Get all Post IDs for the current WP_Query

post.php

<?php

add_action( 'loop_end', 'tonya_get_list_of_post_ids_in_current_loop' );
/**
 * Get a list of Post IDs for the current Loop.
 * 
 * Hooks into 'loop_end' which occurs when WP_Query is done with the
 * loop (see wp-includes/query.php).
 *
 * @since  1.0.0
 * 
 * @param  WP_Query obj $query 	WP_Query object instance
 * @return void
 */ 
function tonya_get_list_of_post_ids_in_current_loop( $query ) {
		$ids = array();
		foreach ( $query->posts as $post ) {
			$ids[] = $post->ID;
		}
		
		//* Now do something with it.
		
}