Re: [boost] C++11 Metaprogramming

Subject: Re: [boost] C++11 Metaprogramming
From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2012-04-03 13:59:55

On Apr 3, 2012, at 1:44 PM, Dave Abrahams wrote:

> 
> on Tue Apr 03 2012, Howard Hinnant <howard.hinnant-AT-gmail.com> wrote:
> 
>>> That's lovely, but... exactly how does noexcept play into the
>>> metaprogramming picture?
>> 
>> For example, I used it to create an is_nothrow_swappable<A> trait (http://llvm.org/svn/llvm-project/libcxx/trunk/include/type_traits) so that I could write:
>> 
>> struct B
>> {
>>    A a;
>> };
>> 
>> void swap(B& x, B& y) noexcept(__is_nothrow_swappable<A>::value)
>> {
>>    swap(x.a, y.a);
>> }
>> 
>> instead of:
>> 
>> using std::swap;
>> void swap(B& x, B& y) noexcept(
>>                        noexcept(
>>                            swap(declval<A&>(),
>>                                 declval<A&>())))
>> {
>>    swap(x.a, y.a);
>> }
>> 
>> The latter gets very tedious when B has several members and/or bases.
> 
> But isn't this a better way to relieve the tedium?
> 
>  #define RETURNS(...)                                              \
>  noexcept(noexcept(decltype(__VA_ARGS__)(std::move(__VA_ARGS__)))) \
>  -> decltype(__VA_ARGS__)                                          \
>  { return (__VA_ARGS__); }                                         \
>  typedef int RETURNS_CAT(RETURNS_, __LINE__)
> 
>  #define RETURNS_CAT_0(x, y) x ## y
>  #define RETURNS_CAT(x, y) RETURNS_CAT_0(x,y)
> 
> ...
> 
>  auto swap(B& x, B& y) RETURNS(swap(x.a,y.a), swap(x.b,y.b), ...);

I hadn't seen that one before.  I guess Bjarne isn't getting rid of the preprocessor yet...

Howard