What does setup_postdata ($post ) do?

The codex defines it as ‘Set up global post data. Helps to format custom query results for using Template tags.’ I don’t really understand this.

Here is a code example :

global $post;
$args = array( 'numberposts' => -1);
$posts = get_posts($args);
foreach( $posts as $post) : setup_postdata($post);
echo $post->ID;

Please can you explain?


Template tag functions rely on global variables to access the post being processed and retrieve data from it or related to it.

The main variable among these is $post, which holds the post object itself.

In your example it’s not explicit, but what is happening is that your loop assigns data to $post. If its name wasn’t $post, you would need to name it explicitly (global $post; $post = $some_other_post;).

However, there are a bunch of other globals and the purpose of setup_postdata() is to fill them with data. If you look at the source, these are:

global $id, $authordata, $currentday, $currentmonth, $page, $pages, $multipage, $more, $numpages;

Note that in most cases you should be calling wp_reset_postdata() afterwards to return globals to their original state.

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