How are strings represented in Go and why are they immutable?

A Go string is a two-word struct similar to a slice header but without a capacity field: a ptr (unsafe.Pointer to the UTF-8 bytes) and a len (byte count). Strings are immutable — the bytes they point to cannot be modified through any string operation.

// String header: {ptr unsafe.Pointer, len int}
s := "hello"
fmt.Println(len(s))    // 5 (bytes, not runes)
fmt.Println(s[0])      // 104 — byte value of 'h'
// s[0] = 'H'          // compile error: cannot assign to s[0]

// UTF-8: len counts bytes, range counts runes
emoji := "Go 🚀"
fmt.Println(len(emoji))              // 7 (G=1, o=1, space=1, rocket=4 bytes)
for i, r := range emoji {
    fmt.Printf("%d: %c\n", i, r)  // i is byte offset, r is rune value
}

// String concatenation creates a NEW backing array each time
// Avoid in loops — use strings.Builder instead
var b strings.Builder
for _, s := range words {
    b.WriteString(s)
    b.WriteByte(' ')
}
result := b.String() // single allocation

// string <-> []byte conversion
// Each conversion copies the bytes (different backing arrays)
bytes := []byte(s)   // copy into a new mutable byte slice
s2    := string(bytes) // copy back — new immutable string

// Zero-copy cast with unsafe (avoid unless in hot path + well-understood)
// Not recommended for general use

Immutability allows strings to be safely shared without copying — multiple strings can point to the same backing byte array (e.g., a slice of a longer string). String constants are interned into read-only memory in the binary, so no heap allocation is needed for them. The strings.Builder type avoids repeated allocation by maintaining a []byte buffer and converting to string only at the end.

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