Unless otherwise configured, the client libraries use Application Default Credentials to authenticate with Google Cloud Services. While this works for most applications, in some cases you may need to override this default. You can do so by providing the UnifiedCredentialsOption The following example shows how to explicitly load a service account key file:
[](std::string const& keyfile) {
auto is = std::ifstream(keyfile);
is.exceptions(std::ios::badbit); // Minimal error handling in examples
auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
auto options =
google::cloud::Options{}.set<google::cloud::UnifiedCredentialsOption>(
google::cloud::MakeServiceAccountCredentials(contents));
return google::cloud::appengine_v1::ApplicationsClient(
google::cloud::appengine_v1::MakeApplicationsConnection(options));
}
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complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-14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\u003eThis page provides documentation for overriding default authentication credentials when using Google Cloud client libraries, specifically focusing on App Engine.\u003c/p\u003e\n"],["\u003cp\u003eThe latest version, 2.37.0-rc, is available, with numerous other past versions listed from 2.36.0 down to 2.11.0.\u003c/p\u003e\n"],["\u003cp\u003eThe default authentication method utilizes Application Default Credentials, but can be overridden using \u003ccode\u003eUnifiedCredentialsOption\u003c/code\u003e, as shown in the example using a service account key file.\u003c/p\u003e\n"],["\u003cp\u003eExamples of overriding credentials for various App Engine client classes, like \u003ccode\u003eApplicationsClient\u003c/code\u003e and \u003ccode\u003eVersionsClient\u003c/code\u003e, are provided with links.\u003c/p\u003e\n"],["\u003cp\u003eFor further information on creating credentials objects, consult the "Authentication Components" link provided in the documentation.\u003c/p\u003e\n"]]],[],null,[]]
