Re: [boost] [type_traits] is_complete<T> possible realization

Subject: Re: [boost] [type_traits] is_complete<T> possible realization
From: John Maddock (boost.regex_at_[hidden])
Date: 2012-11-09 12:17:09

>>
>> What if I do in another TU is_complete<incomplete, __LINE__> at the same
>> line number and when incomplete is actually complete?
>
> You can always add some more misc template parameters:
>
> void foo1() {
>    struct unique_foo1_tag;
>    cout
>            << is_complete<incomplete, __LINE__, unique_foo1_tag>::value
>            << is_complete<incomplete_template<0> , __LINE__,
> unique_foo1_tag>::value
>            << is_complete<incomplete_template<1> , __LINE__,
> unique_foo1_tag>::value
>            << is_complete<complete, __LINE__, unique_foo1_tag>::value
>            << is_complete<complete_abstract, __LINE__, 
> unique_foo1_tag>::value
>            << endl;
> }
>
> And you can also use __COUNTER__ on MSVC.

__COUNTER__ won't do what you want across multiple TU's.  The problem runs 
deeper:

Translation Units a.cpp and b.cpp each instantiate a function foo<T> which 
uses is_complete<U>, but U is complete in TU a.cpp, and incomplete in b.cpp 
(because they include different headers).  You now have an insidious ODR 
violation which would be very hard to diagnose and track down - which 
version of foo<T> is the correct one?

John.