Re: [Boost-bugs] [Boost C++ Libraries] #9497: mpq_rational asserts when used in variant

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #9497: mpq_rational asserts when used in variant
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2013-12-25 12:03:20

#9497: mpq_rational asserts when used in variant
-------------------------------------------+----------------------------
  Reporter:  András Kucsma <r0maikx02b@…>  |      Owner:  johnmaddock
      Type:  Bugs                          |     Status:  new
 Milestone:  To Be Determined              |  Component:  multiprecision
   Version:  Boost 1.55.0                  |   Severity:  Problem
Resolution:                                |   Keywords:
-------------------------------------------+----------------------------
Changes (by apolukhin):

 * cc: antoshkka@… (added)
 * owner:  apolukhin => johnmaddock
 * status:  assigned => new
 * component:  variant => multiprecision

Comment:

 It's not a bug, boost::variant tries to satisfy the newer-empty guarantee:
 * `boost::multiprecision::mpq_rational` has no noexcept default
 constructor because default constructor of `gmp_rational` is not marked
 with noexcept
 * `Foo` has no noexcept default constructor
 * because of previous two points variant can not use
 `boost::multiprecision::mpq_rational` or `Foo` as fallback types
 * copy constructor of `Foo` is not noexcept, there is no fallback type in
 variant, so it attempts '''to copy/move content of variant to backup
 storage''' before doing an assignment of `Foo`.
 * ...

 Looks like `gmp_rational` calls only pure C methods. Can the default
 constructor of `gmp_rational` be marked with noexcept?

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/9497#comment:7>
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