cpp-httplib v0.42.0
DRAFT

S09. Add Pre-Processing to All Routes

Sometimes you want the same logic to run before every request — auth checks, logging, rate limiting. Register those with set_pre_routing_handler().

Basic usage

svr.set_pre_routing_handler(
  [](const httplib::Request &req, httplib::Response &res) {
    std::cout << req.method << " " << req.path << std::endl;
    return httplib::Server::HandlerResponse::Unhandled;
  });

svr.set_pre_routing_handler(
  [](const httplib::Request &req, httplib::Response &res) {
    std::cout << req.method << " " << req.path << std::endl;
    return httplib::Server::HandlerResponse::Unhandled;
  });

The pre-routing handler runs before routing. It catches every request — including ones that don't match any handler.

The HandlerResponse return value is key:

  • Return Unhandled → continue normally (routing and the actual handler run)
  • Return Handled → the response is considered complete, skip the rest

Use it for authentication

Put your shared auth check in one place.

svr.set_pre_routing_handler(
  [](const httplib::Request &req, httplib::Response &res) {
    if (req.path.rfind("/public", 0) == 0) {
      return httplib::Server::HandlerResponse::Unhandled; // no auth needed
    }

    auto auth = req.get_header_value("Authorization");
    if (auth.empty()) {
      res.status = 401;
      res.set_content("unauthorized", "text/plain");
      return httplib::Server::HandlerResponse::Handled;
    }

    return httplib::Server::HandlerResponse::Unhandled;
  });

svr.set_pre_routing_handler(
  [](const httplib::Request &req, httplib::Response &res) {
    if (req.path.rfind("/public", 0) == 0) {
      return httplib::Server::HandlerResponse::Unhandled; // no auth needed
    }

    auto auth = req.get_header_value("Authorization");
    if (auth.empty()) {
      res.status = 401;
      res.set_content("unauthorized", "text/plain");
      return httplib::Server::HandlerResponse::Handled;
    }

    return httplib::Server::HandlerResponse::Unhandled;
  });

If auth fails, return Handled to respond with 401 immediately. If it passes, return Unhandled and let routing take over.

For per-route auth

If you want different auth rules per route rather than a single global check, set_pre_request_handler() is a better fit. See S11. Authenticate per route with a pre-request handler.

Note: If all you want is to modify the response, set_post_routing_handler() is the right tool. See S10. Add response headers with a post-routing handler.

ESC