We made conversion operators constant for bsoncxx types to make it possible to write like this:

const bsoncxx::types::value& value{ ... }; // some value extracted from a document element
bsoncxx::stdx::string_view str_view{ value.get_utf8() };

Without const qualifier I have to use intermediate non-constant variable:

bsoncxx::types::b_utf8 utf8{ value.get_utf8() };
bsoncxx::stdx::string_view str_view{ utf8 };