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

From: Andrei Alexandrescu (andrei_at_[hidden])
Date: 2008-04-09 13:11:14

Howard Hinnant wrote:
> On Apr 8, 2008, at 9:40 PM, Steven Watanabe wrote:
>> AMDG
>>
>> Howard Hinnant wrote:
>>>> template <class T, class U>
>>>> decltype(true ? T() : U()) Min(T&& a, U&& b) {
>>>>   return b < a ? b : a;
>>>> }
>>>>
>> Should this really work if T of U is a reference type and thus cannot
>> be default constructed?
> 
> You're right.  Not by current language rules.  The reported error  
> message could have been better.

I knew this is going to come. That's why I'd tried this with conceptg++:

typedef const int& cintr;
typedef int& cint;

int main()
{
     int a;
     int x = cintr();
     int y = cint();
}

Turns out this program compiles fine. I assumed the compiler was right,
and I was also self-pressured to make Min's definition self-contained,
so I just went with that trick.

> By coincidence I'm dealing with a similar problem here: http://home.twcny.rr.com/hinnant/cpp_extensions/time2_demo.html 
>   , and dealing with it via a promote trait, quite similar to what  
> I've seen in boost.  I.e. the Min signature might look like:
> 
>     template <class T, class U> typename promote<T, U>::type Min(T&&  
> a, U&& b);
> 
> With the *default* (and preferably standardized) promote trait looking  
> like:
> 
> template <class T, class U>
> struct promote
> {
> private:
>      static T t;
>      static U u;
> public:
>      typedef decltype(true ? static_cast<T&&>(t) :  
> static_cast<U&&>(u)) type;
> };
> 
> // promote is specializable for client-defined types

So it does look like there is a simple solution to min and max! Any
interest in submitting a revised 2199?

Oh, and in order to save space and default-constructor-related
aggravation, you may want to use static functions in lieu of static
variables. In the process, I also changed name to what I think is a
better one:

template <class T, class U>
struct common_type
{
private:
      static T t();
      static U u();
public:
      typedef decltype(true ? static_cast<T&&>(t()) :
static_cast<U&&>(u())) type;
};

Actually, heck, just have the functions return the rvalue ref directly:

template <class T, class U>
struct common_type
{
private:
      static T&& t();
      static U&& u();
public:
      typedef decltype(true ? t() : u()) type;
};

Then you define Min like this:

template <class T, class U>
typename common_type<T, U>::type
Min(T&& a, U&& b) {
     return b < a ? b : a;
}

and you're home free. At least my few tests pass fine.

I think you're dead on that common_type/promote belongs to the standard.
I'd defined a template called CommonType in the D standard library:

http://www.digitalmars.com/d/2.0/phobos/std_traits.html#CommonType

and it turned out to be mightily useful. (Notice that it takes a
variable number of types.)

Andrei