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 gcs = ::google::cloud::storage;
  [](std::string const& filename, std::string const& bucket_name,
     std::string const& object_name) {
    auto is = std::ifstream(filename);
    is.exceptions(std::ios::badbit);
    auto json_string =
        std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
    auto credentials =
        google::cloud::MakeServiceAccountCredentials(json_string);
    auto client = gcs::Client(
        google::cloud::Options{}.set<google::cloud::UnifiedCredentialsOption>(
            credentials));
    PerformSomeOperations(client, bucket_name, object_name);
  }

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.

See also: Authentication Components - for more information on the factory functions to create google::cloud::Credentials objects.