How does Go\'s copy function handle overlapping slices?

Understanding the Copy Function in Go

The copy function in Go, as documented, is a built-in function that facilitates the copying of elements from a source slice to a destination slice. It operates on slices of the same element type and can handle cases where the source and destination overlap.

How Copy Works

The copy function takes two parameters: a destination slice dst and a source slice src. It returns an integer representing the number of elements successfully copied. The number of elements copied is determined by the minimum length of the source and destination slices.

Examples

Let's illustrate the functionality of copy with some examples:

<code class="go">src := []int{10, 11, 12, 13, 14}
dst := []int{0, 1, 2, 3, 4}

// Copy all elements from src to dst.
n := copy(dst, src)
fmt.Println(n, src, dst) // Output: 5 [10 11 12 13 14] [10 11 12 13 14]</code>

In this example, all five elements from src will be copied into dst, leaving both slices with the same elements.

<code class="go">dst = []int{0, 1}

// Copy only as many elements as the shorter of src and dst.
n = copy(dst, src)
fmt.Println(n, src, dst) // Output: 2 [10 11 12 13 14] [10 11]</code>

In this case, dst only has two elements, so only two elements are copied, resulting in dst containing the first two elements of src.

<code class="go">src = []int{10, 11}
dst = []int{0, 1, 2, 3, 4}

// Copy only as many elements as the shorter of src and dst.
n = copy(dst, src)
fmt.Println(n, src, dst) // Output: 2 [10 11] [10 11 2 3 4]</code>

Similarly, when src has fewer elements than dst, only the elements available in src are copied, and the remaining elements in dst remain unchanged.

特殊案例：拷贝字符串到字节片

值得注意的是，copy()还可以将字符串中的字节拷贝到字节片([]byte)中：

<code class="go">str := "Hello, World!"
data := make([]byte, 5)

// Copy 5 bytes from the UTF-8 representation of str into data.
n = copy(data, str)
fmt.Println(n, str, data) // Output: 5 Hello, World! [72 101 108 108 111]</code>

Overlap

Copy can handle situations where the source and destination slices overlap. In such cases, the elements in the overlapping portion will be copied, leaving the order of the elements in the destination slice unchanged.

Conclusion

The copy function in Go provides a convenient way to copy elements from one slice to another. It allows for flexibility in handling cases with differing slice lengths and overlaps, making it a useful tool for manipulating data in slices.

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