cpp-httplib/docs-src/pages/en/cookbook/s12-user-data.md

title, order, status

title	order	status
S12. Pass Data Between Handlers with res.user_data	31	draft

Say your pre-request handler decodes an auth token and you want the route handler to use the result. That "data handoff between handlers" is what res.user_data is for — it holds values of arbitrary types.

Basic usage

struct AuthUser {
  std::string id;
  std::string name;
  bool is_admin;
};

svr.set_pre_request_handler(
  [](const httplib::Request &req, httplib::Response &res) {
    auto token = req.get_header_value("Authorization");
    auto user = decode_token(token); // decode the auth token
    res.user_data.set("user", user);
    return httplib::Server::HandlerResponse::Unhandled;
  });

svr.Get("/me", [](const httplib::Request &req, httplib::Response &res) {
  auto *user = res.user_data.get<AuthUser>("user");
  if (!user) {
    res.status = 401;
    return;
  }
  res.set_content("Hello, " + user->name, "text/plain");
});

user_data.set() stores a value of any type, and user_data.get<T>() retrieves it. If you give the wrong type you get nullptr back — so be careful.

Typical value types

Strings, numbers, structs, std::shared_ptr — anything copyable or movable works.

res.user_data.set("user_id", std::string{"42"});
res.user_data.set("is_admin", true);
res.user_data.set("started_at", std::chrono::steady_clock::now());

Where to set, where to read

The usual flow is: set it in set_pre_routing_handler() or set_pre_request_handler(), read it in the route handler. Pre-request runs after routing, so you can combine it with req.matched_route to set values only for specific routes.

A gotcha

user_data lives on Response, not Request. That's because handlers get Response& (mutable) but only const Request&. It looks odd at first, but it makes sense once you think of it as "the mutable context shared between handlers."

Warning: user_data.get<T>() returns nullptr when the type doesn't match. Use the exact same type on set and get. Storing as AuthUser and fetching as const AuthUser won't work.