Does Deleting a Map Entry Lead to Memory Leaks When Holding Pointers?

Does Deleting a Map Entry Cause Memory Leaks?

Introduction:
In a recent discussion, it was raised that deleting elements from a slice of pointers could potentially lead to memory leaks. This question extends that inquiry to maps, specifically whether deleting an entry from a map holding pointers results in similar behavior.

Checking the Implementation:
To determine the truthfulness of this claim, let's investigate the source code for map deletion in Go's runtime: runtime/hashmap.go (function mapdelete()). Examination reveals that both the key and value are cleared upon deletion (#600, #602).

Zeroing Implications:
Clearing these values effectively severs the connection between the map and the pointed objects. This ensures that even though the values themselves may be pointers, the map no longer references them. As a result, the pointed objects become eligible for garbage collection if they have no other references.

Practical Example:
To verify this behavior, let's construct a test case:

<code class="go">type point struct {
    X, Y int
}

var m = map[int]*point{}

func main() {
    fillMap()
    delete(m, 1)
    runtime.GC()
    time.Sleep(time.Second)
    fmt.Println(m)
}

func fillMap() {
    p := &amp;point{1, 2}
    runtime.SetFinalizer(p, func(p *point) {
        fmt.Printf("Finalized: %p %+v\n", p, p)
    })
    m[1] = p
    fmt.Printf("Put in map: %p %+v\n", p, p)
}</code>

Output:

Put in map: 0x1040a128 &{X:1 Y:2}
Finalized: 0x1040a128 &{X:1 Y:2}
map[]

In this example, the pointer value (p) is put into the map and a finalizer is set to be called upon garbage collection. After deleting the entry from the map and forcing garbage collection, the finalizer is invoked, proving that the pointer was indeed removed from the map.

Conclusion:
Based on the source code analysis and practical example, we can conclude that deleting an entry from a map of pointers does not cause memory leaks. The Go runtime ensures proper garbage collection by zeroing the cleared values, allowing the pointed objects to be reclaimed when no other references exist.

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