C17. Handle Error Codes

cli.Get(), cli.Post(), and friends return a Result. When the request fails — can't reach the server, times out, etc. — the result is "falsy". To get the specific reason, use Result::error().

Basic check

httplib::Client cli("http://localhost:8080");
auto res = cli.Get("/api/data");

if (res) {
  // the request was sent and a response came back
  std::cout << "status: " << res->status << std::endl;
} else {
  // the network layer failed
  std::cerr << "error: " << httplib::to_string(res.error()) << std::endl;
}

httplib::Client cli("http://localhost:8080");
auto res = cli.Get("/api/data");

if (res) {
  // the request was sent and a response came back
  std::cout << "status: " << res->status << std::endl;
} else {
  // the network layer failed
  std::cerr << "error: " << httplib::to_string(res.error()) << std::endl;
}

Use if (res) to check success. On failure, res.error() returns a httplib::Error enum value. Pass it to to_string() to get a human-readable description.

Common errors

Value	Meaning
`Error::Connection`	Couldn't connect to the server
`Error::ConnectionTimeout`	Connection timeout (`set_connection_timeout`)
`Error::Read` / `Error::Write`	Error during send or receive
`Error::Timeout`	Overall timeout set via `set_max_timeout`
`Error::ExceedRedirectCount`	Too many redirects
`Error::SSLConnection`	TLS handshake failed
`Error::SSLServerVerification`	Server certificate verification failed
`Error::Canceled`	A progress callback returned `false`

Network errors vs. HTTP errors

Even when res is truthy, the HTTP status code can still be 4xx or 5xx. These are two different things.

auto res = cli.Get("/api/data");
if (!res) {
  // network error (no response received at all)
  std::cerr << "network error: " << httplib::to_string(res.error()) << std::endl;
  return 1;
}

if (res->status >= 400) {
  // HTTP error (response received, but the status is bad)
  std::cerr << "http error: " << res->status << std::endl;
  return 1;
}

// success
std::cout << res->body << std::endl;

auto res = cli.Get("/api/data");
if (!res) {
  // network error (no response received at all)
  std::cerr << "network error: " << httplib::to_string(res.error()) << std::endl;
  return 1;
}

if (res->status >= 400) {
  // HTTP error (response received, but the status is bad)
  std::cerr << "http error: " << res->status << std::endl;
  return 1;
}

// success
std::cout << res->body << std::endl;

Keep them separated in your head: network-layer errors go through res.error(), HTTP-level errors through res->status.

To dig deeper into SSL-related errors, see C18. Handle SSL errors.