Go: What is the difference between interface and struct?

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

Interfaces in Go are implicitly satisfied — no implements keyword. Any type with the right methods satisfies the interface. This enables polymorphism, dependency injection, and clean testing.

1type Repository interface {
2    FindByID(id int) (*User, error)
3    Save(user *User) error
4}
5
6type PostgresRepo struct{ db *sql.DB }
7func (r *PostgresRepo) FindByID(id int) (*User, error) {
8    // database query
9}
10func (r *PostgresRepo) Save(user *User) error {
11    // insert/update
12}

Structs are the concrete types that implement interfaces. They hold data, support embedding for composition, and can have methods with either receiver type.

1type User struct {
2    ID   int
3    Name string
4}
5
6type UserRepo struct {
7    users map[int]*User  // embeds storage
8}

When to use:

• Interface — abstraction, testing, decoupling.
• Struct — concrete data, domain models, configs.

Common mistakes:

• Defining interfaces where implementations live — define where consumed.
• Overusing any — loses type safety.
• Nil interface trap: interface with typed nil pointer ≠ nil.
• Large interfaces — keep to 1-3 methods.