$post_id or $post->ID in wordpress meta box construction - wordpress

Question

Can someone explain why when creating meta boxes the callback needs the post id to be passed via $post->ID, however with the action hook 'save_post', the function can pass $post_id. A general explanation of when to use which one would help clear up some issues I've been having, thanks.
ex:
function show_custom_meta_box($post) {
$meta = get_post_meta($post->ID, 'custom_meta_class', true);
// Use nonce for verification
echo '<input type="hidden" name="custom_meta_box_nonce" value="'.wp_create_nonce(basename(__FILE__)).'" />';
// Begin the field table and loop
echo '<table class="form-table">';
echo '<tr>
<th><label for="custom-meta-class">Custom Meta Box</label></th>
<td>
<input class="widefat" type="text" name="custom-meta-class" id="custom-meta-class" value="'.$meta.'" size="50" />';
echo '</td></tr>';
echo '</table>'; // end table
}
and for the $post_id
function save_custom_meta($post) {
if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;
/* Verify the nonce before proceeding. */
if ( !isset( $_POST['custom_meta_box_nonce'] ) || !wp_verify_nonce( $_POST['custom_meta_box_nonce'], basename( __FILE__ ) ) )
return $post_id;
$post_type = get_post_type_object( $post->post_type );
/* Check if the current user has permission to edit the post. */
if ( !current_user_can( $post_type->cap->edit_post, $post_id ) )
return $post_id;
/* Get the posted data and sanitize it for use as an HTML class. */
$new_meta_value = ( isset( $_POST['custom-meta-class'] ) ? sanitize_html_class( $_POST['custom-meta-class'] ) : '' );
/* Get the meta key. */
$meta_key = 'custom_meta_class';
/* Get the meta value of the custom field key. */
$meta_value = get_post_meta( $post_id, $meta_key, true );
if ( $new_meta_value && '' == $meta_value )
add_post_meta( $post_id, $meta_key, $new_meta_value, true );
/* If the new meta value does not match the old value, update it. */
elseif ( $new_meta_value && $new_meta_value != $meta_value )
update_post_meta( $post_id, $meta_key, $new_meta_value );
/* If there is no new meta value but an old value exists, delete it. */
elseif ( '' == $new_meta_value && $meta_value )
delete_post_meta( $post_id, $meta_key, $meta_value );
}

Answer 1

I don't have an answer but I have the exact same problem. I'm surprised nobody has chimed in in the last 7 years.
Here's my code, it works (in functions.php)
add_action('save_post','extract_citations',100,1);
function extract_citations($post_id){
$the_post = get_post($post_id);
$content = $the_post->post_content;
preg_match_all('/<a\s+.*?href=[\"\']?([^\"\' >]*)[\"\']?[^>]*>(.*?)<\/a>/i',$content,$citation_array);
$citation_urls = $citation_array[1];
update_post_meta($post_id,'citations',serialize($citation_urls));
}
Here is code that does NOT work, despite being theoretically correct:
add_action('save_post','extract_citations',100,1);
function extract_citations($post){
$the_post = get_post($post->ID);
$content = $the_post->post_content;
preg_match_all('/<a\s+.*?href=[\"\']?([^\"\' >]*)[\"\']?[^>]*>(.*?)<\/a>/i',$content,$citation_array);
$citation_urls = $citation_array[1];
update_post_meta($post->ID,'citations',serialize($citation_urls));
}

Answer 2

I know this is a very old thread, but I wanted to add my thoughts.
Let’s pretend that the Post ID is 1724
In the first example you are inserting 1724 directly into the function so line 3 would become
$the_post = get_post(1724);
While in the second example of are asking for the ID of the post number so your line 3 would become
$the_post = get_post(1703->ID);
The thing to remember is that when you call the function within your site the number is added which becomes $post_id, it doesn’t actually matter what word you put in the brackets of the function.