In Go, copying slices requires attention to the correct way to avoid errors or performance issues. 1. It is recommended to use the built-in copy function for copy. The syntax is copy(dst, src). If the copy length is smaller, the target slice needs to allocate capacity in advance, otherwise the copy content will be truncated; 2. For slices of structure or pointer type, only copy is used as a shallow copy, and you need to manually traverse and create a new object to achieve deep copy to ensure that the new and old slices are independent; 3. Using the slice expression newSlice := oldSlice[:] will share the underlying array, modifying each other and affecting each other, and are not suitable for the completely isolated copy requirements.
In Go, copying a slice looks simple, but if you don't pay attention to details, it is easy to make mistakes or cause performance problems. Copying a slice correctly not only moves data from one place to another, but also makes sure that new and old slices do not affect each other.
Use built-in copy functions
Go provides built-in copy function, which is the recommended way to copy slices. Its syntax is:
copy(dst, src)
This function will copy the data in src into dst , and the copy length will be the smaller of the two.
Two things to note when using:
- The target slice (dst) needs to be allocated in advance;
- If the target slice capacity is insufficient, the copied content will be truncated.
For example:
src := []int{1, 2, 3}
dst := make([]int, 2)
copy(dst, src) // dst becomes [1, 2] Because dst only has 2 capacity, you can only copy the first two elements.
You need to pay attention to the underlying array to create a deep copy
Although copy can copy the content of slice, if you are dealing with slices of structure or pointer type, just copy is not enough.
For example, you have a slice of type []*User , and each element in it is a pointer. At this time, even if copy is used, the pointer address is simply copied, not the object itself. The old and new slices still share objects in the same underlying array, modifying one will affect the other.
Workaround: You need to manually traverse the source slice and create a new copy for each element:
type User struct {
Name string
}
src := []*User{{Name: "Alice"}, {Name: "Bob"}}
dst := make([]*User, len(src))
for i := range src {
dst[i] = &User{Name: src[i].Name}
}In this way, dst and src are completely independent.
Can slice expressions also "copy"? Don't be misled
Sometimes you will see this writing method:
newSlice := oldSlice[:]
This does generate a new slice header, but it points to the same underlying array . In other words, if you modify an element in newSlice, oldSlice will also change!
This situation is suitable for temporary operational data, but this is not suitable if you want two slices to be completely isolated. If you want to really copy, you should combine the aforementioned copy or copy elements one by one.
Basically that's it. Go's slice copy seems simple, but to truly achieve "correctness", you still have to see clearly whether you want to share data, shallow copy, or deep copy.
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