C++ 在gflags源代码中使用FLAGS_noname

我最近在读gflags的源代码。这里的评论让我很困惑

它说引入FLAGS_noname是为了确保静态初始化。但据我所知，如果你定义一个全局变量，比如： int x=20

x仍然是静态初始化的。这里有什么必要用旗子

我误解了吗

谢谢

// 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

---

你是对的，比如int i=10；执行静态初始化。因此，可以编写

#define DEFINE_VARIABLE(type, shorttype, name, value, help)             \
  namespace fL##shorttype {                                             \
    /* We always want to export defined variables, dll or no */         \
    GFLAGS_DLL_DEFINE_FLAG type FLAGS_##name = value;                   \
    type FLAGS_no##name = value;                                        \
    static @GFLAGS_NAMESPACE@::FlagRegisterer o_##name( \
      #name, #type, MAYBE_STRIPPED_HELP(help), __FILE__,                \
      &FLAGS_##name, &FLAGS_no##name);                                  \
  }                                                                     \
  using fL##shorttype::FLAGS_##name

但是现在你有了动态初始化，你需要静态初始化

为了确保这两种方法都能正常工作，您需要一个额外的helper变量，这就是您所询问的代码的作用

#define DEFINE_VARIABLE(type, shorttype, name, value, help)             \
  namespace fL##shorttype {                                             \
    /* We always want to export defined variables, dll or no */         \
    GFLAGS_DLL_DEFINE_FLAG type FLAGS_##name = value;                   \
    type FLAGS_no##name = value;                                        \
    static @GFLAGS_NAMESPACE@::FlagRegisterer o_##name( \
      #name, #type, MAYBE_STRIPPED_HELP(help), __FILE__,                \
      &FLAGS_##name, &FLAGS_no##name);                                  \
  }                                                                     \
  using fL##shorttype::FLAGS_##name

#define DEFINE_VARIABLE(type, shorttype, name, value, help)             \
  namespace fL##shorttype {                                             \
    /* We always want to export defined variables, dll or no */         \
    GFLAGS_DLL_DEFINE_FLAG type FLAGS_##name = value;                   \
    type FLAGS_no##name = FLAGS_##name;                                 \
    static @GFLAGS_NAMESPACE@::FlagRegisterer o_##name( \
      #name, #type, MAYBE_STRIPPED_HELP(help), __FILE__,                \
      &FLAGS_##name, &FLAGS_no##name);                                  \
  }                                                                     \
  using fL##shorttype::FLAGS_##name