Package clone provides functions to deep clone any Go data. It also provides a wrapper to protect a pointer from any unexpected mutation.

Clone/Slowly can clone unexported fields and "no-copy" structs as well. Use this feature wisely.

Use go get to install this package.

go get github.com/huandu/go-clone

If we want to clone any Go value, use Clone.

t := &T{...}
v := clone.Clone(t).(*T)
reflect.DeepEqual(t, v) // true

For the sake of performance, Clone doesn't deal with values containing pointer cycles. If we need to clone such values, use Slowly instead.

type ListNode struct {
    Data int
    Next *ListNode
}
node1 := &ListNode{
    Data: 1,
}
node2 := &ListNode{
    Data: 2,
}
node3 := &ListNode{
    Data: 3,
}
node1.Next = node2
node2.Next = node3
node3.Next = node1

// We must use `Slowly` to clone a circular linked list.
node := Slowly(node1).(*ListNode)

for i := 0; i < 10; i++ {
    fmt.Println(node.Data)
    node = node.Next
}

Some struct types can be considered as scalar.

A well-known case is time.Time. Although there is a pointer loc *time.Location inside time.Time, we always use time.Time by value in all methods. When cloning time.Time, it should be OK to return a shadow copy.

Currently, following types are marked as scalar by default.

• time.Time
• reflect.Value

If there is any type defined in built-in package should be considered as scalar, please open new issue to let me know. I will update the default.

If there is any custom type should be considered as scalar, call MarkAsScalar to mark it manually. See MarkAsScalar sample code for more details.

Some pointer values are used as enumerable const values.

A well-known case is elliptic.Curve. In package crypto/tls, curve type of a certificate is checked by comparing values to pre-defined curve values, e.g. elliptic.P521(). In this case, the curve values, which are pointers or structs, cannot be cloned deeply.

Currently, following types are marked as scalar by default.

• elliptic.Curve, which is *elliptic.CurveParam or elliptic.p256Curve.
• reflect.Type, which is *reflect.rtype defined in runtime.

If there is any pointer type defined in built-in package should be considered as opaque, please open new issue to let me know. I will update the default.

If there is any custom pointer type should be considered as opaque, call MarkAsOpaquePointer to mark it manually. See MarkAsOpaquePointer sample code for more details.

There are some "no-copy" types like sync.Mutex, atomic.Value, etc. They cannot be cloned by copying all fields one by one, but we can alloc a new zero value and call methods to do proper initialization.

Currently, all "no-copy" types defined in sync and sync/atomic can be cloned properly using following strategies.

• sync.Mutex: Cloned value is a newly allocated zero mutex.
• sync.RWMutex: Cloned value is a newly allocated zero mutex.
• sync.WaitGroup: Cloned value is a newly allocated zero wait group.
• sync.Cond: Cloned value is a cond with a newly allocated zero lock.
• sync.Pool: Cloned value is an empty pool with the same New function.
• sync.Map: Cloned value is a sync map with cloned key/value pairs.
• sync.Once: Cloned value is a once type with the same done flag.
• atomic.Value: Cloned value is a new atomic value with the same value.

If there is any type defined in built-in package should be considered as "no-copy" types, please open new issue to let me know. I will update the default.

If default clone strategy doesn't work for a struct type, we can call SetCustomFunc to implement custom clone logic. Clone and Slowly can be used in custom clone functions.

See SetCustomFunc sample code for more details.

Package clone provides Wrap/Unwrap functions to protect a pointer value from any unexpected mutation. It's useful when we want to protect a variable which should be immutable by design, e.g. global config, the value stored in context, the value sent to a chan, etc.

// Suppose we have a type T defined as following.
//     type T struct {
//         Foo int
//     }
v := &T{
    Foo: 123,
}
w := Wrap(v).(*T) // Wrap value to protect it.

// Use w freely. The type of w is the same as that of v.

// It's OK to modify w. The change will not affect v.
w.Foo = 456
fmt.Println(w.Foo) // 456
fmt.Println(v.Foo) // 123

// Once we need the original value stored in w, call `Unwrap`.
orig := Unwrap(w).(*T)
fmt.Println(orig == v) // true
fmt.Println(orig.Foo)  // 123

// Or, we can simply undo any change made in w.
// Note that `Undo` is significantly slower than `Unwrap`, thus
// the latter is always preferred.
Undo(w)
fmt.Println(w.Foo) // 123

Here is the performance data running on my MacBook Pro.

MacBook Pro (15-inch, 2019)
Processor: 2.6 GHz Intel Core i7

go 1.15.8
goos: darwin
goarch: amd64
pkg: github.com/huandu/go-clone

BenchmarkSimpleClone-12          8325153               137 ns/op              32 B/op          1 allocs/op
BenchmarkComplexClone-12          540330              2190 ns/op            1488 B/op         24 allocs/op
BenchmarkUnwrap-12              12075483                96.8 ns/op             0 B/op          0 allocs/op
BenchmarkSimpleWrap-12           3233422               373 ns/op              80 B/op          2 allocs/op
BenchmarkComplexWrap-12           757730              1498 ns/op             752 B/op         16 allocs/op

This package is licensed under MIT license. See LICENSE for details.