Go: What is the difference between map and struct?

Map — heterogeneous key-value store with runtime type checking. Struct — homogeneous collection of named fields with compile-time type checking.

Maps are ideal when keys are not known ahead of time. They use hash tables internally, giving O(1) average-case lookups. Maps are reference types — passing one to a function shares the same data.

1config := map[string]string{
2    "log_level": "debug",
3    "port":      "8080",
4}
5config[os.Getenv("KEY")] = os.Getenv("VALUE")  // dynamic key

Structs give you compile-time safety — the compiler knows every field name and type. This catches typos, enables IDE autocompletion, and makes code self-documenting.

1type Config struct {
2    LogLevel string `json:"log_level"`
3    Port     string `json:"port"`
4}
5
6cfg := Config{LogLevel: "debug", Port: "8080"}
7cfg.Log_level = "info"  // compile error — typo caught!

When to use:

• Map — dynamic keys, caching, counting, configuration from unstructured data.
• Struct — domain models, configuration, API payloads, database records.

Common mistakes:

• Using map[string]interface{} to model complex data — use a struct with json tags.
• Reading from a map without checking the ok value — silently returns zero value.
• Using maps in concurrent contexts without synchronization.
• Forgetting that maps cannot be compared with == (except against nil).