[boost] Re: question re has_trivial_contructor type trait

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2004-12-02 22:54:07

On Dec 2, 2004, at 9:46 PM, Matt Austern wrote:

> On Thu, 2 Dec 2004 21:02:10 -0500, Howard Hinnant 
> <hinnant_at_[hidden]> wrote:
>
>> Metrowerks::has_trivial_copy_ctor<T>::value
>>
>> for a year or two now (depending on platform).  Some of the STL
>> containers use this information for optimizations.  The implementation
>> of this trait is tied to the Metrowerks compiler in a non-standard-C++
>> fashion (e.g. a compiler intrinsic).
>
> This is probably getting to be more standards-related than
> boost-related, but...  Assuming it isn't proprietary information, it
> would be useful if you could say a little bit about what your
> intrinsic looks like.  As you may know, the core people, especially
> EDG, would like to see those sorts of intrinsics standardized.
> Everyone is going to have to implement some kind of interface that
> makes type traits implementable, and the EDG people figure that
> everyone might as well all implement the same thing.

No problem.  I have my doubts about needing to standardize at the 
language level, that's what std::libs are for:  We write non-portable 
code so you don't have to.  (that's my motto)  :-)  Although a 
de-facto, unofficial standard would certainly be convenient for us 
vendors.

But here is a copy/paste:

        template <class T>
        struct class_has_trivial_copy_ctor
        {
        #if __MWERKS__ >= 0x3100
                static const bool value = __builtin_trivial_members(T) & 0x4;
        #else
                static const bool value = false;  // hook
        #endif
        };

Notes:  class_has_trivial_copy_ctor is not a client level interface, 
has_trivial_copy_ctor is.  At the class_has_trivial_copy_ctor level, T 
is guaranteed to be a struct or a class (not a union, scalar, 
reference, array, void or function) (fwiw).

-Howard