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complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-22 UTC."],[[["\u003cp\u003eThis webpage provides documentation and examples for Google Cloud Spanner C++ client library versions, ranging from 2.11.0 to 2.36.0, with 2.37.0-rc being the latest release candidate.\u003c/p\u003e\n"],["\u003cp\u003eThe primary focus is on showing how to override the default authentication configuration for applications that cannot use Application Default Credentials, specifically with a service account key file.\u003c/p\u003e\n"],["\u003cp\u003eThe example provided demonstrates how to use \u003ccode\u003egoogle::cloud::UnifiedCredentialsOption\u003c/code\u003e and \u003ccode\u003egoogle::cloud::MakeServiceAccountCredentials\u003c/code\u003e to explicitly load a service account key file for authentication.\u003c/p\u003e\n"],["\u003cp\u003eThe documentation includes links to further examples for \u003ccode\u003eDatabaseAdminClient\u003c/code\u003e and \u003ccode\u003eInstanceAdminClient\u003c/code\u003e, showing how to use these with a service account.\u003c/p\u003e\n"]]],[],null,[]]
