invalid argument in foreach when trying to list custom post types in archive.php

I try to list pages of custom post type in archive.php, and I get this error message:

Invalid argument supplied for foreach()

$post_type = get_post_type( get_the_ID() );
$pages = get_pages(array( 'post_type' => $post_type ) );
    foreach ($pages as $page ) {
        echo '<div class = "prop-sidebar-section">';
        echo $page->post_title;
        echo '</div>';
    }

the weird thing is, that with get_pages(array( 'post_type' => 'page'); is working, and if I use get_pages(array( 'post_type' => 'post') ); is not.

Answers:

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Method 1

get_pages() is intended only for hierarchical post types like page. For non-hierarchical post types like post, get_pages() will return a false, hence that explains why get_pages( array( 'post_type' => 'post' ) ) “doesn’t work”, i.e. the returned value is not an array, so it can’t be used with foreach:

// Good: 'page' is a hierarchical post type.
$pages = get_pages( array( 'post_type' => 'page' ) );
// Output: $pages is an array. Can do foreach ( $pages ... )

// Not good: 'post' is a non-hierarchical post type.
$posts = get_pages( array( 'post_type' => 'post' ) );
// Output: $posts is a FALSE. Can't do foreach ( $posts ... )
// because foreach ( false ... ) is invalid.

So for non-hierarchical post types, you would use get_posts() or make a new instance of WP_Query (i.e. new WP_Query()):

$posts = get_posts( array( 'post_type' => 'post' ) );

// or..
$query = new WP_Query( array( 'post_type' => 'post' ) );
$posts = $query->posts;

Method 2

I am assuming your code is not within a wordpress loop,
something like while(have_posts()): the_post();
which means get_the_ID will return null,
try var_dump(get_the_ID); if it is null thats why your code is not working.

and to get your post type you can use this:

var_dump($wp_query->query,get_queried_object()); die;

$wp_query->query will have post_type component for custom post types. That will not be there for post post types. get_queried_object will return quite a bit of data for custom post types but null for post post type.

Method 3

Run this code and will return all you got, then you can check if there your $post_type is returning ‘page’ or ‘your_custom_post_type’, and also check all available with the function get_pages();

$post_type = get_post_type( get_the_ID() );
echo '<pre>';
print_r($post_type);
echo '<br> ####################################### <br>';
$pages = get_pages(array( 'sort_order'   => 'ASC',
    'sort_column'  => 'post_title',
    'hierarchical' => 1,) );
echo '<pre>';
print_r($pages);

In my case I only have $post_type returning ‘page’ and $pages have a lot of info but all post_types are ‘page’. Hope that can help some how!..

And a few minutes later after reading Sally CJ answer, I had use the get_posts instead and works like a charm.

$post_type = get_post_type( get_the_ID() );
$pages = get_posts( array( 'post_type' => $post_type ) );
    foreach ($pages as $page ) {
        echo '<div class = "prop-sidebar-section">';
        echo $page->post_title;
        echo '</div>';
    }

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0