What is the difference between interface and struct?

Question

Accepted Answer

**Interface** — contract of method signatures defining behavior. **Struct** — named, typed fields defining data.

Interfaces enable polymorphism through implicit satisfaction. Any type with the right methods satisfies the interface — no declaration needed.

```go
type Formatter interface {
    Format() string
}

type JSONFormatter struct{ Data interface{} }
func (f JSONFormatter) Format() string {
    b, _ := json.Marshal(f.Data)
    return string(b)
}

type TextFormatter struct{ Text string }
func (f TextFormatter) Format() string {
    return f.Text
}
```

Structs hold data and implement interfaces. They compose via embedding and support JSON/DB tags.

```go
type Base struct{ ID int }
type User struct {
    Base
    Name string
}
```

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

**Common mistakes:**
- Large interfaces.
- Defining where implemented instead of consumed.
- Nil interface trap.
- Overusing `any`.

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