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complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-14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 services.\u003c/p\u003e\n"],["\u003cp\u003eThe client libraries typically use Application Default Credentials for authentication, but users can override this by using the \u003ccode\u003eUnifiedCredentialsOption\u003c/code\u003e.\u003c/p\u003e\n"],["\u003cp\u003eThe page shows an example of loading a service account key file to authenticate using the \u003ccode\u003eUnifiedCredentialsOption\u003c/code\u003e, and provides code on how to implement this.\u003c/p\u003e\n"],["\u003cp\u003eLinks to examples for several \u003ccode\u003e*Client\u003c/code\u003e classes are provided, including \u003ccode\u003eApplicationsClient\u003c/code\u003e, \u003ccode\u003eAuthorizedCertificatesClient\u003c/code\u003e, \u003ccode\u003eAuthorizedDomainsClient\u003c/code\u003e, and others, demonstrating how to use service account authentication.\u003c/p\u003e\n"],["\u003cp\u003eThe page references the "Best practices for managing service account keys" guide for more details, as well as "Authentication Components" for more information on credentials.\u003c/p\u003e\n"]]],[],null,[]]
