Imagine you are designing a text$-$based game where the player controls a character exploring a mysterious cave. As part of the game mechanics, the player needs to input commands using single characters. Write the C$++$ code for the input validation for these commands.

Your program should prompt the user to enter a single character command. One of the following valid commands: $\text{'}$N$\text{'}($move north$),\text{'}$S$\text{'}($move south$),\text{'}$E$\text{'}($move east$),\text{'}$W$\text{'}($move west$),$ or $\text{'}$Q$\text{'}($quit the game$).$ If the entered input is not one of these characters, display an error message and prompt the user to re$-$enter the command until a valid input is provided.

Remember to indent where necessary or I will count off.