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

Map Initialization: Unraveling the Differences Between Shortcut and Explicit Creation

The question arises: what starkly differentiates the following two approaches to initializing maps in Go:

var m = map[string]int{}

and

var m = make(map[string]int)

Is the former merely a concise method for speedily initializing fields? Are there any potential performance implications to consider?

Dissecting the Differences

The distinction between these two syntaxes lies in their implications for non-empty maps. While make consistently creates an empty map, the first example leverages a unique feature of map literals: the ability to establish non-empty maps directly. For instance:

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

To generalize your example, a non-empty map literal with no initial value pairs, like the following:

m := map[T]U{}

is semantically identical to using make explicitly:

m := make(map[T]U)

However, make provides an additional advantage: it permits the specification of an initial capacity for the map that exceeds the count of initially assigned elements. Consider this example:

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

This syntax allocates sufficient space within the map to accommodate up to 50 items. By proactively reserving memory, future allocations can be minimized when anticipating map growth.

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