How Can I Avoid Memory Leaks When Reslicing Go Slices with Pointers?

Understanding Memory Leaks in Go Slice

When working with slices in Go, it's essential to understand the potential for memory leaks, especially when pointers are involved.

Memory Leaks with Pointers

Consider a slice of pointers to integers ([]*int):

s := []*int{new(int), new(int)}

This slice has a backing array with a length of 2 and contains 2 non-nil pointers pointing to allocated integers outside the array.

Now, if we reslice it:

s = s[:1]

The length becomes 1, but the backing array remains untouched. The unused pointer in the second element is still in memory as part of the array. Since it's not being referred to by any slice, it becomes inaccessible and cannot be freed by the garbage collector, resulting in a memory leak.

Why Not Non-Pointers?

With a slice of non-pointers ([]int):

t := []int{1, 2}

Slicing it will simply hide the elements from the new slice, but they remain in the backing array. Unlike pointers, these elements are part of the array itself and do not reference external memory.

Pointers and Structs

When a slice holds structs that contain pointers, memory leaks can occur:

type Books struct {
    title string
    author string
}
...
var bkSlice = []Books{Book1, Book2}
bkSlice = bkSlice[:1]

Even though the slice only retains Book1, the author and title strings of Book2 are still in memory as part of the array. To prevent this, assign the zero value to Book2 before slicing:

bkSlice[1] = Book{}
bkSlice = bkSlice[:1]

This removes the reference to the strings in Book2, allowing them to be garbage collected.

General Rule

To avoid memory leaks, aim to zero out elements that refer to memory outside the slice's backing array. For instance, structs with fields that are pointers, slices, or other complex data structures should be zeroed before reslicing to cut off external references.

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