Is the 404 page automatically displayed if a loop returns nothing?

Question

If you have an index.php with a loop such as:

if(have_posts()){
    while(have_posts()){
        the_post();
    }
}

And nothing is returned, you would generally do a:

if(have_posts()){
    while(have_posts()){
        the_post();
    }
}else{
    //display message
}

However, what if you have a 404.php? Will that page be displayed if nothing is returned in terms of the loop (assuming I have not written the else part of the loop)?

Answer 1

If you want to force the 404.php template to load if there are no found posts, use the template_include filter:

function wpa84794_template_check( $template ) {
    global $wp_query;
    if ( 0 == $wp_query->found_posts ){
        $template = locate_template(
            array( '404.php', $template ),
            false
        );
    }
    return $template;
}
add_filter( 'template_include', 'wpa84794_template_check' );

Answer 2

Checkout the WordPress Template Hierarchy page, in particular the diagram: http://codex.wordpress.org/Template_Hierarchy#Visual_Overview

In short though the answer is no, the 404.php is only used when the URL does not resolve to any other template type.