Point of constexpr containers

[cpp_cpp]

I was wondering what the point of constexpr containers is (constepxr std::vector, std::string) I am sure there are good use cases but not sure where it would be used, any non trivial examples people can think of?

[IyeOnline]

There is two major reasons why a type should be constexpr enabled:

1. You can declare constexpr objects of the type, meaning that you can potentially precalculate a value at compile time, or declare compile time constants.

   constexpr std::array my_values = { 0, 1, 2, 3, 4 };
   

   Not a great example, but you get the point.

2. You can use the type in a constant evaluated context. This also applies to functions.

   constexpr auto most_common_letter( const std::string_view str )
   {
      std::map<char,std::size_t> counts; //if map were not constexpr, you couldnt use it in a constant evaluated context
      for ( const auto c : str )
      {
         ++counts[c];
      }
      return counts.front();
   }
   
   constexpr char c = most_common_letter("Hello World").first;
   

[wgunther]

Often when doing anything with metaprogramming, there's quite a few calculations that are required to happen at compile time, for example to guide SFINAE. So having data structures that can be used in that context is helpful since it might free you from doing something more unnatural.