You should use std::string_view to pass a parameter to a function (or return a value from a function), instead of std::string const & unless your code needs to call other functions that take std::string parameters (in which case, conversions would be necessary):

    std::string_view get_filename(std::string_view str) 
    { 
      auto const pos1 {str.find_last_of('')}; 
      auto const pos2 {str.find_last_of('.')}; 
      return str.substr(pos1 + 1, pos2 - pos1 - 1); 
    } 

    char const file1[] {R"(c:	estexample1.doc)"}; 
    auto name1 = get_filename(file1); 

    std::string file2 {R"(c:	estexample2)"}; 
    auto name2 = get_filename(file2); 

    auto name3 = get_filename(std::string_view{file1, 16});