Does Deleting a Map Entry of Pointers Cause Memory Leaks in Go?

In Go, Does Deleting an Entry in a Map of Pointers Cause Memory Leak?

While slicing or deleting elements in a slice of pointers may pose a potential memory leak, let's explore whether this holds true for a map as well.

Inspecting the Sources

Checking the Go runtime sources reveals that in the mapdelete() function, both the key and value storage are cleared upon deletion.

<br>558 func mapdelete(t <em>maptype, h </em>hmap, key unsafe.Pointer) {</p>
<pre class="brush:php;toolbar:false">    // ...

600 memclr(k, uintptr(t.keysize))
601 v := unsafe.Pointer(uintptr(unsafe.Pointer(b)) dataOffset bucketCntuintptr(t.keysize) iuintptr(t.valuesize))
602 memclr(v, uintptr(t.valuesize))

    // ...

618 }

This means any pointers in keys or values are zeroed, breaking their references to the map's internal data structures.

Constructing a Test to Verify

To further demonstrate this, we can construct a test:

<br>type point struct {</p>
<pre class="brush:php;toolbar:false">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 := &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)

}

Upon running this test, we observe that the registered finalizer is called, confirming the pointer's removal from the map and subsequent garbage collection.

Conclusion

In Go, deleting an entry from a map of pointers does not cause a memory leak. Both the key and value storage are cleared upon deletion, releasing any pointers they held, ensuring proper garbage collection.

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