boost::system::error_code is the most basic class in Boost.System; it represents operating system-specific errors. Because operating systems typically enumerate errors, boost::system::error_code saves an error code in a variable of type int.

1. error_code

include <boost/system/error_code.hpp>
#include <iostream>

using namespace boost::system;

void fail(error_code &ec)
{
  ec = errc::make_error_code(errc::not_supported);
}

int main()
{
  error_code ec;
  fail(ec);
  std::cout << ec.value() << std::endl;
  std::cout << ec.category() << std::endl;
  return 0;
}

boost::system::errc::not_supported is a number and ec is an object of type boost::system::error_code, the function boost::system::errc::make_error_code() is called. This function creates an object of type boost::system::error_code with the respective error code.

value() member function returns the error code stored in the object.

Error codes of type boost::system::error_code belong to a category that can be retrieved with the member function category(). Errors created with boost::system::errc::make_error_code() automatically belong to the generic category. This is the category errors belong to if they aren’t assigned to another category explicitly.

2. creating error category

#include <boost/system/error_code.hpp>
#include <string>
#include <iostream>

class application_category :
  public boost::system::error_category
{
public:
  const char *name() const noexcept { return "my app"; }
  std::string message(int ev) const { return "error message"; }
};

application_category cat;

int main()
{
  boost::system::error_code ec(129, cat);
  std::cout << ec.value() << std::endl;
  std::cout << ec.category().name() << std::endl;
  return 0;
}

A new error category is defined by creating a class derived from boost::system::error_category. This requires you to define various member functions. At a minimum, the member functions name() and message() must be supplied because they are defined as pure virtual member functions in boost::system::error_category. While name() returns the name of the error category, message() is used to retrieve the error description for a particular error code.

3. error_condition

#include <boost/system/error_code.hpp>
#include <iostream>

using namespace boost::system;

void fail(error_code &ec)
{
  ec = errc::make_error_code(errc::not_supported);
}

int main()
{
  error_code ec;
  fail(ec);
  boost::system::error_condition ecnd = ec.default_error_condition();
  std::cout << ecnd.value() << std::endl;
  std::cout << ecnd.category().name() << std::endl;
  return 0;
}

boost::system::error_condition is used just like boost::system::error_code.

While the class boost::system::error_code is used for platform-dependent error codes, boost::system::error_condition is used to access platform-independent error codes. The member function default_error_condition() translates a platform-dependent error code into a platform-independent error code of type boost::system::error_condition.