The following example uses the comment_post hook to run a function immediately after a comment is posted. The function checks whether the comment is approved and, if so, executes the code specified.

function show_message_function( $comment_ID, $comment_approved ) {
	if( 1 === $comment_approved ){
		//function logic goes here
	}
}
add_action( 'comment_post', 'show_message_function', 10, 2 );

Note that the add_action line includes the priority and the number of parameters (, 10, 2). If we leave the number of parameters out, we will only be able to access to the first parameter ($comment_ID) in our function. We will not be able to access the second parameter ($comment_approved).