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Showing content from https://cloud.google.com/cpp/docs/reference/spanner/2.16.0/spanner-error-handling below:

C++ Client Libraries | Google Cloud

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Error Handling

In general, the library returns a StatusOr 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::future for more details on the type returned by asynchronous operations.

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.

Last updated 2025-08-14 UTC.

[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-14 UTC."],[[["This document details the error handling procedures for the Spanner C++ library, with version 2.37.0-rc being the latest release candidate and 2.16.0 being the documented version at the time of writing."],["The library predominantly uses `StatusOr\u003cT\u003e` to manage potential errors, functioning like a smart pointer that either holds a value of type `T` or a `Status` object detailing an error."],["Developers should verify if a `StatusOr\u003cT\u003e` object contains a value using methods like `.ok()` or `operator bool()` before attempting to access the value to prevent errors, and if it does not contain a value you can access the `Status` error information with `.status()`."],["Alternatively, the `.value()` member function can be utilized to retrieve the contained value, but it will either throw a `RuntimeStatusError` exception or terminate the program if the object does not hold a value."],["The library has other functions that are not wrapped in a `StatusOr\u003cT\u003e` but that will return a `StatusOr\u003cT\u003e` to signal an error, such as wrappers for asynchronous operations that return `future\u003cStatusOr\u003cT\u003e\u003e`."]]],[]]

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