sync.Once in Go

Exactly-once initialization, safe from any number of goroutines.

var (
    setupOnce sync.Once
    table     map[string]int
)

func lookup(key string) int {
    setupOnce.Do(func() {
        table = map[string]int{"basics": 39, "traits": 19}
    })
    return table[key]
}

var expensive = sync.OnceValue(func() int {
    return 40 + 2
})

func answer() int { return expensive() }

How it works

1. Do runs the function the first time and never again.
2. Later callers block until that first run finishes.
3. OnceValue is the newer form that returns the result.

Keywords and builtins used here

• func
• int
• map
• return
• string
• var

The run, in numbers

Lines

17

Characters to type

283

Tokens

82

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 47 seconds.

Type this snippet

Step 3 of 5 in Synchronization, step 16 of 25 in Concurrency.

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