Error Handling

This library never throws exceptions to signal error.

In general, the library returns a StatusOr<T> if an error is possible. Some functions return objects that are not wrapped in a StatusOr<T> but will themselves return a StatusOr<T> to signal an error. For example, wrappers for asynchronous operations return future<StatusOr<T>>.

Applications should check if the StatusOr<T> contains a value before using it, much like how you might check that a pointer is not null before dereferencing it. Indeed, a StatusOr<T> object can be used like a smart-pointer to T, with the main difference being that when it does not hold a T it will instead hold a Status object with extra information about the error.

You can check that a StatusOr<T> contains a value by calling the .ok() method, or by using operator bool() (like with other smart pointers). If there is no value, you can access the contained Status object using the .status() member. If there is a value, you may access it by dereferencing with operator*() or operator->(). As with all smart pointers, callers must first check that the StatusOr<T> contains a value before dereferencing and accessing the contained value. Alternatively, callers may instead use the .value() member function which is defined to throw a RuntimeStatusError if there is no value.

Note

If you're compiling with exceptions disabled, calling .value() on a StatusOr<T> that does not contain a value will terminate the program instead of throwing.

Example

  namespace spanner = ::google::cloud::spanner;
  [](spanner::Client client) {
    auto rows = client.Read("Albums", spanner::KeySet::All(), {"AlbumTitle"});
    // The actual type of `row` is google::cloud::StatusOr<spanner::Row>, but
    // we expect it'll most often be declared with auto like this.
    for (auto const& row : rows) {
      // Use `row` like a smart pointer; check it before dereferencing
      if (!row) {
        // `row` doesn't contain a value, so `.status()` will contain error info
        std::cerr << row.status();
        break;
      }
 
      // The actual type of `song` is google::cloud::StatusOr<std::string>, but
      // again we expect it'll be commonly declared with auto as we show here.
      auto song = row->get<std::string>("AlbumTitle");
 
      // Instead of checking then dereferencing `song` as we did with `row`
      // above, here we demonstrate use of the `.value()` member, which will
      // return a reference to the contained `T` if it exists, otherwise it
      // will throw an exception (or terminate if compiled without exceptions).
      std::cout << "SongName: " << song.value() << "\n";
    }
  }

See also

google::cloud::StatusOr

google::cloud::Status the class used to describe errors.

google::cloud::future for more details on the type returned by asynchronous operations.