How to filter duplicate types from tuple C++

Question

How does one filter duplicate types from a tuple?

For example:

using Tuple = std::tuple
usi         


        

Answer 1

Piotr's code is very concise and should be prefered. Here an extended version for general variadic templated classes which works on C++17 (e.g. std::variant or custom containers):

#include 

// end of recursive call: tuple is forwared using `type`
template 
struct unique_impl {using type = T;};

// recursive call: 1. Consumes the first type of the variadic arguments, 
//                    if not repeated add it to the tuple.  
//                 2. Call this again with the rest of arguments
template  class Tuple, typename... Ts, typename U, typename... Us>
struct unique_impl, U, Us...>
    : std::conditional_t<(std::is_same_v || ...)
                       , unique_impl, Us...>
                       , unique_impl, Us...>> {};

// forward definition
template 
struct unique_tuple;

// class specialization so that tuple arguments can be extracted from type
template class Tuple, typename... Ts>
struct unique_tuple> : public unique_impl, Ts...> {};

Live Demo

If you need C++11 you just need to replace the fold expression (std::is_same_v || ...) to use a self-made disjunction<...> (see cppreference possible implementation).