flags.h

// ABSL_CONST_INIT
//
// A variable declaration annotated with the `ABSL_CONST_INIT` attribute will
// not compile (on supported platforms) unless the variable has a constant
// initializer. This is useful for variables with static and thread storage
// duration, because it guarantees that they will not suffer from the so-called
// "static init order fiasco". Prefer to put this attribute on the most visible
// declaration of the variable, if there's more than one, because code that
// accesses the variable can then use the attribute for optimization.
// Note that this attribute is redundant if the variable is declared constexpr.
#define ABSL_CONST_INIT [[clang::require_constant_initialization]]

// This struct and corresponding overload to MakeDefaultValue are used to
// facilitate usage of {} as default value in ABSL_FLAG macro.
struct EmptyBraces {};

template <typename T>
T* MakeFromDefaultValue(T t) {
  return new T(std::move(t));
}

template <typename T>
T* MakeFromDefaultValue(EmptyBraces) {
  return new T;
}

#define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)   \
  static void* AbslFlagsInitFlag##name() {                                  \
    return absl::flags_internal::MakeFromDefaultValue<Type>(default_value); \
  }

#define ABSL_FLAG(Type, name, default_value, help)                  \
  namespace absl /* block flags in namespaces */ {}                 \
  ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \ // Creating a function to create a default flag
  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help);                  \ 
  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                    \ // Const init initialization (see above)
      ABSL_FLAG_IMPL_FLAGNAME(#name), ABSL_FLAG_IMPL_FILENAME(),    \ // Both compile time pointers.
      &absl::flags_internal::FlagMarshallingOps<Type>,              \ // Compile time pointer to dispatch the operations.
      absl::flags_internal::HelpArg<AbslFlagHelpGenFor##name>(0),   \ // Compile time struct to generate --help values.
      &AbslFlagsInitFlag##name};                                      // Compile time pointer from line 34.