flag.h

// flag.h
template <typename T>
class Flag {
 public:
  T Get() const {
    // See implementation notes in CommandLineFlag::Get().
    union U {
      T value;
      U() {}
      ~U() { value.~T(); }
    };
    U u;

    // Operations are good to check that the flag was declared with the same type.
    // IF THIS COMES FROM SHARED library.
    impl_.Read(&u.value, &flags_internal::FlagOps<T>);
    return std::move(u.value);
  }
}

// flag.cc
void FlagImpl::Read(void* dst, const flags_internal::FlagOpFn dst_op) const {
  absl::ReaderMutexLock l(DataGuard());

  // `dst_op` is the unmarshaling operation corresponding to the declaration
  // visibile at the call site. `op` is the Flag's defined unmarshalling
  // operation. They must match for this operation to be well-defined.
  // THIS IS NOT TRUE ANYMORE AS POINTERS FROM SHARED LIBRARIES ARE
  // DIFFERENT FROM STATIC INITIALIZATION.
  if (ABSL_PREDICT_FALSE(dst_op != op_)) {
    ABSL_INTERNAL_LOG(
        ERROR,
        absl::StrCat("Flag '", Name(),
                     "' is defined as one type and declared as another"));
  }
  CopyConstruct(op_, cur_, dst);
}