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:
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Missing the information I need","missingTheInformationINeed","thumb-down"],["Too complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-14 UTC."],[[["\u003cp\u003eThe latest version of the Spanner documentation is version 2.37.0-rc, but there are many previous versions available for reference, ranging back to version 2.11.0.\u003c/p\u003e\n"],["\u003cp\u003eApplications can override the default authentication mechanism by using \u003ccode\u003egoogle::cloud::UnifiedCredentialsOption\u003c/code\u003e, as shown in the provided code example.\u003c/p\u003e\n"],["\u003cp\u003eThe provided example demonstrates loading a service account key file to override default authentication, but the document advises to review the "Best practices for managing service account keys" guide for more details.\u003c/p\u003e\n"],["\u003cp\u003eExamples are also available for other client classes, such as \u003ccode\u003eDatabaseAdminClient\u003c/code\u003e and \u003ccode\u003eInstanceAdminClient\u003c/code\u003e, which can be found through the provided links.\u003c/p\u003e\n"]]],[],null,[]]
