Some applications cannot use the default authentication mechanism (known as [Application Default Credentials]).

You can override this default using google::cloud::UnifiedCredentialsOption. The following example shows how to explicitly load a service account key file.

  namespace spanner = ::google::cloud::spanner;
  [](std::string const& project_id, std::string const& instance_id,
     std::string const& database_id, std::string const& keyfile) {
    auto is = std::ifstream(keyfile);
    is.exceptions(std::ios::badbit);
    auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
    auto options =
        google::cloud::Options{}.set<google::cloud::UnifiedCredentialsOption>(
            google::cloud::MakeServiceAccountCredentials(contents));
    return spanner::Client(spanner::MakeConnection(
        spanner::Database(project_id, instance_id, database_id), options));
  }

Keep in mind that we chose this as an example because it is relatively easy to understand. Consult the [Best practices for managing service account keys] guide for more details.

Follow these links to find examples for other *Client classes: