Go: What is the difference between interface and struct?

Interface — defines behavior through method signatures, enabling polymorphism. Struct — defines data through named, typed 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 Reader interface {
2    Read(p []byte) (int, error)
3}
4
5type File struct{ Name string }
6func (f *File) Read(p []byte) (int, error) {
7    // read from file
8    return 0, nil
9}
10
11func process(r Reader) error {
12    buf := make([]byte, 1024)
13    _, err := r.Read(buf)
14    return err
15}

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 Config struct {
2    Host string
3    Port int
4}
5
6type Server struct {
7    Config  // embed struct for composition
8    Name string
9}

When to use:

• Interface — abstraction layers, dependency injection, testing (mocking), decoupling packages.
• Struct — all concrete data types: models, configs, messages.

Common mistakes:

• Defining interfaces in the same package as the implementation — prefer defining where consumed.
• Overusing any (empty interface) — loses type safety.
• Nil interface trap: a nil pointer inside an interface is NOT nil — common source of subtle bugs.
• Making interfaces too large — keep them small and composable (1-3 methods).