Go: What is the difference between interface and struct?

Interface — contract of method signatures defining what a type can do. Struct — concrete data layout defining what a type holds.

Interfaces enable polymorphism in Go. Since implementation is implicit, you can create new types that satisfy existing interfaces without modifying the interface definition. This is the foundation of Go's "accept interfaces, return structs" philosophy.

1type Store interface {
2    Get(key string) (string, error)
3    Set(key, value string) error
4}
5
6type MemoryStore struct {
7    data map[string]string
8}
9
10func (m *MemoryStore) Get(key string) (string, error) {
11    v, ok := m.data[key]
12    if !ok { return "", errors.New("not found") }
13    return v, nil
14}
15func (m *MemoryStore) Set(k, v string) error {
16    m.data[k] = v
17    return nil
18}

Structs hold the actual fields. You can embed structs for composition, add JSON/DB tags, and define methods. Structs are concrete — the compiler knows their exact memory layout.

When to use:

• Interface — for abstraction layers, test doubles, decoupling packages.
• Struct — for all data representation: models, configs, messages.

Common mistakes:

• Defining interfaces in the same package as the implementation — prefer defining them where they are consumed.
• Returning error interfaces where a concrete error type would be more informative.
• Making interfaces too large — keep them small and composable (1-3 methods).
• Ignoring that interface values with nil concrete values are not nil themselves.