How to call get_post()?

Question

I have a sidebar on my site and there I want to show the current post's meta information, such as the last modified date, the author and more.

Thus I wrote the following function:

function showPostMetaInfo()
{   
    global $post;

    setup_postdata($post);

    $mypost = get_post();

            $string.= "<table id='meta-info'>"
                    . "<thead>"
                    . "<tr>"
                    . "<th> Meta Type </th>"
                    . "<th> Value"
                    . "</tr>"
                    . "</thead>"
                    . "<tbody>"
                    . "<tr>"
                    . "<td> Title </td>"
                    . "<td>".get_the_title()."</td>"
                    . "</tr>"
                    . "<tr>"
                    . "<td> Author </td>"
                    . "<td>".get_the_author()."</td>"
                    . "</tr>"
                    . "<tr>"
                    . "<td> Published </td>"
                    . "<td>".get_the_date()."</td>"
                    . "</tr>"
                    . "<tr>"
                    . "<td> Last Modified </td>"
                    . "<td>".get_the_modified_date()."</td>"
                    . "</tr>"
                    . "<tr>"
                    . "<td> Categories </td>"
                    . "<td>".listCategories()."</td>"
                    . "</tr>"
                    . "<tr>"
                    . "<td> Tags </td>"
                    . "<td>".listTags()."</td>"
                    . "</tr>"
                    . "</tbody>"
                    . "</table>";

    return $string;
}

However, var_dump($mypost) returns NULL.

From what I understand this is because I call get_post() before the information is ready.

My problem now is that I don't know WHERE and HOW and AFTER WHICH HOOK to call add_action(...) properly. The function responsible for the sidebar is createPageInfo( $type ) where $type can be single, page or null. How to pass that argument $type to add_action properly?

I might have a conceptual misunderstanding but here's roughly how i built my current site:

index.php:

...
get_header();
createMainPanel();
get_sidebar();
get_footer();
...

sidebar.php:

 ...
 if( is_single() || is_page() )
    {
        if( is_single() ){createPageInfo( "single" );}
        elseif ( is_page() ){createPageInfo( "page" );}
    }

    else
    {
        createPageInfo ( null );
    }
 ...

EDIT:

Following Rarst's advice i changed showPostMetaInfo() to the following:

function showPostMetaInfo()
{   
    $mypost = get_post();

    $id = $mypost->ID;
    $title = $mypost->post_title;
    $title2 = get_the_title();
    $date = $mypost->post_date;

    echo $id;
    echo $title;
    echo $title2;
    echo $date;

    $string .= "<p>".$title."</p>";
    return $string;
}

It turns out that $id, $title, $title2, and $date are echoed correctly, but the return of the function is "<p></p>" only...

Now I'm really puzzled...

Answer 1

It's not clear why you mess with the global, under normal circumstances you don't need to.

Out of the box, when viewing a post page, get_post() will return current post object. Regardless of the loop even, since it is set up as part of main query before template is even reached.

The difference between post and page for this purpose is minimal as well.

If get_post() call alone return empty/unexpected results, then the likely issue is that something messes up global context before this point is reached.