[Boost-bugs] [Boost C++ Libraries] #11844: memcpy should only be used on trivially copyable objects

Subject: [Boost-bugs] [Boost C++ Libraries] #11844: memcpy should only be used on trivially copyable objects
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-12-13 07:22:50

#11844: memcpy should only be used on trivially copyable objects
------------------------------+------------------------
 Reporter:  joseph.thomson@…  |      Owner:  igaztanaga
     Type:  Bugs              |     Status:  new
Milestone:  To Be Determined  |  Component:  container
  Version:  Boost 1.60.0      |   Severity:  Problem
 Keywords:                    |
------------------------------+------------------------
 This is a follow up to this ticket:

 https://svn.boost.org/trac/boost/ticket/11229


 Boost.Container currently checks the `is_trivially_copy_constructible` and
 `is_trivially_copy_assignable` type traits to enable `memcpy`
 optimization. However, the standard says that `memcpy` may only be used
 with ''trivially copyable'' types, and being ''trivially copy
 constructible/assignable'' is not a sufficient, or indeed necessary,
 condition for trivial copyability. Trivially copyable types must:

 * Have no non-trivial copy constructors
 * Have no non-trivial copy assignment operators
 * Have no non-trivial move constructors
 * Have no non-trivial move assignment operators
 * Have a trivial destructor

 Note that deleted special functions are classified as trivial. This is
 required, otherwise types such as this would not be trivially copyable:

 {{{
 struct X
 {
     const int x;
 };
 }}}

 This is why I say that trivially copy constructibility is not a necessary
 condition for trivial copyability.

 Note that there is a proposal to modify the definition of trivially
 copyable so that at least one special function has to not be deleted (and
 the destructor must not be deleted):

 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4460.html


 Using `memcpy` with types that aren't trivially copyable is ''undefined
 behaviour'' according to the standard. Thus, the behaviour of
 Boost.Container is technically undefined when used with types that are
 trivially copy constructible/assignable but ''not'' trivially copyable.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11844>
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