lingbin commented on a change in pull request #2969: Improve implementation of
Status
URL: https://github.com/apache/incubator-doris/pull/2969#discussion_r382666077
##########
File path: be/src/common/status.h
##########
@@ -17,114 +17,136 @@ namespace doris {
class Status {
public:
- Status(): _state(nullptr) {}
+ Status() : _state(nullptr) { }
~Status() noexcept { delete[] _state; }
- // copy c'tor makes copy of error detail so Status can be returned by value
- Status(const Status& s)
- : _state(s._state == nullptr ? nullptr : copy_state(s._state)) {
- }
+ // Here _state == nullptr(Status::OK) is the most common case, so use this
as
+ // a fast path. This check can avoid one function call in most cases,
because
+ // _copy_state() is not an inline function.
+ Status(const Status& s) : _state(s._state == nullptr ? nullptr :
_copy_state(s._state)) { }
// same as copy c'tor
Status& operator=(const Status& s) {
// The following condition catches both aliasing (when this == &s),
// and the common case where both s and *this are OK.
if (_state != s._state) {
delete[] _state;
- _state = (s._state == nullptr) ? nullptr : copy_state(s._state);
+ _state = (s._state == nullptr) ? nullptr : _copy_state(s._state);
}
return *this;
}
- // move c'tor
Status(Status&& s) noexcept : _state(s._state) {
s._state = nullptr;
}
- // move assign
Status& operator=(Status&& s) noexcept {
- std::swap(_state, s._state);
+ if (_state != s._state) {
Review comment:
> In general, a move assignment operator should:
> 1. Destroy visible resources (though maybe save implementation detail
resources).
> 2. Move assign all bases and members.
> 3. If the move assignment of bases and members didn't make the rhs
resource-less, then make it so.
Taken from
[https://stackoverflow.com/questions/6687388/why-do-some-people-use-swap-for-move-assignments](https://stackoverflow.com/questions/6687388/why-do-some-people-use-swap-for-move-assignments)
Another example is, somebody may still use the the `Status` object after
move.
```
Status src = Status::RuntimeError();
Status dest = Status::IOError("xxx")
dest = std::move(src);
// here, if we use `std::move` in `Status::operator=`
```
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