[C++] allocation free error Status #44491
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FWIW, absl::Status uses pointer tagging for its junk value: https://github.com/abseil/abseil-cpp/blob/master/absl/status/status.h#L901-L903 |
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### Rationale for this change
It'd be convenient to construct placeholder error Status cheaply.
### What changes are included in this PR?
A constant error Status can now be constructed wrapped in a function like
```c++
Status UninitializedResult() {
static StatusConstant uninitialized_result{StatusCode::UnknownError,
"Uninitialized Result<T>"};
return uninitialized_result;
}
```
Copying the constant status is relatively cheap (no heap interaction), so it's suitable for use as a placeholder error status.
Added `bool Status::State::is_constant` which causes copies to be shallow and skips destruction. Also `Status::state_` is a const pointer now; this helps to ensure that it is obvious when we mutate state_ (as in AddContextLine).
### Are these changes tested?
Minimal unit test added. The main consideration is probably benchmarks to make sure hot paths don't get much slower.
### Are there any user-facing changes?
This API is not currently public; no user-facing changes.
* GitHub Issue: #44491
Lead-authored-by: Benjamin Kietzman <[email protected]>
Co-authored-by: Antoine Pitrou <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>
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Issue resolved by pull request 44493 |
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Describe the enhancement requested
In many situations, it'd be beneficial to have an error Status which was copyable without using the heap. For example, this is a frequent pain point with Result. We want Result to be default constructible even if T isn't, so a defaulted Result is an error but this requires heap allocating a Status::State which we immediately overwrite. Similarly, if the value or error is moved out of a Result we'd like to avoid managing the stored object and wind up heap allocating a flag.
Allowing some Status objects to have static lifetime (for example, by tagging the
state_pointer or addingbool Status::State::is_static) should not have an effect on the critical hot paths of moving a status or skipping deletion of ok status. Copying will get slightly slower since we need to branch onstate_->is_static, but that's already quite slow due to heap interaction.Component(s)
C++
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