Question

Given this template:

template <class A>
struct Something {
    ... // members common to all template instantiations for all A types 
    SpecialType member; // but not this - I want this to be conditional...
}

…I want to use “enable_if” to have the SpecialType member exist conditionally; that is, only when the template is instantiated with A=SpecialCase1 or SpecialCase2 types. In all other cases, I want the SpecialType member to be missing.

In case you’re wondering why, this is about optimization – i.e. not carrying useless payload in the struct. I am a newbie in template metaprogramming, but I understand I need “enable_if” and two “is_same” somehow – not sure exactly how, though…

EDIT: Doing it with generic C++ (i.e. without Boost-specifics) would be a plus.

Answer 1

Well: use a base class.

struct Empty {};

struct SpecialTypeCnt { SpecialType member; };

template <typename A>
struct Something: if_< /* cond */ , SpecialTypeCnt, Empty>::type {
};

Where if_ is defined as:

template <typename, typename, typename E> struct if_ { typedef E type; };

template <typename T, typename E>
struct if_<std::true_type, T, E> { typedef T type; };

(You can also specialize on a boolean)

Now of course, you need to express your condition properly.

---

Having said that, you should probably not use just a struct. Instead you should use a class which provides the operations that need be applied on member. Then you provide a class Null with a default behavior and a class SomeType with the behavior specific to member.

Otherwise you’ll rewrite the condition everywhere you need to “perhaps” modify member, and it gets annoying real quick.