This project it to generate below string val in the auth request, (C++), as I do not know the format of the resulting string:
"std::string val = "GET/realtime" + std::to_string(expires);"
The full C++ code it is used in:
long long int expires = std::chrono::duration_cast(
std::chrono::system_clock::now().time_since_epoch()).count() + 1000;
std::string val = "GET/realtime" + std::to_string(expires);
std::string signature = sign_message(val, secret_key);
json auth_msg = {
{"op", "auth"},
{"args", {api_key, expires, signature}}
};