Тhank you for answer! Here's a test project: #include <unordered_map> #include <system_error> #include <string_view> #include #include int main(int argc, const char *argv[]) { std::unordered_map<int, std::pair<int, std::function<std::expected<std::string_view, std::error_code>(void)>>> var{ {24, {42, [] -> std::expected<std::string_view, std::error_code> { return "Hello, World!"; } }} }; const auto &it{var.find(24)}; if (it == var.end()) [[unlikely]] return 1; return 0; }