C$++$ pls;

gives the hex$2$Dec$($const string& hexString$)$ function that returns a decimal number from a hex string. Implement the hex$2$Dec function to throw a invalid$\_$argument exception if the string is not a hex string. Write a test program that prompts the user to enter a hex number as a string and display the number in decimal. If the function throws an exception, display "Not a hex number".