Go Maps: `map[string]int{}` vs. `make(map[string]int)`: What's the Difference?

Understanding the Difference Between map Initialization with and without make

When dealing with maps in Go, you may encounter two distinct forms of initialization:

1. Using a Map Literal:

var m = map[string]int{}

2. Using the make Function:

var m = make(map[string]int)

Functional Differences:

The primary distinction lies in the way maps are initialized. The second form, using make, always produces an empty map. However, the first form is a unique case of a map literal. Map literals can construct maps with initial key-value pairs. For instance:

m := map[bool]string{false: "FALSE", true: "TRUE"}

Equivalence and Performance:

The generalized version of your example,

m := map[T]U{}

is equivalent to invoking make:

m := make(map[T]U)

In terms of performance, the two approaches behave identically when creating empty maps.

Initial Capacity:

The primary advantage of using make is the ability to specify an initial capacity. This can be done by adding an integer argument to the make function:

m := make(map[T]U, 50)

This initializes a map with space allocated for 50 elements. Pre-allocating can reduce future memory allocations if you anticipate map growth.

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