Subject: Re: [boost] C++11 Metaprogramming
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-04-03 13:44:41
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), ...);
-- Dave Abrahams BoostPro Computing http://www.boostpro.com