Go: What is the difference between interface and struct?

Interface — abstracts behavior by listing method signatures without implementation. Struct — defines concrete data with named, typed fields.

Go interfaces are satisfied implicitly: if your type has all the methods, it satisfies the interface — no declaration needed. This enables loose coupling. For example, io.Reader is satisfied by files, network connections, HTTP response bodies, and more.

1type Reader interface {
2    Read(p []byte) (n int, err error)
3}
4
5// os.File, *strings.Reader, *bytes.Buffer all satisfy Reader
6func process(r Reader) error {
7    buf := make([]byte, 1024)
8    _, err := r.Read(buf)
9    return err
10}

Structs hold real data and can have methods. You compose structs from other structs (embedding) for reuse. Structs are the concrete types that implement interfaces.

1type Config struct {
2    Host string
3    Port int
4}
5
6func (c Config) Address() string {
7    return fmt.Sprintf("%s:%d", c.Host, c.Port)
8}
9
10type Server struct {
11    Config  // embed struct for composition
12    Name string
13}

When to use:

• Interface — for dependency injection, testing (mocking), and defining contracts.
• Struct — for representing domain entities, config, and data models.

Common mistakes:

• Over-engineering with interfaces when a concrete type suffices.
• Checking err == nil after an interface assignment — interface nil checks are tricky if the interface holds a typed nil pointer.
• Exposing structs with exported fields when methods would encapsulate better.