chore: explain use of "performance-unnecessary-value-param" (#1244)

devbww · web-flow · commit 01b696d6bbca · 2020-02-05T15:54:43.000-05:00
For future reference, document the clang-tidy warning suppression.
diff --git a/google/cloud/spanner/transaction.h b/google/cloud/spanner/transaction.h
@@ -219,6 +219,9 @@ Transaction MakeSingleUseTransaction(T&& opts) {
 }
 
 template <typename Functor>
+// Pass `txn` by value, despite being used only once. This avoids the
+// possibility of `txn` being destroyed by `f` before `Visit()` can
+// return. Therefore, ...
 // NOLINTNEXTLINE(performance-unnecessary-value-param)
 VisitInvokeResult<Functor> Visit(Transaction txn, Functor&& f) {
   return txn.impl_->Visit(std::forward<Functor>(f));

Original file line number	Diff line number	Diff line change
`@@ -219,6 +219,9 @@ Transaction MakeSingleUseTransaction(T&& opts) {`
`219`	`219`	`}`
`220`	`220`
`221`	`221`	`template <typename Functor>`
	`222`	+// Pass `txn` by value, despite being used only once. This avoids the
	`223`	+// possibility of `txn` being destroyed by `f` before `Visit()` can
	`224`	`+// return. Therefore, ...`
`222`	`225`	`// NOLINTNEXTLINE(performance-unnecessary-value-param)`
`223`	`226`	`VisitInvokeResult<Functor> Visit(Transaction txn, Functor&& f) {`
`224`	`227`	`return txn.impl_->Visit(std::forward<Functor>(f));`