Pairwise bool and in c++ template

Question

I am writing a template that takes a arbitrary number of arguments and find the Boolean AND on these value.

template <bool... Vs> struct meta_bool_and;

template <bool V> struct meta_bool_and : std::integral_constant<bool, V> {}; 

template <bool V, bool... Vs> 
struct meta_bool_and : std::integral_constant<bool, V && meta_bool_and<Vs...>::value> {}; 

However, I failed to compile by the following message

 error: redeclared with 2 template parameters
 struct meta_bool_and : std::integral_constant<bool, V && meta_bool_and<Vs...>::value> {}; 

How can I fix this problem?

Answer 1

You have written a re-definition instead of a partial specialization. In order to provide a specialization, you must specify what properties you specialize on.

This will work:

#include <type_traits>

template <bool... Vs> struct meta_bool_and;

template <bool V> struct meta_bool_and<V> : std::integral_constant<bool, V> {};
//                                    ^^^

template <bool V, bool... Vs> 
struct meta_bool_and<V, Vs...> : std::integral_constant<bool, V && meta_bool_and<Vs...>::value> {}; 
//                  ^^^^^^^^^^

As an improvement, think whether you want to support the empty conjunction (typically defined as true). If so, don't specialize on meta_bool_and<bool> but on meta_bool_and<> (derived from std::true_type).