What is the difference between Go's struct embedding and struct composition?

Question

Jayant Kumar · Accepted Answer

Table of Contents
Introduction
In Go, struct embedding and struct composition are powerful techniques used for organizing and reusing code. Both methods enable the creation of complex data structures but differ in their approach and use cases. This guide explores the differences between struct embedding and struct composition, providing insights into how each technique is utilized in Go programming.
Struct Embedding vs. Struct Composition
Struct Embedding

Definition:

Struct embedding in Go allows one struct to include another struct as an embedded field. This technique enables code reuse and promotes composition by leveraging the embedded struct's fields and methods.

Characteristics:

Direct Access: Fields and methods of the embedded struct are directly accessible from the embedding struct.
Inheritance-Like Behavior: Provides a form of inheritance by embedding one struct within another, allowing access to its members without explicit delegation.
Automatic Promotion: Fields and methods of the embedded struct are promoted to the outer struct.

Examples:
package main

import "fmt"

// Base struct
type Address struct {
    Street string
    City   string
}

// Struct embedding
type Person struct {
    Address  // Embedded field
    Name     string
    Age      int
}

func main() {
    // Initialize using embedded struct
    p := Person{
        Address: Address{
            Street: "123 Main St",
            City:   "Anytown",
        },
        Name: "Alice",
        Age:  30,
    }

// Access embedded struct fields directly
    fmt.Println(p.Street) // Output: 123 Main St
    fmt.Println(p.City)   // Output: Anytown
    fmt.Println(p.Name)   // Output: Alice
}

Explanation: Address is embedded in Person. Fields of Address (like Street and City) are accessed directly on Person without needing to reference Address explicitly.

Struct Composition

Definition:

Struct composition involves including one or more structs as fields within another struct. Unlike embedding, struct composition does not automatically promote fields and methods of the embedded structs; they are accessed through their fields.

Characteristics:

Explicit Access: Fields and methods of the composed struct must be accessed through the field name of the containing struct.
Flexible Relationships: Provides flexibility to compose structs with different relationships and access patterns.
No Promotion: Unlike embedding, struct composition does not provide automatic promotion of fields or methods.

Examples:
package main

import "fmt"

// Base struct
type Address struct {
    Street string
    City   string
}

// Struct composition
type Person struct {
    Addr Address  // Composed field
    Name string
    Age  int
}

func main() {
    // Initialize using composed struct
    p := Person{
        Addr: Address{
            Street: "123 Main St",
            City:   "Anytown",
        },
        Name: "Alice",
        Age:  30,
    }

// Access composed struct fields explicitly
    fmt.Println(p.Addr.Street) // Output: 123 Main St
    fmt.Println(p.Addr.City)   // Output: Anytown
    fmt.Println(p.Name)        // Output: Alice
}

Explanation: Address is composed within Person as a field named Addr. To access fields of Address, you need to use p.Addr.Street and p.Addr.City.

Key Differences

Access Method:

Struct Embedding: Allows direct access to embedded struct fields and methods.
Struct Composition: Requires access through the field name of the composed struct.

Field Promotion:

Struct Embedding: Automatically promotes fields and methods of the embedded struct to the outer struct.
Struct Composition: No automatic promotion; fields are accessed through the composed struct field.

Use Cases:

Struct Embedding: Ideal for situations where you want to leverage or extend existing types without needing explicit field references.
Struct Composition: Useful for creating more flexible data structures where you want to control the composition and access pattern explicitly.

Conclusion
Both struct embedding and struct composition are valuable techniques in Go for organizing and reusing code. Struct embedding provides a way to extend and reuse types with automatic field and method promotion, while struct composition offers greater flexibility in structuring data without automatic promotion. Understanding the differences between these techniques helps in designing clean, maintainable, and efficient Go code.

What is the difference between Go's struct embedding and struct composition?

Table of Contents

Introduction

Struct Embedding vs. Struct Composition

Struct Embedding

Struct Composition

Key Differences

Conclusion

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