input/async: use std::exchange()

src/input/AsyncInputStream.cxx

@@ -50,8 +50,7 @@ AsyncInputStream::~AsyncInputStream()
 void
 AsyncInputStream::SetTag(Tag *_tag) noexcept
 {
-	delete tag;
-	tag = _tag;
+	delete std::exchange(tag, _tag);
 }
 
 void
@@ -77,11 +76,9 @@ AsyncInputStream::Resume()
 void
 AsyncInputStream::Check()
 {
-	if (postponed_exception) {
-		auto e = std::move(postponed_exception);
-		postponed_exception = std::exception_ptr();
-		std::rethrow_exception(e);
-	}
+	if (postponed_exception)
+		std::rethrow_exception(std::exchange(postponed_exception,
+						     std::exception_ptr()));
 }
 
 bool
@@ -154,9 +151,7 @@ AsyncInputStream::SeekDone() noexcept
 Tag *
 AsyncInputStream::ReadTag()
 {
-	Tag *result = tag;
-	tag = nullptr;
-	return result;
+	return std::exchange(tag, nullptr);
 }
 
 bool