Class google::cloud::StatusOr (2.10.1)

Holds a value or a Status indicating why there is no value.

StatusOr<T> represents either a usable T value or a Status object explaining why a T value is not present. Typical usage of StatusOr<T> looks like usage of a smart pointer, or even a std::optional<T>, in that you first check its validity using a conversion to bool (or by calling StatusOr::ok()), then you may dereference the object to access the contained value.

It is undefined behavior (UB) to dereference a StatusOr<T> that is not "ok". For example:

StatusOr<Foo> foo = FetchFoo();
if (!foo) {  // Same as !foo.ok()
  // handle error and probably look at foo.status()
} else {
  foo->DoSomethingFooey();  // UB if !foo
}

Alternatively, you may call the StatusOr::value() member function, which is defined to: (1) throw an exception if there is no T value, or (2) crash the program if exceptions are disabled. It is never UB to call value().

StatusOr<Foo> foo = FetchFoo();
foo.value().DoSomethingFooey();  // May throw/crash if there is no value

Functions that can fail will often return a StatusOr<T> instead of returning an error code and taking a T out-param, or rather than directly returning the T and throwing an exception on error. StatusOr<T> is used so that callers can choose whether they want to explicitly check for errors, crash the program, or throw exceptions.

Since constructors do not have a return value, they should be designed in such a way that they cannot fail by moving the object's complex initialization logic into a separate factory function that itself can return a StatusOr<T>. For example:

class Bar {
 public:
  Bar(Arg arg);
  ...
};
StatusOr<Bar> MakeBar() {
  ... complicated logic that might fail
  return Bar(std::move(arg));
}

StatusOr<T> supports equality comparisons if the underlying type T does.

Constructors

StatusOr

Initializes with an error status (StatusCode::kUnknown).

StatusOr

Parameter
Name Description
StatusOr const &

StatusOr

Parameter
Name Description
other StatusOr &&

StatusOr

Creates a new StatusOr<T> holding the error condition rhs.

Post-conditionsok()== false and status()== rhs.
Parameter
Name Description
rhs Status

the status to initialize the object.

StatusOr

Creates a new StatusOr<T> holding the value rhs.

Post-conditionsok()== true and value()== rhs.
Parameter
Name Description
rhs T &&

the value used to initialize the object.

StatusOr

Parameter
Name Description
rhs T const &

Operators

operator*

Returns
Type Description
T &

operator*

Returns
Type Description
T const &

operator*

Returns
Type Description
T &&

operator*

Returns
Type Description
T const &&

operator->

Returns
Type Description
T *

operator->

Returns
Type Description
T const *

operator=

Parameter
Name Description
StatusOr const &
Returns
Type Description
StatusOr &

operator=

Parameter
Name Description
other StatusOr &&
Returns
Type Description
StatusOr &

operator=

Assigns the given non-OK Status to this StatusOr<T>.

Parameter
Name Description
status Status
Returns
Type Description
StatusOr &

operator=

Assign a T (or anything convertible to T) into the StatusOr.

Parameters
Name Description
rhs U &&
typename U
Returns
Type Description
std::enable_if<!std::is_same< StatusOr, typenamestd::decay< U >::type >::value, StatusOr >::type &

operator bool

Returns true when this holds a value.

Functions

value

Returns
Type Description
T &

value

Returns
Type Description
T const &

value

Returns
Type Description
T &&

value

Returns
Type Description
T const &&

status

Returns
Type Description
Status const &

status

Returns
Type Description
Status &&

ok

Returns true when this holds a value.

Returns
Type Description
bool