Go: What is the difference between map and struct?

Map — dynamic key-value store, runtime-checked, reference type. Struct — fixed set of named fields, compile-time-checked, value type.

Maps grow dynamically and support arbitrary comparable keys. They're reference types — all copies point to the same data. Maps are not safe for concurrent writes.

1env := make(map[string]string)
2for _, v := range os.Environ() {
3    k, val, _ := strings.Cut(v, "=")
4    env[k] = val
5}
6if port, ok := env["PORT"]; ok {
7    fmt.Println("Port:", port)
8}

Structs define a fixed schema at compile time. The compiler catches type errors, and IDEs provide autocomplete. Structs are value types.

1type EnvVars struct {
2    Port   string
3    Debug  bool
4    DBHost string
5}

When to use:

• Map — dynamic keys, caching, configuration.
• Struct — known-shape data, models, configs.

Common mistakes:

• map[string]interface{} instead of structs.
• Reading without ok check.
• Concurrent writes without synchronization.
• Nil maps are readable but not writable.