[Boost-users] MPL and conditional compilation

From: Alan M. Carroll (amc_at_[hidden])
Date: 2005-08-25 21:09:13

OK, I worked it out. I couldn't get this to compile:

        
typedef
typename boost::mpl::if_c<IS_OUTER,
        
        
typename PAYLOAD::key_type,
        
        
PAYLOAD
        
>::type
testing;

The problem is that I can't have the type specification
PAYLOAD::key_type, it has to be disconnected to allow lazy evaluation to
work. The following compiles and works:

        
template <
bool F , typename T >
        
struct
get_key_type_if { typedef typename T type; };
        
template
< typename T >
        
struct
get_key_type_if<true, T> { typedef typename T::key_type type;
};

        
typedef

typename
 get_key_type_if<IS_OUTER,
PAYLOAD>::type testing;

Ah, finally, I have an MPL variant that works:

        
struct
 get_key_type {
template
 <
typename
 T >
struct
 apply {
typedef

typename
 T::key_type type; }; };
        
typedef

typename
 boost::mpl::eval_if_c<IS_OUTER,
        
        
typename
 boost::mpl::apply<get_key_type,
PAYLOAD>,
        
        
typename
 boost::mpl::identity<PAYLOAD>
        
>::type
testing;

Is this as simple as it gets?