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Overloading property access

From: Object-Oriented Programming with PHP

Video: Overloading property access

Within PHP, overloading means to dynamically create a property or method that has not been declared, or is not visible in the current scope. When I say create, it's figurative. PHP will act like they exist and can be accessed, even though in reality they don't. These dynamic entities are processed using magic methods. Object properties have two events that can be invoked when accessing a missing or out-of-scope property: when a script _gets a property, and when a script _sets a property. They can be useful to do things like use a method or function to determine the value to be returned, or execute some sort of failsafe behavior when trying to set a property that doesn't exist.

Overloading property access

Within PHP, overloading means to dynamically create a property or method that has not been declared, or is not visible in the current scope. When I say create, it's figurative. PHP will act like they exist and can be accessed, even though in reality they don't. These dynamic entities are processed using magic methods. Object properties have two events that can be invoked when accessing a missing or out-of-scope property: when a script _gets a property, and when a script _sets a property. They can be useful to do things like use a method or function to determine the value to be returned, or execute some sort of failsafe behavior when trying to set a property that doesn't exist.

Both the get and the set magic methods have public visibility, so you don't have to specify scope when declaring them. In the Address class, a magic _get method would be useful for calculating a missing postal code. For example, if the postal code was null, but there is a city in subdivision, I could attempt to look up the value in a table. However, as the postal code is currently public, the magic _get method won't be triggered. Therefore, we'll change the scope of the postal code to protected, in order to be able to use the magic _get.

I'll demo the database lookup later. For the time being, we'll put in a placeholder and focus on magic methods. Open the Address class, then change the scope of postal code to protected. Remember to prepend the property name with an underscore (_) to visually indicate the scope. Then start editing the file after the last property declaration. Add a placeholder function for the database lookup for the postal code. As there is no need to expose this function outside of the address class, protected as well. protected function _ postal_code_guess, return LOOKUP. Add proper PHP documentation, guess the postal code given the subdivision and city name, to do.

Replace with a database LOOKUP. Now that you have a postal code placeholder, you can now leverage the magic _get method. They take only one argument, the name of the property in the form of a string. Place the Magic _get method above the protected method, postal_code_guess. function__get name. Then, documentation Magic _ get. Name as a string, and returns next.

Insert the logic for the special behavior for the postal code property. Postal code lookup if I set, if no postal_code, and set postal code to _postal_code_guess. After the special condition for postal code, you'll expose any protected properties that start with an underscore as read-only. Remember, this will get around scope restrictions.

So, only do this if you are aware of the potential consequences, such as accidentally exposing internal data. In this case, there is no harm in exposing all protected properties as read-only. Attempt to return a protected property by name. You cannot call a property using a concatenated string, so create a variable that starts with the underscore followed by the name. $protected_property_name=_ followed by $name. if property_exists in this with the property name, return this protected_property_name.

If the property does not exist, trigger a PHP error and return null. trigger_error Undefined property via _ get, followed by the name. return NULL. If you were to test the script now, you would get a failure when you attempted to set the postal code, as it is currently protected. Therefore, you will also need to define a magic _set to handle that circumstance.

Beneath the magic _get, define a magic _set method. The magic _set method has two arguments. The first is the name of the property as a string, and the second is a mixed value to be set. function__set name, value. Add the PHP documentation, string for name, mixed for value. Add the check for the postal code property, and allow anything to manipulate it. In this example, stick with a simple if statement, as there is only one logical check.

If there were multiple property names that you are checking for, a switch statement may be more readable. Allow anything to set the postal code. If postal code is the name, this name = value. return. If the property is not postal code, trigger contextually appropriate error. Unable to access property; trigger error. trigger_error ('Undefined or unallowed property via _ _set (); followed by the name); Save the address class, then open the demo file. At the end, remove the unintentional failed display of address ID. Then, let's test the new magic _get method by unsetting the postal code, and displaying the result.

echo

Testing magic _get and _ set; unset ($address postal_code); echo $address_display();. Save the demo file, then refresh your browser. The first instance of the postal code shows up as expected, and the second instance with no postal code now shows the word LOOKUP. In the next video, you can set the time created, leveraging a construction magic method.

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This video is part of

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Object-Oriented Programming with PHP

37 video lessons · 17931 viewers

Jon Peck
Author

 
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  1. 4m 10s
    1. Welcome
      39s
    2. Exercise files
      2m 16s
    3. What you should know
      1m 15s
  2. 7m 47s
    1. What is an object?
      2m 6s
    2. What is a class?
      2m 7s
    3. Why should you use object-oriented programming?
      2m 14s
    4. The history of object-oriented PHP
      1m 20s
  3. 16m 40s
    1. Defining a class
      2m 58s
    2. Defining class properties
      3m 26s
    3. Creating a method and exploring object context with $this
      2m 50s
    4. Instantiating an object and accessing its contents
      3m 19s
    5. Specifying the visibility scope
      4m 7s
  4. 15m 51s
    1. What is a magic method, and do I need one?
      2m 23s
    2. Overloading property access
      6m 37s
    3. Customizing object construction
      4m 34s
    4. Standardizing object rendering as a string
      2m 17s
  5. 20m 54s
    1. Using the static keyword
      3m 36s
    2. Leveraging scope resolution operators
      1m 10s
    3. Setting constant values
      2m 47s
    4. Implementing static methods
      5m 43s
    5. Creating a database class
      7m 38s
  6. 26m 19s
    1. Extending your class
      6m 8s
    2. Abstracting classes
      5m 57s
    3. Sharing interfaces using polymorphism
      4m 39s
    4. Overriding methods, properties, and constants
      3m 25s
    5. Cloning and comparing objects
      2m 51s
    6. Referencing objects
      3m 19s
  7. 14m 52s
    1. Leveraging standard class objects
      2m 42s
    2. Retrieving objects from the database
      5m 41s
    3. Error handling with exceptions
      2m 6s
    4. Customizing PHP exceptions
      4m 23s
  8. 8m 2s
    1. Identifying the singleton pattern
      1m 42s
    2. Using the factory method pattern
      1m 51s
    3. Implementing a strategy pattern
      4m 29s
  9. 2m 57s
    1. Looking forward to namespaces
      47s
    2. Next steps
      1m 6s
    3. Goodbye
      1m 4s

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