Why use std::type_index instead of std::type_info*

Question

I need to key some data in a map by a type. Currently I have something like this:

struct TypeInfoComparer {   bool operator()(std::type_info const* a, std::type_info const* b) const   {     return a->before(*b);   }; };  std::map<std::type_info const*, Foo, TypeInfoComparer> d_fooByTypeId; 

Which I can then look up from using (for example, in a template method having <typename T>:

auto pair = d_fooByTypeId.find(&typeid(T)); 

However today I was reading about std::type_index which seems to be intended for use in such a case as this.

I'm interested in improving my C++ knowledge. Can someone please explain whether I should modify my code to use std::type_index, and why? Is there a reason beyond being able to remove the TypeInfoComparer?

Answer 1

type_index is "a simple wrapper for type_info which can be used as an index type in associative containers (23.4) and in unordered associative containers (23.5)". If you use type_index instead of type_info*, you will free yourself from having to provide an explicit comparator in your maps. The only cost is that you need to #include <typeindex>.

Another benefit is that it will allow you to switch to (or also use) hashmaps (aka unordered_maps).

On the whole, since it simplifies your code, I'd say "go for it".

Answer 2

I don't think using a pointer to the result returned from typeid(x) is guaranteed to always yield the same result. In particular it seems problematic to guarantee the same object to be returned when shared libraries are used. The intended use of std::type_info for sorting is to use before() member. The class std::type_index wraps this into a simoler interface.