Map types are reference types. var m map[string]int doesn't point to an initialized map. What doe this mean?

Question

I have read on the golang blog: https://blog.golang.org/go-maps-in-action that:

var m map[string]int

Map types are reference types, like pointers or slices, and so the value of m above is nil; it doesn't point to an initialized map. A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic; don't do that. To initialize a map, use the built in make function:

m = make(map[string]int)

The make function allocates and initializes a hash map data structure and returns a map value that points to it.

I have a hard time understanding some parts of this:

• What does var m map[string]int do?
• Why do I need to write m = make(map[string]int) but not i = make(int)

Answer 1

What does var m map[string]int do?

It tells the compiler that m is a variable of type map[string]int, and assigns "The Zero Value" of the type map[string] int to m (that's why m is nil as nil is The Zero Value of any map).

Why do I need to write m = make(map[string]int) but not i = make(int)

You don't need to. You can create a initialized map also like this:

m = map[string]int{}

which does exactly the same.

The difference between maps and ints is: A nil map is perfectly fine. E.g. len() of a nil map works and is 0. The only thing you cannot do with a nil map is store key-value-pairs. If you want to do this you'll have to prepare/initialize the map. This preparation/initialization in Go is done through the builtin make (or by a literal map as shown above). This initialization process is not needed for ints. As there are no nil ints this initialization would be total noise.

Note that you do not initialize the variable m: The variable m is a map of strings to ints, initialized or not. Like i is a variable for ints. Now ints are directly usable while maps require one more step because the language works that way.

Answer 2

What does var m map[string]int do?

You can think about it like pointer with nil value, it does not point to anything yet but able to point to concrete value.

Why do I need to write m = make(map[string]int) but not i = make(int)

https://golang.org/doc/effective_go.html#allocation_make

Back to allocation. The built-in function make(T, args) serves a purpose different from new(T). It creates slices, maps, and channels only, and it returns an initialized (not zeroed) value of type T (not *T). The reason for the distinction is that these three types represent, under the covers, references to data structures that must be initialized before use. A slice, for example, is a three-item descriptor containing a pointer to the data (inside an array), the length, and the capacity, and until those items are initialized, the slice is nil. For slices, maps, and channels, make initializes the internal data structure and prepares the value for use. For instance,

make([]int, 10, 100)

allocates an array of 100 ints and then creates a slice structure with length 10 and a capacity of 100 pointing at the first 10 elements of the array. (When making a slice, the capacity can be omitted; see the section on slices for more information.) In contrast, new([]int) returns a pointer to a newly allocated, zeroed slice structure, that is, a pointer to a nil slice value.

These examples illustrate the difference between new and make.

var p *[]int = new([]int)       // allocates slice structure; *p == nil; rarely useful
var v  []int = make([]int, 100) // the slice v now refers to a new array of 100 ints

// Unnecessarily complex:
var p *[]int = new([]int)
*p = make([]int, 100, 100)

// Idiomatic:
v := make([]int, 100)

Remember that make applies only to maps, slices and channels and does not return a pointer. To obtain an explicit pointer allocate with new or take the address of a variable explicitly.

Answer 3

All words have the same length of 32 bits (4 bytes) or 64 bits (8 bytes), depending on the processor and the operating system. They are identified by their memory address (represented as a hexadecimal number).

All variables of primitive types like int, float, bool, string ... are value types, they point directly to the values contained in the memory. Also composite types like arrays and structs are value types. When assigning with = the value of a value type to another variable: j = i, a copy of the original value i is made in memory.

More complex data which usually needs several words are treated as reference types. A reference type variable r1 contains the address (a number) of the memory location where the value of r1 is stored (or at least the 1st word of it):

For reference types when assigning r2 = r1, only the reference (the address) is copied and not the value!!. If the value of r1 is modified, all references of that value (like r1 and r2) will be reflected.

In Go pointers are reference types, as well as slices, maps and channels. The variables that are referenced are stored in the heap, which is garbage collected.

In the light of the above statements it's clear why the article states:

To initialize a map, use the built in make function.

The make function allocates and initializes a hash map data structure and returns a map value that points to it. This means you can write into it, compare to

var m map[string]int

which is readable, resulting a nil map, but an attempt to write to a nil map will cause a runtime panic. This is the reason why it's important to initialize the map with make.

m = make(map[string]int)