在 Google protobuf 中将默认枚举值设置为“未指定”的目的是什么？

Question

我可以看到，在 Google Chrome (Chromium) 源中，第一个枚举值（默认）被声明为“未指定”：

// The type of a subresource filtering rule.
enum RuleType {
  RULE_TYPE_UNSPECIFIED = 0;

  RULE_TYPE_COMMENT = 1;  // Comment rule.
  RULE_TYPE_URL = 2;      // Network level filtering rule based on URL pattern.
  RULE_TYPE_CSS = 3;      // Element hiding rule based on a CSS selector.
};

...

// Types of anchors that can be used to constrain where a URL pattern must
// begin/end in the URL in order to be considered a match.
enum AnchorType {
  ANCHOR_TYPE_UNSPECIFIED = 0;

  // Acts like a '*' wildcard at the respective end of a pattern.
  ANCHOR_TYPE_NONE = 1;
  // The pattern must match from the start/until the end of the URL.
  ANCHOR_TYPE_BOUNDARY = 2;
  // The pattern must match starting with the TLD+n of the URL's domain, but the
  // scheme and subdomains (if any) can be arbitrary.
  ANCHOR_TYPE_SUBDOMAIN = 3;
};
...

https://cs.chromium.org/chromium/src/components/url_pattern_index/proto/rules.proto

这样做的目的是什么？

考虑到它是protobuf2，因此字段可以用optionaland标记，required并且unspecified只需将字段设置为可选即可删除枚举值。

比方说： https: //cs.chromium.org/chromium/src/components/url_pattern_index/proto/rules.proto ?l=22 ：

// The format of a URL pattern.
enum UrlPatternType {
  URL_PATTERN_TYPE_UNSPECIFIED = 0;

  // A pattern without special characters, e.g. "example.com".
  URL_PATTERN_TYPE_SUBSTRING = 1;

  // The pattern contains one or more wildcards, namely '*' and/or '^'
  // characters. The '*' matches any sequence of characters, while the '^'
  // matches a separator, i.e. anything but a letter, a digit, or one of [-._%].
  URL_PATTERN_TYPE_WILDCARDED = 2;

  // The pattern is a regular expression.
  URL_PATTERN_TYPE_REGEXP = 3;
};

但该字段是optional： https: //cs.chromium.org/chromium/src/components/url_pattern_index/proto/rules.proto ?l=150 ：

 optional UrlPatternType url_pattern_type = 6;

为什么不传递它而不是传递unspecified值？

Answer 1

也许最重要的原因是如果您将来想迁移到 proto3，请提供一条干净的路径，因为这是唯一直接兼容的选项。

然而，它也简化了在许多平台上的使用，因为不同语言之间的枚举差异很大。它还使默认值非常明显地被认为“可能是错误的”——同样：默认的新实例的初始化方式在平台/语言之间可能会有很大差异，并且您希望的控制级别可能在某些平台上不可用。