枚举的基础 int 是否有保证宽度？
在 N2347 的示例中，新强类型枚举 (C++11) 与当前 (C++03) 相比的一个优势是通过一个“旧”枚举的示例来解释的：
enum Version { Ver1 = 1, Ver2 = 2 };
struct Packet {
Version ver;
// ... more data ...
// bad, size can vary by implementation
Version getVersion() const { return ver; }
};
我想知道：这里的“变化”意味着编译器甚至可以选择一种8位表示形式，即枚举元素仅占用一个单词？或者是否有最小长度保证并且选择只能在int和long之间？
换句话说，如果我有一个像这样的 Version 枚举：
enum VersionXyz { Ver1 = 100, Ver2 = 200 };
struct Packet {
VersionXyz ver;
// ... more data ...
};
这是否可能只需要 Packet 中的 8bit 。软件更新后：
enum VersionXyz { Ver1 = 100, Ver2 = 200, Ver3=300 };
现在 Packet 不再具有相同的大小，因为枚举已增长？
In the example in N2347 one advantage if the new stongly typed enums (C++11) compared the current (C++03) is explained by an example with an "old" enum:
enum Version { Ver1 = 1, Ver2 = 2 };
struct Packet {
Version ver;
// ... more data ...
// bad, size can vary by implementation
Version getVersion() const { return ver; }
};
I wonder: means "varying" here that the compiler can even choose a 8bit representation, i.e. that the enum-elements occupy only one word? Or is there a minimum length guaranteed and the choice may only be between int and long?
In other words, if I have a Version enum like this:
enum VersionXyz { Ver1 = 100, Ver2 = 200 };
struct Packet {
VersionXyz ver;
// ... more data ...
};
Is it possible that this takes only 8bit in Packet. And after a software update to:
enum VersionXyz { Ver1 = 100, Ver2 = 200, Ver3=300 };
now Packet has not the same size anymore because the enum has grown?
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C++03 标准的相关段落是 §7.2.5：
所以它是实现定义的。唯一可以保证的是，如果您定义一个所有值都适合 int 的枚举，则枚举类型的大小不会大于 int。 （但是由于
int的大小也是实现定义的，所以这并没有给你太多信息。）The relevant paragraph of the C++03 standard is §7.2.5:
So it's implementation defined. The only thing you are guaranteed is that if you define an enum where all values fit into an
int, then the enumeration type's size won't be greater thanint. (But sinceint's size is also implementation-defined, that doesn't give you much.)如果您知道您的枚举列表只会达到一定的大小，那么在使用时可能值得对其进行类型转换，并且还可以断言是否超过了该大小，至少这就是我在发送数据包时使用枚举的方式
If you know that your enum list is only ever going to be a certain size it might be worth type casting it when used, and also have an assert for if that size is ever exceeded, at least thats how I use enums when sending packets