Re: [Boost-users] [multi-index][foreach] Iterating over a sequence index (worked in 1.46, broke in 1.47)

Subject: Re: [Boost-users] [multi-index][foreach] Iterating over a sequence index (worked in 1.46, broke in 1.47)
From: Eric Niebler (eric_at_[hidden])
Date: 2011-07-31 17:18:44

On 7/31/2011 1:34 PM, Michel MORIN wrote:
>> Michel MORIN <mimomorin <at> gmail.com> writes:
>>> GCC 4.6 (in C++03 mode) changes the behavior of rvalue conversion.
>>> This affects the way of lvalue/rvalue detection in Boost.Foreach;
>>> then compilation error occurs.
>>>
>>> To detect detecting lvalue/rvalue, Boost.Foreach tries to copy
>>> `sequenced_index` (though the copy is never executed…).
>>> However, the copy ctor of `sequenced_index` is protected,
>>> and so the compile error emitted.
> 
> This description might be insufficient. So let me explain it again.
> 
> GCC 4.6 changes the behavior of rvalue conversion.
> This affects the way of lvalue/rvalue detection in Boost.Foreach:
>   * On GCC 4.6, Boost.Foreach detects it at run time.
>   * On GCC 4.5 and prior versions, Boost.Foreach detects it at compile time.
> (Note that, in Boost 1.46 and prior versions, Boost.Foreach incorrectly uses
> compile-time detection on GCC 4.6.)
> 
> When detecting lvalue/rvalue at run time, Boost.Foreach tries to copy
> `sequenced_index`. However, its copy ctor is protected, and so
> the compile error emitted. To avoid this, we need to tell Boost.Foreach
> not to copy `sequenced_index` by specializing `is_noncopyable`
> or overloading `boost_foreach_is_noncopyable`.
> 
> Compile-time lvalue/rvalue detection does not cause any problem with
> `sequenced_index`. Considering the fact that many compilers (VC++ 7.1
> and newer versions, GCC 3.4-4.5) support compile-time detection,

"Support" isn't the right word. They have compiler bugs that Foreach can
exploit to get the information it needs. Gcc had such bugs until v4.6,
but now it doesn't, hence the trouble.

> it is not surprising that the compatibility issue between Boost.MultiIndex
> and Boost.Foreach did not occur in the past. But it's just that the problem
> did not surface in the past; on compilers that use run-time detection,
> the problem has existed from the beginning.

Exactly right. Thank you Michel.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com