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Overriding existing functions

From: WordPress 3: Building Child Themes

Video: Overriding existing functions

Depending on how well a parent theme is built, you can use a series of different techniques to either override or overrule functions in that parent theme. By overriding or overruling the functions, you're telling WordPress to ignore the function that's in the parent theme or augment the function that's in the parent theme to produce the result you want. How to override a function depends on how well the theme is written, and also how that function is included in the theme. Most well-written themes wrap functions in conditional statements so that they can easily be overwritten by child themes.

Overriding existing functions

Depending on how well a parent theme is built, you can use a series of different techniques to either override or overrule functions in that parent theme. By overriding or overruling the functions, you're telling WordPress to ignore the function that's in the parent theme or augment the function that's in the parent theme to produce the result you want. How to override a function depends on how well the theme is written, and also how that function is included in the theme. Most well-written themes wrap functions in conditional statements so that they can easily be overwritten by child themes.

When they are not wrapped in these statements you're usually given either a filter or an action hook that you can hook into or change the actions you can interact with it. Let's a take a look at some of these things in the twentytwelve theme, so you can see how you can interact with it. If you go down to line 28, you'll see an example of how a function is included so that you can override it with your child theme. The function here sets a default content width for the site, meaning if you add an image and it's larger than the value set here--625 pixels--it'll automatically be scrunched down to fit within 625 pixels.

What happens here is WordPress says, if this value is not already set, then use this value. That's because of how functions.php interacts with the functions.php file in your child theme. Unlike with a style sheet where the parent theme style sheet is called first and then the child theme style sheet is called, when it comes to functions.php, the child theme functions.php file is called first and then the parent theme functions.php is called afterwards. Meaning, if you have set content_width in your child theme, this condition says it's already set; therefore, we're not going to set it in the parent theme.

On line 228 you see a different type of conditional statement. Here it says if function exists, or rather if not function exists, meaning if the child theme does not have the function twentytwelve_content_nav, then use this function instead. That also means if you want to create a different content nav function in your child theme, you simply create it inside your child theme and then it will automatically override the function in the parent theme. On line 177 you see a different type of interactive feature.

This is a filter. It says add_filter, wp_title. A filter is different from a function, in that when you have a filter you can pass information to that filter and change the behavior of the function. So this filter interacts with the function above it, this one, twentytwelve_wp_title. It's up here, this whole function. And what you can do in your child theme is add information with a filter that then gets passed into this function and you can change to function through that filter. So it's a more advanced option when you want to change maybe a word or a link or something like that in that function without having to rebuild the function in the process.

Overriding or removing functions allows you to take control of the functionality of your child theme and do things not originally intended by the parent theme author. It can be very useful, but be aware that when you start messing with functions weird things tend to happen, so move slowly. Fortunately, by doing all of this in a child theme you're never going to do anything that will actually break the site, and everything is always reversible.

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This video is part of

Image for WordPress 3: Building Child Themes
WordPress 3: Building Child Themes

45 video lessons · 35035 viewers

Morten Rand-Hendriksen
Author

 
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  1. 6m 59s
    1. Welcome
      1m 6s
    2. Using the exercise files
      1m 34s
    3. What you need to know before watching this course
      1m 29s
    4. Getting the right tools for theme creation
      2m 50s
  2. 17m 2s
    1. What is a WordPress child theme and when should you use it?
      2m 38s
    2. Picking a parent theme
      3m 55s
    3. Making sure you have the Twenty Twelve parent theme
      1m 50s
    4. Creating and activating a basic child theme
      4m 20s
    5. Importing parent theme styles
      4m 19s
  3. 16m 2s
    1. Using the developer tools
      3m 53s
    2. Modifying existing styles
      4m 24s
    3. Adding space between paragraphs
      4m 7s
    4. Changing font family styles
      3m 38s
  4. 31m 47s
    1. Understanding the WordPress template hierarchy
      3m 12s
    2. Modifying existing templates
      2m 33s
    3. Moving the header image
      4m 29s
    4. Adding Related Posts feature to posts
      6m 26s
    5. Creating custom page templates
      5m 43s
    6. Using conditional statements for customized effects
      5m 41s
    7. Creating custom header, footer, and sidebar templates
      3m 43s
  5. 17m 5s
    1. Understanding the different index pages and what they do
      4m 6s
    2. Adding author, date, and time information to the index loop
      7m 15s
    3. Changing the appearance of category index pages
      5m 44s
  6. 43m 5s
    1. Introducing functions.php
      3m 24s
    2. Overriding existing functions
      3m 23s
    3. Adding pagination to index pages
      5m 49s
    4. Adding to existing functions
      3m 21s
    5. Adding a new footer menu to Twenty Twelve
      6m 24s
    6. Adding a new widgetized area to pages
      4m 9s
    7. Adding static content to the sidebar
      7m 44s
    8. Replacing existing functions
      2m 36s
    9. Adding a Google font through a function
      6m 15s
  7. 10m 24s
    1. Adding new featured image sizes
      5m 41s
    2. Adding featured images to posts and pages
      4m 43s
  8. 31m 1s
    1. Adding a welcome message to the front page
      1m 22s
    2. Displaying page content in an index page
      7m 42s
    3. Hooking in a featured image
      4m 34s
    4. Making the welcome message responsive
      6m 27s
    5. Restricting content to the first page of the blog
      4m 22s
    6. Adding a jQuery function to show or hide the welcome message
      6m 34s
  9. 10m 23s
    1. Adding a custom favicon
      3m 58s
    2. Adding a custom screenshot
      2m 29s
    3. Adding footer information
      3m 56s
  10. 7m 14s
    1. What to do when a child theme crashes your website
      4m 38s
    2. Updating parent and child themes
      2m 36s

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