Re: [boost] [type trait extension] I hate volatile...

Subject: Re: [boost] [type trait extension] I hate volatile...
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-07-22 13:54:23

2011/7/22 Frédéric Bron <frederic.bron_at_[hidden]>
[...]

> I found another issue with g++ (all versions up to 4.6.0) (works fine
> with msvc 10
> and intel 12): although g++ behaves well with fundamental types
> returned by value,
> the result of postfix operator++ applied to an int volatile is detected as
> void!
> The standard states in 13.6/3 (Built-in operators):
> "For every pair (T, VQ), where T is an arithmetic type, and VQ is
> either volatile
> or empty, there exist candidate operator functions of the form
> VQ T& operator++(VQ T&);
> T operator++(VQ T&, int);"
>
> That means that for an "int volatile" variable the corresponding built-in
> operator should be:
> int operator++(int volatile &, int);
> and according to the table above this should be correctly detected as
> returning
> non-void. Note that prefix operator++ works fine.
> This is the program that fails (writes true) with g++ (4.5.2 and 4.6.0):
>
> #include <iostream>
> #include <iomanip>
> #include <boost/type_traits/detail/yes_no_type.hpp>
>
> namespace detail {\
>   struct returns_void_t {};
>   static ::boost::type_traits::yes_type returns_void(returns_void_t);\
>   static ::boost::type_traits::no_type returns_void(int);\
> }
> template <typename T> int operator,(const T&, ::detail::returns_void_t);
> template <typename T> int operator,(const volatile T&,
> ::detail::returns_void_t);
>
> #define RETURNS_VOID(Expr)\
>   sizeof(::boost::type_traits::yes_type)\
>   ==\
>   sizeof(::detail::returns_void(((Expr), ::detail::returns_void_t())))
>
> int main() {
>   int volatile one_int_volatile;
>   std::cout<<std::boolalpha<<"RETURNS_VOID:
> "<<(RETURNS_VOID((one_int_volatile++)))<<'\n';
>   return 0;
> }
>

Indeed, this is weird.  I would hope that operator--(int volatile&, int) at
least returns a consistent result :)

At the end of the day, it's sounding like you should just paste a huge
warning somewhere in the library concerning volatile-qualified by-value
return types.

- Jeff