Clean Code: Structs, Methods, and Composition Over Inheritance [Part 2]

This is the second article in my clean code series. You can read the first part here. This is the second article in my clean code series. You can read the first part here. here https://hackernoon.com/clean-code-functions-and-error-handling-in-go-from-chaos-to-clarity-part-1?embedable=true https://hackernoon.com/clean-code-functions-and-error-handling-in-go-from-chaos-to-clarity-part-1?embedable=true Introduction: Why OOP in Go Isn't What You Think I've seen hundreds of developers try to write Go like Java, creating inheritance hierarchies that don't exist and fighting the language every step of the way. "Go has no classes!" — the first shock for developers with Java/C# background. The second — "How to live without inheritance?!". Relax, Go offers something better: composition through embedding, interfaces without explicit implementation, and clear rules for methods. Common struct/method mistakes I've observed: Using value receivers with mutexes: ~25% cause data races Mixing receiver types: ~35% of struct methods Creating getters/setters for everything: ~60% of structs Trying to implement inheritance: ~40% of new Go developers Using value receivers with mutexes: ~25% cause data races Mixing receiver types: ~35% of struct methods Creating getters/setters for everything: ~60% of structs Trying to implement inheritance: ~40% of new Go developers After 6 years of working with Go, I can say: the difference between fighting the language and flowing with it usually comes down to understanding structs and methods properly. Receivers: The Go Developer's Main Dilemma Value vs Pointer Receiver This is question #1 in interviews and code reviews. Here's a simple rule that covers 90% of cases: // Value receiver - for immutable methods func (u User) FullName() string { return fmt.Sprintf("%s %s", u.FirstName, u.LastName) } // Pointer receiver - when changing state func (u *User) SetEmail(email string) error { if !isValidEmail(email) { return ErrInvalidEmail } u.Email = email u.UpdatedAt = time.Now() return nil } // Value receiver - for immutable methods func (u User) FullName() string { return fmt.Sprintf("%s %s", u.FirstName, u.LastName) } // Pointer receiver - when changing state func (u *User) SetEmail(email string) error { if !isValidEmail(email) { return ErrInvalidEmail } u.Email = email u.UpdatedAt = time.Now() return nil } Rules for Choosing a Receiver type Account struct { ID string Balance decimal.Decimal mutex sync.RWMutex } // Rule 1: If even one method requires a pointer receiver, // ALL methods should use pointer receiver (consistency) // BAD: mixed receivers func (a Account) GetBalance() decimal.Decimal { // value receiver return a.Balance } func (a *Account) Deposit(amount decimal.Decimal) { // pointer receiver a.Balance = a.Balance.Add(amount) } // GOOD: consistent receivers func (a *Account) GetBalance() decimal.Decimal { a.mutex.RLock() defer a.mutex.RUnlock() return a.Balance } func (a *Account) Deposit(amount decimal.Decimal) error { if amount.LessThanOrEqual(decimal.Zero) { return ErrInvalidAmount } a.mutex.Lock() defer a.mutex.Unlock() a.Balance = a.Balance.Add(amount) return nil } type Account struct { ID string Balance decimal.Decimal mutex sync.RWMutex } // Rule 1: If even one method requires a pointer receiver, // ALL methods should use pointer receiver (consistency) // BAD: mixed receivers func (a Account) GetBalance() decimal.Decimal { // value receiver return a.Balance } func (a *Account) Deposit(amount decimal.Decimal) { // pointer receiver a.Balance = a.Balance.Add(amount) } // GOOD: consistent receivers func (a *Account) GetBalance() decimal.Decimal { a.mutex.RLock() defer a.mutex.RUnlock() return a.Balance } func (a *Account) Deposit(amount decimal.Decimal) error { if amount.LessThanOrEqual(decimal.Zero) { return ErrInvalidAmount } a.mutex.Lock() defer a.mutex.Unlock() a.Balance = a.Balance.Add(amount) return nil } When to Use Pointer Receiver Method modifies state Struct contains mutex (otherwise it will be copied!) Large struct (avoid copying) Consistency (if at least one method requires pointer) Method modifies state Method modifies state Struct contains mutex (otherwise it will be copied!) Struct contains mutex Large struct (avoid copying) Large struct Consistency (if at least one method requires pointer) Consistency // Struct with mutex ALWAYS pointer receiver type Cache struct { data map[string]interface{} mu sync.RWMutex } // DANGEROUS: value receiver copies mutex! func (c Cache) Get(key string) interface{} { // BUG! c.mu.RLock() // Locking a COPY of mutex defer c.mu.RUnlock() return c.data[key] } // CORRECT: pointer receiver func (c *Cache) Get(key string) interface{} { c.mu.RLock() defer c.mu.RUnlock() return c.data[key] } // Struct with mutex ALWAYS pointer receiver type Cache struct { data map[string]interface{} mu sync.RWMutex } // DANGEROUS: value receiver copies mutex! func (c Cache) Get(key string) interface{} { // BUG! c.mu.RLock() // Locking a COPY of mutex defer c.mu.RUnlock() return c.data[key] } // CORRECT: pointer receiver func (c *Cache) Get(key string) interface{} { c.mu.RLock() defer c.mu.RUnlock() return c.data[key] } Constructors and Factory Functions Go doesn't have constructors in the classical sense, but there's the New* idiom: New* // BAD: direct struct creation func main() { user := &User{ ID: generateID(), // What if we forget? Email: "test@test.com", // CreatedAt not set! } } // GOOD: factory function guarantees initialization func NewUser(email string) (*User, error) { if !isValidEmail(email) { return nil, ErrInvalidEmail } return &User{ ID: generateID(), Email: email, CreatedAt: time.Now(), UpdatedAt: time.Now(), }, nil } // BAD: direct struct creation func main() { user := &User{ ID: generateID(), // What if we forget? Email: "test@test.com", // CreatedAt not set! } } // GOOD: factory function guarantees initialization func NewUser(email string) (*User, error) { if !isValidEmail(email) { return nil, ErrInvalidEmail } return &User{ ID: generateID(), Email: email, CreatedAt: time.Now(), UpdatedAt: time.Now(), }, nil } Functional Options Pattern For structs with many optional parameters: type Server struct { host string port int timeout time.Duration maxConns int tls *tls.Config } // Option - function that modifies Server type Option func(*Server) // Factory functions for options func WithTimeout(timeout time.Duration) Option { return func(s *Server) { s.timeout = timeout } } func WithTLS(config *tls.Config) Option { return func(s *Server) { s.tls = config } } func WithMaxConnections(max int) Option { return func(s *Server) { s.maxConns = max } } // Constructor accepts required parameters and options func NewServer(host string, port int, opts ...Option) *Server { server := &Server{ host: host, port: port, timeout: 30 * time.Second, // defaults maxConns: 100, } // Apply options for _, opt := range opts { opt(server) } return server } // Usage - reads like prose server := NewServer("localhost", 8080, WithTimeout(60*time.Second), WithMaxConnections(1000), WithTLS(tlsConfig), ) type Server struct { host string port int timeout time.Duration maxConns int tls *tls.Config } // Option - function that modifies Server type Option func(*Server) // Factory functions for options func WithTimeout(timeout time.Duration) Option { return func(s *Server) { s.timeout = timeout } } func WithTLS(config *tls.Config) Option { return func(s *Server) { s.tls = config } } func WithMaxConnections(max int) Option { return func(s *Server) { s.maxConns = max } } // Constructor accepts required parameters and options func NewServer(host string, port int, opts ...Option) *Server { server := &Server{ host: host, port: port, timeout: 30 * time.Second, // defaults maxConns: 100, } // Apply options for _, opt := range opts { opt(server) } return server } // Usage - reads like prose server := NewServer("localhost", 8080, WithTimeout(60*time.Second), WithMaxConnections(1000), WithTLS(tlsConfig), ) Encapsulation Through Naming Go has no private/public keywords. Instead — the case of the first letter: type User struct { ID string // Public field (Exported) Email string password string // Private field (Unexported) createdAt time.Time // Private } // Public method func (u *User) SetPassword(pwd string) error { if len(pwd) < 8 { return ErrWeakPassword } hashed, err := bcrypt.GenerateFromPassword([]byte(pwd), bcrypt.DefaultCost) if err != nil { return fmt.Errorf("hash password: %w", err) } u.password = string(hashed) return nil } // Private helper func (u *User) validatePassword(pwd string) error { return bcrypt.CompareHashAndPassword([]byte(u.password), []byte(pwd)) } // Public method uses private one func (u *User) Authenticate(pwd string) error { if err := u.validatePassword(pwd); err != nil { return ErrInvalidCredentials } return nil } type User struct { ID string // Public field (Exported) Email string password string // Private field (Unexported) createdAt time.Time // Private } // Public method func (u *User) SetPassword(pwd string) error { if len(pwd) < 8 { return ErrWeakPassword } hashed, err := bcrypt.GenerateFromPassword([]byte(pwd), bcrypt.DefaultCost) if err != nil { return fmt.Errorf("hash password: %w", err) } u.password = string(hashed) return nil } // Private helper func (u *User) validatePassword(pwd string) error { return bcrypt.CompareHashAndPassword([]byte(u.password), []byte(pwd)) } // Public method uses private one func (u *User) Authenticate(pwd string) error { if err := u.validatePassword(pwd); err != nil { return ErrInvalidCredentials } return nil } Composition Through Embedding Instead of inheritance, Go offers embedding. This is NOT inheritance, it's composition: // Base struct type Person struct { FirstName string LastName string BirthDate time.Time } func (p Person) FullName() string { return fmt.Sprintf("%s %s", p.FirstName, p.LastName) } func (p Person) Age() int { return int(time.Since(p.BirthDate).Hours() / 24 / 365) } // Employee embeds Person type Employee struct { Person // Embedding - NOT inheritance! EmployeeID string Department string Salary decimal.Decimal } // Employee can override Person's methods func (e Employee) FullName() string { return fmt.Sprintf("%s (%s)", e.Person.FullName(), e.EmployeeID) } // Usage emp := Employee{ Person: Person{ FirstName: "John", LastName: "Doe", BirthDate: time.Date(1990, 1, 1, 0, 0, 0, 0, time.UTC), }, EmployeeID: "EMP001", Department: "Engineering", } fmt.Println(emp.FullName()) // John Doe (EMP001) - overridden method fmt.Println(emp.Age()) // 34 - method from Person fmt.Println(emp.FirstName) // John - field from Person // Base struct type Person struct { FirstName string LastName string BirthDate time.Time } func (p Person) FullName() string { return fmt.Sprintf("%s %s", p.FirstName, p.LastName) } func (p Person) Age() int { return int(time.Since(p.BirthDate).Hours() / 24 / 365) } // Employee embeds Person type Employee struct { Person // Embedding - NOT inheritance! EmployeeID string Department string Salary decimal.Decimal } // Employee can override Person's methods func (e Employee) FullName() string { return fmt.Sprintf("%s (%s)", e.Person.FullName(), e.EmployeeID) } // Usage emp := Employee{ Person: Person{ FirstName: "John", LastName: "Doe", BirthDate: time.Date(1990, 1, 1, 0, 0, 0, 0, time.UTC), }, EmployeeID: "EMP001", Department: "Engineering", } fmt.Println(emp.FullName()) // John Doe (EMP001) - overridden method fmt.Println(emp.Age()) // 34 - method from Person fmt.Println(emp.FirstName) // John - field from Person Embedding Interfaces type Reader interface { Read([]byte) (int, error) } type Writer interface { Write([]byte) (int, error) } // ReadWriter embeds both interfaces type ReadWriter interface { Reader Writer } // Struct can embed interfaces for delegation type LoggedWriter struct { Writer // Embed interface logger *log.Logger } func (w LoggedWriter) Write(p []byte) (n int, err error) { n, err = w.Writer.Write(p) // Delegate to embedded Writer w.logger.Printf("Wrote %d bytes, err: %v", n, err) return n, err } // Usage var buf bytes.Buffer logged := LoggedWriter{ Writer: &buf, logger: log.New(os.Stdout, "WRITE: ", log.LstdFlags), } logged.Write([]byte("Hello, World!")) type Reader interface { Read([]byte) (int, error) } type Writer interface { Write([]byte) (int, error) } // ReadWriter embeds both interfaces type ReadWriter interface { Reader Writer } // Struct can embed interfaces for delegation type LoggedWriter struct { Writer // Embed interface logger *log.Logger } func (w LoggedWriter) Write(p []byte) (n int, err error) { n, err = w.Writer.Write(p) // Delegate to embedded Writer w.logger.Printf("Wrote %d bytes, err: %v", n, err) return n, err } // Usage var buf bytes.Buffer logged := LoggedWriter{ Writer: &buf, logger: log.New(os.Stdout, "WRITE: ", log.LstdFlags), } logged.Write([]byte("Hello, World!")) Method Chaining (Builder Pattern) type QueryBuilder struct { table string columns []string where []string orderBy string limit int } // Each method returns *QueryBuilder for chaining func NewQuery(table string) *QueryBuilder { return &QueryBuilder{ table: table, columns: []string{"*"}, } } func (q *QueryBuilder) Select(columns ...string) *QueryBuilder { q.columns = columns return q } func (q *QueryBuilder) Where(condition string) *QueryBuilder { q.where = append(q.where, condition) return q } func (q *QueryBuilder) OrderBy(column string) *QueryBuilder { q.orderBy = column return q } func (q *QueryBuilder) Limit(n int) *QueryBuilder { q.limit = n return q } func (q *QueryBuilder) Build() string { query := fmt.Sprintf("SELECT %s FROM %s", strings.Join(q.columns, ", "), q.table) if len(q.where) > 0 { query += " WHERE " + strings.Join(q.where, " AND ") } if q.orderBy != "" { query += " ORDER BY " + q.orderBy } if q.limit > 0 { query += fmt.Sprintf(" LIMIT %d", q.limit) } return query } // Usage - reads like SQL query := NewQuery("users"). Select("id", "name", "email"). Where("active = true"). Where("created_at > '2024-01-01'"). OrderBy("created_at DESC"). Limit(10). Build() // SELECT id, name, email FROM users WHERE active = true AND created_at > '2024-01-01' ORDER BY created_at DESC LIMIT 10 type QueryBuilder struct { table string columns []string where []string orderBy string limit int } // Each method returns *QueryBuilder for chaining func NewQuery(table string) *QueryBuilder { return &QueryBuilder{ table: table, columns: []string{"*"}, } } func (q *QueryBuilder) Select(columns ...string) *QueryBuilder { q.columns = columns return q } func (q *QueryBuilder) Where(condition string) *QueryBuilder { q.where = append(q.where, condition) return q } func (q *QueryBuilder) OrderBy(column string) *QueryBuilder { q.orderBy = column return q } func (q *QueryBuilder) Limit(n int) *QueryBuilder { q.limit = n return q } func (q *QueryBuilder) Build() string { query := fmt.Sprintf("SELECT %s FROM %s", strings.Join(q.columns, ", "), q.table) if len(q.where) > 0 { query += " WHERE " + strings.Join(q.where, " AND ") } if q.orderBy != "" { query += " ORDER BY " + q.orderBy } if q.limit > 0 { query += fmt.Sprintf(" LIMIT %d", q.limit) } return query } // Usage - reads like SQL query := NewQuery("users"). Select("id", "name", "email"). Where("active = true"). Where("created_at > '2024-01-01'"). OrderBy("created_at DESC"). Limit(10). Build() // SELECT id, name, email FROM users WHERE active = true AND created_at > '2024-01-01' ORDER BY created_at DESC LIMIT 10 Thread-Safe Structs // BAD: race condition type Counter struct { value int } func (c *Counter) Inc() { c.value++ // Race when accessed concurrently! } // GOOD: protected with mutex type SafeCounter struct { mu sync.Mutex value int } func (c *SafeCounter) Inc() { c.mu.Lock() defer c.mu.Unlock() c.value++ } func (c *SafeCounter) Value() int { c.mu.Lock() defer c.mu.Unlock() return c.value } // EVEN BETTER: using atomic type AtomicCounter struct { value atomic.Int64 } func (c *AtomicCounter) Inc() { c.value.Add(1) } func (c *AtomicCounter) Value() int64 { return c.value.Load() } // BAD: race condition type Counter struct { value int } func (c *Counter) Inc() { c.value++ // Race when accessed concurrently! } // GOOD: protected with mutex type SafeCounter struct { mu sync.Mutex value int } func (c *SafeCounter) Inc() { c.mu.Lock() defer c.mu.Unlock() c.value++ } func (c *SafeCounter) Value() int { c.mu.Lock() defer c.mu.Unlock() return c.value } // EVEN BETTER: using atomic type AtomicCounter struct { value atomic.Int64 } func (c *AtomicCounter) Inc() { c.value.Add(1) } func (c *AtomicCounter) Value() int64 { return c.value.Load() } Anti-patterns and How to Avoid Them 1. Getters/Setters for All Fields // BAD: Java-style getters/setters type User struct { name string age int } func (u *User) GetName() string { return u.name } func (u *User) SetName(name string) { u.name = name } func (u *User) GetAge() int { return u.age } func (u *User) SetAge(age int) { u.age = age } // GOOD: export fields or use methods with logic type User struct { Name string age int // private because validation needed } func (u *User) SetAge(age int) error { if age 150 { return ErrInvalidAge } u.age = age return nil } func (u *User) Age() int { return u.age } // BAD: Java-style getters/setters type User struct { name string age int } func (u *User) GetName() string { return u.name } func (u *User) SetName(name string) { u.name = name } func (u *User) GetAge() int { return u.age } func (u *User) SetAge(age int) { u.age = age } // GOOD: export fields or use methods with logic type User struct { Name string age int // private because validation needed } func (u *User) SetAge(age int) error { if age 150 { return ErrInvalidAge } u.age = age return nil } func (u *User) Age() int { return u.age } 2. Huge Structs // BAD: God Object type Application struct { Config Config Database *sql.DB Cache *redis.Client HTTPServer *http.Server GRPCServer *grpc.Server Logger *log.Logger Metrics *prometheus.Registry // ... 20 more fields } // GOOD: separation of concerns type App struct { config *Config services *Services servers *Servers } type Services struct { DB Database Cache Cache Auth Authenticator } type Servers struct { HTTP *HTTPServer GRPC *GRPCServer } // BAD: God Object type Application struct { Config Config Database *sql.DB Cache *redis.Client HTTPServer *http.Server GRPCServer *grpc.Server Logger *log.Logger Metrics *prometheus.Registry // ... 20 more fields } // GOOD: separation of concerns type App struct { config *Config services *Services servers *Servers } type Services struct { DB Database Cache Cache Auth Authenticator } type Servers struct { HTTP *HTTPServer GRPC *GRPCServer } Practical Tips Always use constructors for structs with invariants Be consistent with receivers within a type Prefer composition over inheritance (which doesn't exist) Embedding is not inheritance, it's delegation Protect concurrent access with a mutex or channels Don't create getters/setters without necessity Always use constructors for structs with invariants Always use constructors Be consistent with receivers within a type Be consistent Prefer composition over inheritance (which doesn't exist) Prefer composition Embedding is not inheritance, it's delegation Embedding is not inheritance Protect concurrent access with a mutex or channels Protect concurrent access Don't create getters/setters without necessity Don't create getters/setters Struct and Method Checklist Constructor New* for complex initialization Consistent receivers (all pointer or all value) Pointer receiver for structs with a mutex Private fields for encapsulation Embedding instead of inheritance Thread-safety when needed Minimal getters/setters Constructor New* for complex initialization New* Consistent receivers (all pointer or all value) Pointer receiver for structs with a mutex Private fields for encapsulation Embedding instead of inheritance Thread-safety when needed Minimal getters/setters Conclusion Structs and methods in Go are an exercise in simplicity. No classes? Great, less complexity. No inheritance? Perfect, the composition is clearer. The key is not to drag patterns from other languages but to use Go idioms. In the next article, we'll dive into interfaces — the real magic of Go. We'll discuss why small interfaces are better than large ones, what interface satisfaction means, and why "Accept interfaces, return structs" is the golden rule. How do you handle the transition from OOP languages to Go's composition model? What patterns helped you the most? Share your experience in the comments!