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.

Interfaces are implicitly satisfied in Go — if a type has all the required methods, it implements the interface. No declaration needed. This enables loose coupling and easy testing.

1type Speaker interface {
2    Speak() string
3}
4
5type Dog struct{ Name string }
6func (d Dog) Speak() string {
7    return "Woof"
8}
9
10type Cat struct{ Name string }
11func (c Cat) Speak() string {
12    return "Meow"
13}
14
15func makeSound(s Speaker) {
16    fmt.Println(s.Speak())
17}

Structs are concrete types with named fields. You attach methods to them and can embed other structs for composition.

1type Config struct {
2    Debug bool
3    Port  int
4}

When to use:

• Interface — abstraction, dependency injection, testing (mocking).
• Struct — concrete data, domain models, configs.

Common mistakes:

• Defining interfaces where implementations live — define where consumed.
• Overusing any (empty interface) — loses type safety.
• Nil interface trap: an interface holding a typed nil pointer is NOT nil.
• Making interfaces too large — keep to 1-3 methods.