatible types have the same alignment.

struct { T m; }和T与布局不兼容.

如指出的那样

As pointed here, in order for two elements to be layout compatible then they both have to be standard-layout types, and their non-static data members must occur with the same types and in the same order.

struct { T m; }仅包含T，但是T是T，因此它不能包含T作为其第一个非静态数据成员.

struct { T m; } contains just a T, but T is a T so it cannot contain a T as its first non-static data member.

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