Question

I am trying to figure out how to get a C++ template to use a look-up table to perform its functions, but at compile time not run time. I am having trouble putting it into words, so here’s an example, and the ugly template + preprocessor macro combination that I have so far:

template<class T_POD, class T_Corba>
inline void c_to_corba_base(T_POD &In, CORBA::Any &Out) {
    Out <<= static_cast<T_Corba>(In);
}

#define C_TO_CORBA_PAIR(T_POD, T_CORBA) \
inline void c_to_corba(T_POD &In, CORBA::Any &Out) { \
    c_to_corba_base<T_POD, T_CORBA>(In, Out); \
}

C_TO_CORBA_PAIR(short, CORBA::Short)
C_TO_CORBA_PAIR(long, CORBA::Long)
C_TO_CORBA_PAIR(double, CORBA::Double)
// etc.

So you can see, it typecasts A to B to get C. C is always CORBA::Any. But B depends on A (known at compile time).

I’ve done some research and it looks like Boost::MPL::bind may do what I need (and we already require Boost) but I do not understand the syntax. It could have all been done in the macro, but I’d rather have it as “real” templates if it can be done.

Any suggestions?

Answer 1

Is this better?

template<typename> struct CorbaTypeMap;
template<> struct CorbaTypeMap<short>  { typedef CORBA::Short  type; };
template<> struct CorbaTypeMap<long>   { typedef CORBA::Long   type; };
template<> struct CorbaTypeMap<double> { typedef CORBA::Double type; };

template<typename T_POD>
inline void c_to_corba(T_POD &In, CORBA::Any &Out) {
    Out <<= static_cast< /* typename */ CorbaTypeMap<T_POD>::type >(In);
}

I don’t think you’ll need that typename keyword, because static_cast always needs a type, but if you get an error that’ll probably be the fix.