Adding class to the comment form in Wordpress

Question

I want to add a class to the form, not form items. I've looked on http://codex.wordpress.org/Function_Reference/comment_form but there is no mention of adding a class to the form.

Answer 1

UPDATE:

Wordpress finally supports the possibility to add classes to the comments form. See Nabil Kadimi's answer to get an example.

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My outdated answer:

Because Wordpress still hasn't supported this option, I've worked out the following workaround:

<?php
    ob_start();
    comment_form();
    echo str_replace('class="comment-form"','class="comment-form your-custom-class"',ob_get_clean());
?>

Now the standard class comment-form will be replaced by itself plus the custom class.

Answer 2

In the documentation for the comment_form() function:

WordPress 4.4.0 Introduced the 'class_form' [...] arguments.

So you would do:

// Output the comment form with a custom class:
comment_form ( array( 'class_form' => 'my_custom_class' ) );

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One a second thought

I prefer using hooks:

/**
 * Callback function for the `comment_form_defaults` filter hook
 *
 * @param Array $defaults Defaults.
 * @return Array          Defaults modified.
 */
function se_8476425_modify_comment_form_defaults( $defaults ) {
    $defaults[ 'class_form' ] = 'class1 class2 class3';
    return $defaults;
};

add_filter( 'comment_form_defaults', 'se_8476425_modify_comment_form_defaults' );

This solution is more generic as you can use it to modify the default function behavior and themes you don't "own".