What is the difference between a struct and an interface in Go?

Q: What is the difference between a struct and an interface in Go?

Learn the answer to "What is the difference between a struct and an interface in Go?" with detailed explanations, code examples, and best practices on DeployU.

The Scenario

You are a backend engineer at a fintech company. You are designing a new service that needs to model a variety of different financial instruments, such as stocks, bonds, and options.

You are not sure whether to use a struct or an interface to model the financial instruments.

The Challenge

Explain the difference between a struct and an interface in Go. What are the pros and cons of each approach, and which one would you choose for this use case?

Wrong Approach

A junior engineer might try to use a struct for everything. They might not be aware of the benefits of using interfaces for abstraction and polymorphism.

Addresses symptoms, not root cause

Right Approach

A senior engineer would know that a combination of structs and interfaces is the best approach for this use case. They would be able to explain the difference between the two and would have a clear plan for how to use them to model the financial instruments.

Step 1: Understand the Key Differences

Feature	Struct	Interface
Purpose	To define a collection of fields.	To define a collection of method signatures.
Implementation	Concrete, you create instances of a struct.	Abstract, a type implements an interface by implementing all its methods.
Use Cases	When you need to represent a collection of data.	When you need to define a common set of behaviors for different types.

Step 2: Choose the Right Tool for the Job

For our use case, we should use a combination of structs and interfaces.

We will use structs to represent the concrete financial instruments, such as Stock, Bond, and Option.
We will use an interface to define a common set of behaviors for all financial instruments, such as GetValue().

Step 3: Code Examples

Here’s how we can use a combination of structs and interfaces to model the financial instruments:

package main

import "fmt"

type FinancialInstrument interface {
    GetValue() float64
}

type Stock struct {
    Ticker string
    Price  float64
}

func (s Stock) GetValue() float64 {
    return s.Price
}

type Bond struct {
    Name  string
    Value float64
}

func (b Bond) GetValue() float64 {
    return b.Value
}

func printValue(fi FinancialInstrument) {
    fmt.Println(fi.GetValue())
}

func main() {
    s := Stock{Ticker: "AAPL", Price: 150.0}
    b := Bond{Name: "US Treasury", Value: 1000.0}

    printValue(s) // 150
    printValue(b) // 1000
}

By using a combination of structs and interfaces, we can write code that is flexible, extensible, and easy to maintain.

Systematic, production-ready debugging

Practice Question

You want to write a function that can accept any type that has a `String()` method. Which of the following would you use?

Questions