Go Basics: Why nil Slices accept new values, but nil Maps don't

New values can be added to Nil Slice but adding new values to Nil Map throws a runtime error. Let's see why

var slice []int // nil slice
slice = append(slice, 1) // append works as expected
fmt.Println(slice) // [1]

var m map[string]int // nil map
m["one"] = 1 // 🚨 Runtime panic: assignment to entry in nil map

Why appending to a Nil Slice works:
Slice is a reference to an underlying array and dynamically-sized data structure. It consists of 3 components

• Pointer to underlying array
• Length of slice
• Capacity of slice

When we declare a nil slice, the length and capacity will be zero and it doesn't point to any underlying array. Basically append operation checks the capacity and if it's not sufficient then it allocates new underlying array with enough capacity to store the new elements and returns a new slice that points to newly created array. So append works as expected.

Why adding to a Nil Map doesn't work:
Map in golang basically acts as a hash table and the internal data structures needs to be initialized first before we can store the key-value pair.

When we declare a map as nil (var m map[KeyType]ValueType), it is uninitialized and not yet ready to store key-value pairs. So this will create a runtime error when we try to add values to a nil map. To avoid such errors create map using make

var m map[string]int // nil map

m = make(map[string]int) // initialize it
m["one"] = 1 // map is now initialized, so we can add values
print(m) // map[one:1]

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To learn more about such topics along with code examples, feel free to checkout my Github repo where I regularly commit about Go core-concepts & best practices.

Sai7xp / learn-golang

GoLang Basics to Advanced. Including Design Patterns, SOLID Principles & more