CMB2 front end forms - shortcode takes id of any metabox registered and outputs the appropriate frontend form.

front-end-forms.php

<?php

// @todo post editing

/**
 * Gets the cmb instance
 *
 * @param string $metabox_id The id of the metabox as it is registered with `new_cmb2_box` e.g. `architect`
 * @return CMB2 object
 */
function th_get_frontend_form($metabox_id) {
    $object_id  = 'fake-oject-id';
    return cmb2_get_metabox($metabox_id, $object_id);
}

/**
 * This function will output a front-end CMB2 form on any page. The shortcode should be entered as below:
 *
 * [frontend-form id="architect"]
 *
 * where `id` is the id of the metabox is it is registered with the `new_cmb2_box` function.
 *
 * @param  array  $atts Array of shortcode attributes
 * @return string       Form html
 */
function th_frontend_form_shortcode( $atts = array() ) {

    // Check we have the id attribute and it's a valid
    if (!$atts['id'] && $cmb = th_get_frontend_form($atts['id'])) {
        return false;
    }

    // Get CMB2 metabox object
    $cmb = th_get_frontend_form($atts['id']);

    // Get $cmb object_types
    $post_types = $cmb->prop( 'object_types' );

    // Current user
    $user_id = get_current_user_id();

    // Parse attributes
    $atts = shortcode_atts( array(
        'post_author' => $user_id ? $user_id : 1, // Current user, or admin
        'post_status' => 'pending',
        'metabox_id'  => $atts['id'],
        'post_type'   => reset( $post_types ), // Only use first object_type in array
    ), $atts, 'frontend-form' );

    /*
     * Let's add these attributes as hidden fields to our cmb form
     * so that they will be passed through to our form submission
     */
    foreach ( $atts as $key => $value ) {
        $cmb->add_hidden_field( array(
            'field_args'  => array(
                'id'    => "atts[$key]",
                'type'  => 'hidden',
                'default' => $value,
            ),
        ) );
    }

    // Initiate our output variable
    $output = '';

    // Get any submission errors
    if ( ( $error = $cmb->prop( 'submission_error' ) ) && is_wp_error( $error ) ) {
        // If there was an error with the submission, add it to our ouput.
        $output .= '<h3>' . sprintf( __( 'There was an error in the submission: %s', 'wds-post-submit' ), '<strong>'. $error->get_error_message() .'</strong>' ) . '</h3>';
    }

    // If the post was submitted successfully, notify the user.
    if ( isset( $_GET['post_submitted'] ) && ( $post = get_post( absint( $_GET['post_submitted'] ) ) ) ) {

        // Get submitter's name
        $name = get_post_meta( $post->ID, 'submitted_author_name', 1 );
        $name = $name ? ' '. $name : '';

        // Add notice of submission to our output
        $output .= '<h3>' . sprintf( __( 'Thank you%s, your new post has been submitted and is pending review by a site administrator.', 'wds-post-submit' ), esc_html( $name ) ) . '</h3>';
    }

    // Get our form
    $output .= cmb2_get_metabox_form( $cmb, 'fake-oject-id', array( 'save_button' => __( 'Submit', 'wds-post-submit' ) ) );

    return $output;
}
add_shortcode('frontend-form', 'th_frontend_form_shortcode');

/**
 * Handles form submission on save. Redirects if save is successful, otherwise sets an error message as a cmb property
 *
 * @return void
 */
function th_handle_frontend_submission() {
//    var_dump($_POST); die;

    // If no form submission, bail
    if ( empty( $_POST ) || ! isset( $_POST['submit-cmb'], $_POST['object_id'] ) ) {
        return false;
    }
    // Get CMB2 metabox object
    $cmb = th_get_frontend_form($_POST['atts']['metabox_id']);
    $post_data = array();

    // Get our shortcode attributes and set them as our initial post_data args
    if ( isset( $_POST['atts'] ) ) {
        foreach ( (array) $_POST['atts'] as $key => $value ) {
            $post_data[ $key ] = sanitize_text_field( $value );
        }
        unset( $_POST['atts'] );
    }

    // Check security nonce
    if ( ! isset( $_POST[ $cmb->nonce() ] ) || ! wp_verify_nonce( $_POST[ $cmb->nonce() ], $cmb->nonce() ) ) {
        return $cmb->prop( 'submission_error', new WP_Error( 'security_fail', __( 'Security check failed.' ) ) );
    }

    // Check title submitted
    if ( empty( $_POST['title'] ) ) {
        return $cmb->prop( 'submission_error', new WP_Error( 'post_data_missing', __( 'New post requires a title.' ) ) );
    }

    // And that the title is not the default title
    if ( $cmb->get_field( 'title' )->default() == $_POST['title'] ) {
        return $cmb->prop( 'submission_error', new WP_Error( 'post_data_missing', __( 'Please enter a new title.' ) ) );
    }

    /**
     * Fetch sanitized values
     */
    $sanitized_values = $cmb->get_sanitized_values( $_POST );

    // Set our post data arguments
    $post_data['post_title']   = $sanitized_values['title'];
    unset( $sanitized_values['title'] );

    $post_data['post_content'] = $sanitized_values['submitted_post_content'];
    unset( $sanitized_values['submitted_post_content'] );

    // Create the new post
    $new_submission_id = wp_insert_post( $post_data, true );

    // If we hit a snag, update the user
    if ( is_wp_error( $new_submission_id ) ) {
        return $cmb->prop( 'submission_error', $new_submission_id );
    }

    /**
     * Other than post_type and post_status, we want
     * our uploaded attachment post to have the same post-data
     */
    unset( $post_data['post_type'] );
    unset( $post_data['post_status'] );

    // Try to upload the featured image
    $img_id = th_frontend_form_photo_upload( $new_submission_id, $post_data );

    // If our photo upload was successful, set the featured image
    if ( $img_id && ! is_wp_error( $img_id ) ) {
        set_post_thumbnail( $new_submission_id, $img_id );
    }

    // Loop through remaining (sanitized) data, and save to post-meta
    foreach ( $sanitized_values as $key => $value ) {
        if ( is_array( $value ) ) {
            $value = array_filter( $value );
            if( ! empty( $value ) ) {
                update_post_meta( $new_submission_id, $key, $value );
            }
        } else {
            update_post_meta( $new_submission_id, $key, $value );
        }
    }

    /*
     * Redirect back to the form page with a query variable with the new post ID.
     * This will help double-submissions with browser refreshes
     */

    wp_redirect( esc_url_raw( add_query_arg( 'post_submitted', $new_submission_id ) ) );
    exit;
}
add_action( 'cmb2_after_init', 'th_handle_frontend_submission' );

/**
 * Handles uploading a file to a WordPress post
 *
 * @param  int   $post_id              Post ID to upload the photo to
 * @param  array $attachment_post_data Attachement post-data array
 */
function th_frontend_form_photo_upload( $post_id, $attachment_post_data = array() ) {

    // Make sure the right files were submitted
    if (
        empty( $_FILES )
        || ! isset( $_FILES['submitted_post_thumbnail'] )
        || isset( $_FILES['submitted_post_thumbnail']['error'] ) && 0 !== $_FILES['submitted_post_thumbnail']['error']
    ) {
        return;
    }

    // Filter out empty array values
    $files = array_filter( $_FILES['submitted_post_thumbnail'] );

    // Make sure files were submitted at all
    if ( empty( $files ) ) {
        return;
    }

    // Make sure to include the WordPress media uploader API if it's not (front-end)
    if ( ! function_exists( 'media_handle_upload' ) ) {
        require_once( ABSPATH . 'wp-admin/includes/image.php' );
        require_once( ABSPATH . 'wp-admin/includes/file.php' );
        require_once( ABSPATH . 'wp-admin/includes/media.php' );
    }

    // Upload the file and send back the attachment post ID
    return media_handle_upload( 'submitted_post_thumbnail', $post_id, $attachment_post_data );
}