Re: [boost] boost::result_of and C++03

From: Eric Niebler (eric_at_[hidden])
Date: 2008-04-09 20:30:05

Resending...

Eric Niebler wrote:
> If I were to write an identity that works today *and* tomorrow, I would 
> do it like this:
> 
> struct identity
> {
>    // for C++03 compatibility
>    template<typename Sig>
>    struct result;
> 
>    template<typename This, typename Arg>
>    struct result<This(Arg)>
>    {
>      typedef Arg type;
>    };
> 
>    template<typename Arg>
>    Arg &operator()(Arg &arg) const
>    {
>      return arg;
>    }
> 
>    template<typename Arg>
>    Arg const &operator()(Arg const &arg) const
>    {
>      return arg;
>    }
> 
> #if BOOST_HAS_RVALUE_REFS
>    template<typename Arg>
>    Arg operator()(Arg &&arg) const
>    {
>      return std::move(arg);
>    }
> #endif
> };
> 
> And now in both C++03 and C++0x, I can be sure the following code works:
> 
>      int i = 0;
>      int const j = 0;
> 
>      // rvalue
>      result_of<identity(int)>::type k = identity()(1);
>      assert( 1 == k );
> 
>      // lvalue
>      result_of<identity(int &)>::type l = identity()(i);
>      assert( &l == &i );
> 
>      // const lvalue
>      result_of<identity(int const &)>::type m = identity()(j);
>      assert( &m == &j );
> 
> As Giovanni points out, in C++0x "The values ti are lvalues when the 
> corresponding type Ti is a reference type, and rvalues otherwise."
> 
> So Peter, is this wrong?

I'm glad we know how to write the identity function object in C++0x. 
What about in C++03?

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com