Go: What is the difference between interface and struct?

Interface — defines behavior through method signatures, enabling polymorphism. Struct — defines data through named fields, providing concrete implementations.

In Go, interfaces are satisfied implicitly — any type that has the right methods automatically satisfies the interface. This means you can add new implementations without touching the interface definition.

1type Logger interface {
2    Log(message string)
3}
4
5type ConsoleLogger struct{ Prefix string }
6func (l ConsoleLogger) Log(msg string) {
7    fmt.Printf("[%s] %s\n", l.Prefix, msg)
8}
9
10func process(log Logger) {
11    log.Log("starting...")  // depends on interface, not concrete type
12}

Structs are the building blocks — they hold data and have methods. You can embed structs for composition (similar to inheritance). Structs are concrete, typed, and compile-time checked.

1type Base struct{ ID int }
2type User struct {
3    Base           // embed for ID field
4    Name string
5}

When to use:

• Interface — abstraction layers, dependency injection, test doubles.
• Struct — all concrete data types: models, configs, messages.

Common mistakes:

• Defining interfaces where the implementation lives — define where they're consumed.
• Overusing any (empty interface) — loses type safety.
• Not realizing a nil pointer inside an interface is not nil — common source of subtle bugs.
• Creating huge interfaces — keep them small and composable.