Behavioral

Memento

Captures and restores an object's internal state.

Undo systemsCheckpointsState snapshots

Understanding Memento

The Memento pattern captures an object\'s internal state as a snapshot that can be restored later, without exposing the object's implementation details. This is the pattern behind undo systems, checkpoints, and state history. In Go, the originator creates a memento struct containing a copy of its state, and a caretaker stores these mementos in a stack or list. When the user requests an undo, the caretaker retrieves the previous memento and passes it back to the originator to restore its state.

Key Concepts

  • •Originator — the object whose state needs to be saved and restored — creates and consumes mementos
  • •Memento — an opaque snapshot of the originator\'s state — typically an unexported struct in Go
  • •Caretaker — stores mementos but never examines or modifies them — just holds the history stack
  • •Encapsulation preserved — memento captures state without exposing the originator\'s internal structure

When to Use

✅ Use when
  • • You need undo/redo functionality for complex objects
  • • You want to create checkpoints or savepoints for recovery
  • • You need to capture and restore state without violating encapsulation
  • • You\'re building a version history or state timeline feature
⚠️ Avoid when
  • • The object\'s state is large and frequent snapshots would consume too much memory
  • • A simpler approach like copying a struct is sufficient in Go
  • • You only need to track one level of undo — saving the previous value is simpler

Structure

Originator (creates/restores)
↓ Save() / Restore()
Memento (state snapshot)
↓ stored by
Caretaker (history stack)

How It Works

Originator
Editor
state: "Hello"
📦 Memento
"Hello"
Caretaker
History
M
Object has state
1

Object State

Object has internal state that can change.

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Basic Implementation

Text editor with undo history:

main.go
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Real-World Example: Game Save System

Game with save slots:

main.go
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