constexpr explicit symbol_code( std::string_view str )
      :value(0)
      {
         if( str.size() > 7 ) {
            eosio::check( false, "string is too long to be a valid symbol_code" );
         }
         for( auto itr = str.rbegin(); itr != str.rend(); ++itr ) {
            if( *itr < 'A' || *itr > 'Z') {
               eosio::check( false, "only uppercase letters allowed in symbol_code string" );
            }
            value <<= 8;
            value |= *itr;
         }
      }

格式

<=7位，A-Z