How do generics work in Go 1.18+ and how do they affect performance?

Go 1.18 introduced type parameters (generics), allowing functions and types to be parameterised over types constrained by interfaces. Go uses a GCShape-based implementation: rather than generating a separate binary for each concrete type (full monomorphisation), Go creates a dictionary-based dispatch for types with the same GC shape (memory layout), reducing binary size at a small runtime cost.

// Generic function with type constraint
func Min[T constraints.Ordered](a, b T) T {
    if a < b { return a }
    return b
}

fmt.Println(Min(3, 5))         // int
fmt.Println(Min(3.14, 2.71))   // float64
fmt.Println(Min("foo", "bar")) // string

// Generic type
type Stack[T any] struct {
    items []T
}
func (s *Stack[T]) Push(item T) { s.items = append(s.items, item) }
func (s *Stack[T]) Pop() (T, bool) {
    var zero T
    if len(s.items) == 0 { return zero, false }
    n := len(s.items) - 1
    item := s.items[n]
    s.items = s.items[:n]
    return item, true
}

// Custom interface constraint
type Number interface {
    ~int | ~int32 | ~int64 | ~float32 | ~float64
}
func Sum[T Number](nums []T) T {
    var total T
    for _, n := range nums { total += n }
    return total
}

// ~ means: any type whose underlying type is int
type MyInt int // ~int matches MyInt
fmt.Println(Sum([]MyInt{1, 2, 3})) // 6

Performance: For pointer-sized types (interfaces, pointers, slices), Go generates a single implementation shared via a dictionary (like Java's type erasure). For value types (int, float64), Go can generate specialised code paths. In practice, generic functions are slightly slower than hand-written type-specific functions in some workloads but eliminate code duplication. Use generics when the algorithm is the same across types and the alternative is copy-paste or reflection.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.