Go: What is the difference between map and struct?

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

Maps store arbitrary keys of a single type mapped to values of another type. Keys must be comparable (==). Maps are reference types — passing a map to a function shares the same data. They are not safe for concurrent writes without synchronization.

1ages := map[string]int{
2    "Alice": 30,
3    "Bob":   25,
4}
5ages["Charlie"] = 35   // add
6delete(ages, "Bob")    // remove
7v, ok := ages["Alice"] // safe lookup

Structs define a fixed schema at compile time. Each field has a name and type — the compiler catches typos and type mismatches. Structs are value types and safe for concurrent reads (though mutation requires synchronization).

1type Person struct {
2    Name string
3    Age  int
4}
5p := Person{Name: "Alice", Age: 30}
6p.Age = 31  // compile-time safe

When to use:

• Map — dynamic keys, configuration, caching, grouping by runtime-determined key.
• Struct — domain models, API payloads, any fixed-shape data.

Common mistakes:

• Using map[string]interface{} for everything — lose type safety and discoverability.
• Accessing a map key without checking the ok value — silently returns the zero value.
• Concurrent map writes cause a runtime panic — use sync.Map or a mutex.
• Comparing maps with == — only valid against nil; use reflection or manual comparison for content.