[Boost-users] boost::enable_if : usage clarification

From: Suresh T (suresh.t_at_[hidden])
Date: 2005-08-08 03:22:01

hi 

I am using the boost::enable_if for selection of types. Something similar 
to below

template<std::size_t Size,class Enable = void>
struct type_select{
};

template<std::size_t Size>
struct type_select< Size,
                        typename boost::enable_if < bool_ < (size <= 8) > 
>
                    >::type
                    >{ 
  typedef typename char  type; 
};

template<std::size_t Size>
struct type_select< Size,
                        typename boost::enable_if < bool_ < (size > 8) > >
                    >::type
                    >{ 
  typedef typename int  type; 
};

The conditions are based on the first parameter size. The normal usage 
would be something like      type_select<10>::type;

My question is : If i explicitly use the second parameter ( by passing 
void) will the correct specialization be still used
i.e, if i use say   type_select<6,void>::type,       will it resolve to 
"char" in the above case.

Please note:  I was using the normal way  (without the void) for my code 
on linux+gcc without any problems 
My problem came up when i tried to move my  code to windows + vc7.1. 
The vc7.1 compiler needs the second argument  (in my usage shown below ) 
else it gives an error. The results are as desired, but i would like
to know whether this usage is correct.

Below is a snippet where the vc7.1 compiler gives a problem with the 
usage. 
Note type_select is not directly used but is used as a  default type for 
USER_TYPE parameter

template <std::size_t size ,
         template<std::size_t size,typename E = void > class USER_TYPE 
         >
class my_class
          < size, 
           USER_TYPE = select_type   //select_type is a default parameter
          >  {

public:
       //The commented code line below does not work  for VC7.1
       // typedef typename USER_TYPE<size>::type  data_type;     //THIS 
DOES NOT WORK  with vc7.1 

       typedef typename USER_TYPE<size,void>::type  data_type;   //THIS 
WORKS 

};
 
Thanks for clarification in advance..

---------------------------------------------------------------------------------------------------------
Rgds,
Suresh