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

can explain why when creating meta boxes callback needs post id passed via $post->id, action hook 'save_post', function can pass $post_id. general explanation of when use 1 clear 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 verification echo '<input type="hidden" name="custom_meta_box_nonce" value="'.wp_create_nonce(basename(__file__)).'" />';      // begin field table , 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 $post_id

function save_custom_meta($post) {      if( defined( 'doing_autosave' ) && doing_autosave ) return;     /* verify 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 current user has permission edit post. */     if ( !current_user_can( $post_type->cap->edit_post, $post_id ) )         return $post_id;     /* posted data , sanitize use html class. */     $new_meta_value = ( isset( $_post['custom-meta-class'] ) ? sanitize_html_class( $_post['custom-meta-class'] ) : '' );     /* meta key. */     $meta_key = 'custom_meta_class';     /* meta value of 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 new meta value not match 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 no new meta value old value exists, delete it. */     elseif ( '' == $new_meta_value && $meta_value )         delete_post_meta( $post_id, $meta_key, $meta_value ); } 

the action hook save_post returns $post_id.

show_custom_meta_box not wordpress hook, custom function. uses get_post_meta , function requires $post_id.

there no need give $post. can edit code uses id, not post object:

function show_custom_meta_box($post_id) {     $meta = get_post_meta($post_id, 'custom_meta_class', true);  }