WooCommerce | Create a shortcode for WooCommerce product links, which adds a class based on the if the product is out of stock or in stock.

woocommerce-product-link-shortcode-stock.php

<?php 

/* 
Create a shortcode that will wrap content, and will return a product link
based on the product ID that is added to the shortcode
Usage: [stock_product_link id="123"]The text, image, etc. you want to link[/stock_product_link]
*/
if( !function_exists('custom_shortcode_stock_product_link') ) {
	function custom_shortcode_stock_product_link( $atts, $content = null ) {
		// Shortcode Attributes
		$atts = shortcode_atts(
			array(
				'id' => '',
			),
			$atts, 'stock_product_link'
		);
		$product_id = $atts['id'];
		// Get an instance of the WC_Product object
		$product = wc_get_product( $product_id );
		// Display custom class, based on if product is in or out of stock
		$class = $product->is_in_stock() ? ' class="piece-in"' : ' class="piece-out"';
		// The product link
		$product_link = $product->get_permalink();
	  	// The product add-to-cart link
		$product_cart_link = $product->add_to_cart_url();
		// The output
		return '<a xlink:href="'.$product_cart_link.'"'.$class.'>'.$content.'</a>';
	}
	add_shortcode( 'stock_product_link', 'custom_shortcode_stock_product_link' );
}

/* 
Create a shortcode that will wrap content, and will return an element
with a specific class, based on if the product ID that is added to the shortcode is IN STOCK
Usage: [stock_product_piece id="123"]The text, image, etc. you want to link[/stock_product_piece]
*/
if( !function_exists('custom_shortcode_stock_product_piece') ) {
	function custom_shortcode_stock_product_piece( $atts, $content = null ) {
		// Shortcode Attributes
		$atts = shortcode_atts(
			array(
				'id' => '',
			),
			$atts, 'stock_product_piece'
		);
		$product_id = $atts['id'];
		// Get an instance of the WC_Product object
		$product = wc_get_product( $product_id );
		// Display custom class, based on if product is in or out of stock
		if ( $product->is_in_stock() ) {
			return $content;
		}
	  	else {
		 	return;
		}
	}
	add_shortcode( 'stock_product_piece', 'custom_shortcode_stock_product_piece' );
}