Go: What is the difference between interface and struct?

Interface — defines a set of method signatures (behavior contract). Struct — defines named, typed fields (data layout).

Interfaces enable polymorphism through implicit satisfaction. Any type with the required methods automatically implements the interface — no keyword needed. This makes it easy to swap implementations and write tests.

1type Transport interface {
2    Send(data []byte) error
3    Receive() ([]byte, error)
4}
5
6type HTTPTransport struct{ BaseURL string }
7func (h *HTTPTransport) Send(data []byte) error {
8    _, err := http.Post(h.BaseURL, "application/json", bytes.NewReader(data))
9    return err
10}
11func (h *HTTPTransport) Receive() ([]byte, error) {
12    resp, err := http.Get(h.BaseURL)
13    if err != nil { return nil, err }
14    return io.ReadAll(resp.Body)
15}

Structs hold the data and implement interfaces. You can embed structs for composition. Structs are concrete — the compiler knows their exact fields and types.

1type Config struct {
2    Host string
3    Port int
4}
5type Server struct {
6    Config  // embed for composition
7    Name string
8}

When to use:

• Interface — abstraction, dependency injection, testability.
• Struct — concrete data types, models, configs.

Common mistakes:

• Defining interfaces in the implementation package — define where consumed.
• Overusing any — loses type safety.
• Nil interface trap: an interface holding a typed nil pointer is NOT nil.
• Large interfaces with many methods — keep them small and focused.