////
Copyright 2018 Peter Dimov

Distributed under the Boost Software License, Version 1.0.

See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt
////

[#changes]
# Revision History
:idprefix:

## Changes in Boost 1.74

* `operator bool()` now returns `failed()` instead of `value() != 0`.

## Changes in Boost 1.69

* Boost.System is now header-only. A stub library is still built for
  compatibility, but linking to it is no longer necessary.
* Even more functions have been marked `constexpr`.
* The destructor of `error_category` is now protected and no longer
  virtual. This is a _potentially breaking change_ but its impact
  is expected to be limited.
* `error_category` now has a constructor that accepts a 64 bit identifier,
  enabling distinct category objects to compare equal.
* The constructors of `error_category` are now protected.
* A non-allocating, nonthrowing overload of `message` has been added.
* A virtual function `failed` has been added, allowing categories for
  which success is not synonymous with 0.
* The deprecated `boost::system::throws` object has been removed.
* `boost::throws()` is now deprecated and its use is discouraged.
* The constructor of `system_error` taking a single `error_code` argument
  is now explicit.
* `system_error::code()` now returns by value.

## Changes in Boost 1.68

On a {cpp}14 compiler, many Boost.System functions and member functions
are now `constexpr`, and `error_code` and `error_condition` are literal
classes.

In addition to enabling use in constant expressions (and `constexpr`
functions), this significantly improves the quality of the generated code.

As a result of this change, however, now using Boost.System from {cpp}14
or {cpp}17 code requires that the library be also built with {cpp}14 or
above. This is the default on GCC 6 and newer, but not on GCC 5 or Clang.
One can build Boost for {cpp}14 by passing the `cxxstd=14` option to `b2`.

(Previous versions allowed code built against any {cpp} standard to link
with Boost.System built against any {cpp} standard. In 1.68, code using
any {cpp} standard can link with Boost.System built with {cpp}14 or above,
but if Boost.System is built with {cpp}11 or below, only code also built
with {cpp}11 and below can link to it successfully.)

## Changes in Boost 1.65

On a {cpp}11 compiler, Boost.System now provides implicit conversions
from `boost::system::error_category`, `error_code`, and `error_condition`
to their standard equivalents from `<system_error>`.

This allows libraries to expose a {cpp}11 interface and report errors
via `std::error_code` even when using Boost.System, directly or through a
dependency such as Boost.ASIO.
