Go: What is the difference between map and struct?

Map — dynamic key-value store with O(1) lookups, runtime-checked. Struct — fixed, compile-time-checked collection of named fields.

Maps are perfect for dynamic data where keys aren't known ahead of time. They grow automatically and support delete. Maps are reference types — sharing between goroutines needs synchronization.

1cache := make(map[string]*Product)
2for _, p := range products {
3    cache[p.SKU] = p
4}
5delete(cache, "DISCONTINUED-SKU")
6if p, ok := cache[sku]; ok {
7    fmt.Println(p.Name)
8}

Structs define a compile-time schema — the compiler catches type errors, IDEs provide autocomplete, and memory layout is predictable. Structs compose via embedding.

1type Product struct {
2    SKU   string  `json:"sku"`
3    Name  string  `json:"name"`
4    Price float64 `json:"price"`
5}

When to use:

• Map — dynamic keys, caching, counting, configuration.
• Struct — domain models, API payloads, known-shape data.

Common mistakes:

• map[string]interface{} instead of structs — lose type safety.
• Reading without ok check — zero value masks missing data.
• Concurrent writes without mutex — runtime panic.
• Nil maps are readable but not writable — writing panics.