thank( you );

Another great kata from you. Thanks, DM!

Haha! You know it is possible, but your cached bitset works better. Bravo!

👍

C-style cast makes it 98% close. XD
Your idea with ranges sounds great.

No, using xor does not make the function redundant.

The trick with implicit conversion (boolean to integer):

return s_msg[ n < 10 ];

👍 Very interesting kata. Thanks!

Why? Test several sequences and find a pattern.

UPD: next( &c ) is a valid address for dereference. There will be a data character or a null-terminator.
basic_string is implemented as: [ size_t __cap_ ][ size_t __size_ ][ char* __data_ (ends with \0) ]

UPD: After some research and tests, I've noticed, that Clang 8 implements strings with a null-terminator in the memory layout. Technically, I dereference not std::string::end (which would be UB), but the address in memory, where \0 is located.

thank( you );

NB: if the sizes are different, at() throws an exception.

NOTE Hash map is more efficient, string is more concise.
It's up to you which one to use in such a small problem.
Happy coding, fellow warriors!