std::find has signature like: iterator find( const Key& key );
as std::string can't be constructed from std::string_view implicitly, you can't use std::string_view to search a set/map with key type being std::string directly.
C++14 fills the gap, by providing new overloads of the find method (along with new overloads of count, lower_bound, upper_bound and equal_range.
To activate these overloads, the comparison function object has to define a typedef called is_transparent.
The value of this typedef is not used so it doesn’t matter what it is equal to, as long as it is defined.