7 Proxy (Structural) Design Pattern

The Proxy Design Pattern is a structural design pattern that provides a surrogate or placeholder for another object to control access to it. A proxy class represents and controls access to the real object while adding additional behavior if necessary.

It a structural design pattern that provides a surrogate or placeholder object which controls access to another object.

It is commonly used when direct access to the object is expensive, unsafe, or needs control.

In simple words:

A proxy acts like a middleman between the client and the real object.

Real-life Analogy

1 Bank ATM

• You don't directly talk to the bank's main server.
• Instead, the ATM acts as a proxy.
• It checks your credentialys (PIN, card validity), and only if allowed, forwards your request to the bank system.

ATM = Proxy

Bank system = Real object

2 Security Guard at a Building

• You want to enter an office.
• The security guard (proxy) checks your ID card or pass.
• If valid, you can enter and interact with the office.

Guard = Proxy

Office building = Real object

Problem It Solves

Sometimes, direct access to an object is not desirable or practical.

Why? Because the object might be:

1. Expensive the create (e.g., loading large files or connecting to a remote server).
2. Sensitive (e.g., needs security checks before use).
3. Remote (e.g., object lives on a different machine or server).
4. Needs extra features (e.g., caching, logging, lazy initialization).

Without Proxy (Problem):

Imagine you have a large image file:

RealImage img("big_photo.png");
img.display();

• Even if you never display it, the contructor loads it from disk.
• Wastes time & memory if you don't always need it.
• If access should be restricted (say only admins), there's no way to prevent unauthorized users from calling display().

With Proxy (Solution):

You can create a ProxyImage that:

• Lazily loads the RealImage only when display() is actually called.
• Check access control (only authorized users can call display()).
• Caches the loaded image, so future calls don't reload it.

The client still just uses img.display(), but now access is controlled and optimized.

Types of Proxy

1. Virtual Proxy: Controls access to an object that is resource-intensive to create (e.g., loading large images).
2. Protection Proxy: Controls access based on permissions (e.g., user roles).
3. Remote Proxy: Represents an object in another address space (like RPC).

1️⃣ Virtual Proxy Pattern

The Virtual Proxy controls access to a resource-intensive object by creating it only when needs. This helps improve performance and save memory.

Real-life Analogy

Think of a photo gallery app:

• When you open the gallery, it shows thumbnils first (fast, lightweight).
• The full-size image (big file) is only loaded when you actually click on it.

Thumbnails act like a virtual proxy for the real image.

Without Virtual Proxy

Here, the