Re: [Boost-users] [multi_index] Indices interface type for overload resolution

El 20/11/2019 a las 14:12, Christophe B via Boost-users escribió:

Hi,

I have currently

template std::vector<Key> keys(const std::map &m) { ... }

I'd like to write an overload that works also with the indices interface of multi_index_container.

[...]

template< typename Tag,typename Value,typename IndexSpecifierList,typename Allocator > auto keys(const typename multi_index::index< multi_index::multi_index_container,Tag>::type& m) {     typedef decltype(c.key_extractor()(*c.begin())) key_type; // A better definition would be welcome     std::vector result;     result.reserve(c.size());     for (const auto &elem : c) {         result.push_back(c.key_extractor()(elem));     }     return result; }

But the overload resolution fails.

The second overload is not picked up because Tag, Value, etc. are being used in a nondeduced context: https://stackoverflow.com/questions/1268504/why-is-the-template-argument-ded... So, this approach won't get you anywhere. You have two options: 1. You can use the name of the actual class implementing ordered indices:     template     auto keys(const boost::multi_index::detail::ordered_index& c){...} THIS IS STRONGLY DISCOURAGED, because this name is not documented. Basically, you're relying on an implementation detail. Rather use: 2. With SFINAE and a little ingenuity you can have something like:     template<       typename Index,       std::enable_if_t<         sizeof(typename Index::key_type)&&         sizeof(std::declval<const Index>().key_extractor())       >* =nullptr     >     auto keys(const Index& c)     {       using key_type=typename Index::key_type;       std::vector result;       result.reserve(c.size());       for(const auto &elem:c){         result.push_back(c.key_extractor()(elem));       }       return result;     } Here, we're using SFINAE to activate the overload *only* when Index does have a nested key_type type and a key_extractor member function, which can be taken as a very certain bet that we're indeed dealing with a key-based Boost.MultiIndex index. This is, if you wish, poor man's pre-C++20 concepts. BTW, this overloads also picks hashed and ranked indices. Best, Joaquín M López Muñoz