Object references

Every read and write function that takes $object (wpmcf_get_field(), wpmcf_the_field(), wpmcf_has_field(), wpmcf_update_field(), wpmcf_delete_field(), wpmcf_check_password(), wpmcf_get_password()) accepts the same forms.

Accepted forms

References are case-insensitive and trimmed (' User_3 ' works).

wpmcf_get_field( 'g', 'f' );                        // current post
wpmcf_get_field( 'g', 'f', 42 );                    // post 42
wpmcf_get_field( 'g', 'f', get_queried_object() );  // whatever the page is about, if it is a post/term/user
wpmcf_get_field( 'g', 'f', 'user_3' );
wpmcf_get_field( 'g', 'f', 'term_5' );
wpmcf_get_field( 'g', 'f', 'category_5' );
wpmcf_get_field( 'g', 'f', 'comment_7' );
wpmcf_get_field( 'g', 'f', 'option' );
wpmcf_get_field( 'g', 'f', 3, [ 'context' => 'user' ] );   // same as "user_3"

The context argument wins

An explicit $args['context'] beats the context a reference implies:

wpmcf_get_field( 'g', 'f', 'user_3', [ 'context' => 'post' ] ); // reads POST 3, not user 3

Use one or the other, not both.

Details worth knowing

• Taxonomy prefixes only choose the term context. "category_5" and "post_tag_5" both mean "term 5 of whatever taxonomy it belongs to"; the taxonomy name is not compared with the term (verified: "post_tag_N" returned the value of a category term with that ID). The prefix must be a registered taxonomy, otherwise the reference is invalid.
• Without an ID, only the post context has a "current" object. context => user|term|comment with no ID has nothing to read. The read returns the default; a write returns false.
• The object must exist to write (wpmcf_update_field()): writing to 'user_999999' or post 99999999 returns false so no orphan rows are created. Reading a missing object returns the default.
• Options need no ID. "option" and context => options ignore any ID.
• Widgets need either "widget_{id}" or the three arguments widget_sidebar, widget_id_base and widget_number with context => widget (not exercised end to end in testing).

Invalid references

These are invalid: negative numbers, floats, true, arrays, objects of other classes, strings such as 'foo', 'user' or 'user_abc', and "{unknown}_5".

Nothing throws.

Getting an ID or reference from WordPress

$user  = wp_get_current_user();
wpmcf_get_field( 'profile', 'bio', $user );                       // WP_User works as is
wpmcf_get_field( 'profile', 'bio', 'user_' . get_the_author_meta( 'ID' ) );

if ( is_tax() || is_category() || is_tag() ) {
    wpmcf_get_field( 'brand', 'logo', get_queried_object() );     // WP_Term
}

foreach ( get_comments( [ 'post_id' => get_the_ID() ] ) as $comment ) {
    wpmcf_get_field( 'review', 'rating', $comment );              // WP_Comment
}

Where each context's group comes from

A group only has a table row for objects its location rules include (see Field groups). A group that is attached to posts only never has user values. Reading an object type the group is not attached to returns the default.