c++ - gflags ソースコードでの FLAGS_nono##name の使用

Question

最近、gflags のソース コードを読んでいます。そして、ここのコメントは本当に私を混乱させます。

静的初期化を確実にするために FLAGS_nono##name が導入されたと言っています。しかし、私の知る限り、次のようなグローバル var を定義すると: int x = 20;

x はまだ静的に初期化されています。ここで FLAGS_nono##name の必要性は何ですか。

私はそれを誤解していますか？

ありがとう。

// Each command-line flag has two variables associated with it: one
// with the current value, and one with the default value.  However,
// we have a third variable, which is where value is assigned; it's a
// constant.  This guarantees that FLAG_##value is initialized at
// static initialization time (e.g. before program-start) rather than
// than global construction time (which is after program-start but
// before main), at least when 'value' is a compile-time constant.  We
// use a small trick for the "default value" variable, and call it
// FLAGS_no<name>.  This serves the second purpose of assuring a
// compile error if someone tries to define a flag named no<name>
// which is illegal (--foo and --nofoo both affect the "foo" flag).
#define DEFINE_VARIABLE(type, shorttype, name, value, help)             \
  namespace fL##shorttype {                                             \
    static const type FLAGS_nono##name = value;                         \
    /* We always want to export defined variables, dll or no */         \
    GFLAGS_DLL_DEFINE_FLAG type FLAGS_##name = FLAGS_nono##name;        \
    type FLAGS_no##name = FLAGS_nono##name;                             \
    static @GFLAGS_NAMESPACE@::FlagRegisterer o_##name( \
      #name, #type, MAYBE_STRIPPED_HELP(help), __FILE__,                \
      &FLAGS_##name, &FLAGS_no##name);                                  \
  }                                                                     \
  using fL##shorttype::FLAGS_##name

詳細については、 https://code.google.com/p/gflags/source/browse/src/gflags.h.in#471を参照してください。

別の質問は、上記のコードで使用される FlagRegisterer クラスです。単に関数を定義するのではなく、ここでそのようなクラスとグローバル var の必要性は何なのか疑問に思っています。

class GFLAGS_DLL_DECL FlagRegisterer {
 public:
  FlagRegisterer(const char* name, const char* type,
                 const char* help, const char* filename,
                 void* current_storage, void* defvalue_storage);
};


// --------------------------------------------------------------------
// FlagRegisterer
//    This class exists merely to have a global constructor (the
//    kind that runs before main(), that goes an initializes each
//    flag that's been declared.  Note that it's very important we
//    don't have a destructor that deletes flag_, because that would
//    cause us to delete current_storage/defvalue_storage as well,
//    which can cause a crash if anything tries to access the flag
//    values in a global destructor.
// --------------------------------------------------------------------

FlagRegisterer::FlagRegisterer(const char* name, const char* type,
                               const char* help, const char* filename,
                               void* current_storage, void* defvalue_storage) {
  if (help == NULL)
    help = "";
  // FlagValue expects the type-name to not include any namespace
  // components, so we get rid of those, if any.
  if (strchr(type, ':'))
    type = strrchr(type, ':') + 1;
  FlagValue* current = new FlagValue(current_storage, type, false);
  FlagValue* defvalue = new FlagValue(defvalue_storage, type, false);
  // Importantly, flag_ will never be deleted, so storage is always good.
  CommandLineFlag* flag = new CommandLineFlag(name, help, filename,
                                              current, defvalue);
  FlagRegistry::GlobalRegistry()->RegisterFlag(flag);   // default registry
}