Re: [boost] [test] BOOST_TEST macro and std::max

Subject: Re: [boost] [test] BOOST_TEST macro and std::max
From: Raffi Enficiaud (raffi.enficiaud_at_[hidden])
Date: 2016-01-11 07:46:12

Le 11/01/16 10:51, Andrzej Krzemienski a écrit :
> 2016-01-11 4:11 GMT+01:00 Gennadiy Rozental <rogeeff_at_[hidden]>:
>
>> Benedek Thaler <thalerbenedek <at> gmail.com> writes:
>>
>> I believe at this point we understand WHAT is going on.
>>
>>>    auto const& E = ::boost::test_tools::assertion::seed()->*3u
>>> == ((std::max)(0u, 3u));
>>
>> max returns unsigned int&&, which for some unclear reason is bound to
>> unsigned int const& in expression template E. This obviously becomes
>> dangling reference as soon as we leave this line.
>>
>
> This is an expected behavior in C++: an xvalue (rvalue reference returned
> from a function) can be bound to a const lvalue reference. See example:
> http://melpon.org/wandbox/permlink/yon7EWpj2B5KQNcT
>

This is a nice web site :) (I should update myself)

The example here is more what is happening:
http://melpon.org/wandbox/permlink/R2obgUazntZM9LVT

the "printit" function with an universal ref. The return of std::max 
with two identical types is a const lvalue ref according to the standard.

That ref lifetime is expanded until what point?

The "binary_expr" that is returned with boost.test holds that ref. The 
code is

template<typename T>
binary_expr<ExprType, T, op::EQ<unsigned int, T> >
operator==( T&& rhs )
{
   // ...

Is this ref valid at the return of operator== ?

In the case of

auto const& E = ::boost::test_tools::assertion::seed()->*3u == 
((std::max)(0u, 3u)) ;

that ref would exists until " ; ", but if "E" contains an object having 
this reference, it is dandling after the " ; ".

Is that correct?

Thanks,
Raffi