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Proxy Design Pattern


Difficulty:
1/5


What is a proxy? Dictionary says “The authority to represent someone else”. hmmm… Ponder on that a bit and let's see how the “Name and Conquer” of Proxy applies to software design patterns.

The definition of the Proxy pattern goes like this:

Provides a surrogate or placeholder for another object in order to control access to it.

Let's get the obvious stuff out of the way:

  1. A proxy certainly provides another level of indirection to another “target” object that we basically want to access

That's it. I have nothing else that seems obvious from the definition alone. But after further investigation and coding I realize that a Proxy object is indeed useful when we want to lazy-instantiate an object, or when we want to hide the implementation details of the object. For whatever reason, but this often comes up in software, we represent a back-end highly complicated object with a more user/programmer friendly front-end one. Same story.

There are 3 main scenarios where you would want to create a Proxy object:

  1. Remote proxy: a Proxy object you get when the target object is in a remote server
  2. Virtual proxy: basically a way to cache the object
  3. Protection proxy: hide details of the target object from the client. In this case the clients are only aware of the public interface of the target class; no implementation details are published. Similar to Pimpl.

It is time to emphasize that a Proxy is not just a wrapper/adapter/facade class, as many rookie programmers seem to think at first. So supplementing our original assumption it is not just another wrapper (level of indirection); instead the Proxy object adds additional behavior to control access to the target object. The behavior added is dependent on the usecase.

Design

  • have your target object/class created good and proper
  • the Proxy object may either subclass the target object or contain an instance of it as a member, depending on how you want to structure it
  • add behavior to the Proxy object and inside it call upon the target object's functions and use its state according to your demands

Additional nomenclature surrounding this pattern: SDK (an SDK to the outside developers), Monkey Patching or Object Augmentation pattern.

A related pattern “The Decorator” is also known as a “Smart Proxy”. It is used when you want to add functionality to an object, not by extending that object's type but at runtime (by supplying classes to it).

Proxy is a very frequently used Design Pattern, thus I recommend you get to know it like the back of your hand. It's pretty easy as well so you have no excuse! Study the commented example (code in C++) - it's pretty easy, you'll agree with me; I'm sure. As a last note, don't be hesitant to use it. You can find a multitude of usecases for it.

I used Windows 8.1 x86_64, Visual Studio 2017, Modern C++17 to build the project. It should work on other platforms as well.

Github

Github repository link.

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