error_code example

// This program illustrates how one can plug one's error code
// enumeration into <code>std::error_code</code> facility.


// FILE: flights.h

# include <system_error>

enum class FlightsErrc
{
  // no 0
  NonexistentLocations = 10, // requested airport doesn't exist
  DatesInThePast,            // booking flight for yesterday
  InvertedDates,             // returning before departure
  NoFlightsFound       = 20, // did not find any combination
  ProtocolViolation    = 30, // e.g., bad XML
  ConnectionError,           // could not connect to server
  ResourceError,             // service run short of resources
  Timeout,                   // did not respond in time
};

namespace std
{
  template <>
    struct is_error_code_enum<FlightsErrc> : true_type {};
}

std::error_code make_error_code(FlightsErrc);


// FILE: main.cpp

// # include "flights.h"
# include <cassert>
# include <iostream>

int main ()
{
  std::error_code ec = FlightsErrc::NoFlightsFound;
  assert (ec);
  assert (ec == FlightsErrc::NoFlightsFound);
  assert (ec != FlightsErrc::InvertedDates);
  std::cout << ec << std::endl;
}


// FILE: flights.cpp

// # include "flights.h"

namespace { // anonymous namespace
 
struct FlightsErrCategory : std::error_category
{
  const char* name() const noexcept override;
  std::string message(int ev) const override;
};
 
const char* FlightsErrCategory::name() const noexcept
{
  return "flights";
}
 
std::string FlightsErrCategory::message(int ev) const
{
  switch (static_cast<FlightsErrc>(ev))
  {
  case FlightsErrc::NonexistentLocations:
    return "nonexistent airport name in request";
  
  case FlightsErrc::DatesInThePast:
    return "request for a date from the past";
 
  case FlightsErrc::InvertedDates:
    return "requested flight return date before departure date";
 
  case FlightsErrc::NoFlightsFound:
    return "no filight combination found";
 
  case FlightsErrc::ProtocolViolation:
    return "received malformed request";
 
  case FlightsErrc::ConnectionError:
    return "could not connect to server";
 
  case FlightsErrc::ResourceError:
    return "insufficient resources";
 
  case FlightsErrc::Timeout:
    return "processing timed out";
 
  default:
    return "(unrecognized error)";
  }
}
 
const FlightsErrCategory theFlightsErrCategory {};
 
} // anonymous namespace

std::error_code make_error_code(FlightsErrc e)
{
  return {static_cast<int>(e), theFlightsErrCategory};
}
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