What is the difference between interface and struct?

Question

Accepted Answer

**Interface** — behavior contract via method signatures. **Struct** — data container with named fields.

Interfaces are implicitly satisfied. Any type with the right methods implements the interface.

```go
type Notifier interface {
    Notify(msg string) error
}

type EmailNotifier struct{ Addr string }
func (e EmailNotifier) Notify(msg string) error {
    // send email
    return nil
}
```

Structs hold data and implement interfaces.

```go
type SlackNotifier struct{ Channel string }
func (s SlackNotifier) Notify(msg string) error {
    // send slack message
    return nil
}
```

**When to use:**
- **Interface** — abstraction, DI.
- **Struct** — concrete data.

**Common mistakes:**
- Large interfaces.
- Wrong definition location.
- Nil interface trap.
- Overusing `any`.

Go: What is the difference between interface and struct?