Defining golang struct function using pointer or not - go

Question

Can someone explain to me why appending to an array works when you do this:
func (s *Sample) Append(name string) {
d := &Stuff{
name: name,
}
s.data = append(s.data, d)
}
Full code here
But not when you do this:
func (s Sample) Append(name string) {
d := &Stuff{
name: name,
}
s.data = append(s.data, d)
}
Is there any reason at all why you would want to use the second example.

Answer 1

As mentioned in the FAQ
Should I define methods on values or pointers?
func (s *MyStruct) pointerMethod() { } // method on pointer
func (s MyStruct) valueMethod() { } // method on value
First, and most important, does the method need to modify the receiver? If it does, the receiver must be a pointer. (Slices and maps act as references, so their story is a little more subtle, but for instance to change the length of a slice in a method the receiver must still be a pointer.)
In the examples above, if pointerMethod modifies the fields of s, the caller will see those changes, but valueMethod is called with a copy of the caller's argument (that's the definition of passing a value), so changes it makes will be invisible to the caller.
In your case, func (s Sample) Append(name string) modifies a copy.
laher reminds us in the comments that using a value instead of pointer also means getting a copy, and respecting the immutable nature of an object::
You'd want to use the non-pointer valueMethod when (for nstance) you're returning a [value derived from an] 'immutable' private property.
See "Why are receivers pass by value in Go?":
Can be useful if for instance you have a small immutable object. The caller can know for certain that this method doesn't modify it's receiver.
They can't know this if the receiver is a pointer without reading the code first.

Answer 2

Go slices are a tricky beast. Internally, a variable of slice type (like []int) looks like this:
struct {
data *int // pointer to the data area
len int
cap int
}
When you pass a slice to a function, this structure is passed by value, while the underlying data area (i.e. what data points to) is not copied. The builtin append() function modifies the data area (or generates a new one) and returns a new slice with updated len, data, and cap values. If you want to overwrite anything that is not part of the underlying data area, you need to pass a pointer to the slice or return a modified slice.

Answer 3

Go passes arguments by value, not by reference, unless you use a pointer. So inside the function, you're not modifying s in any outer scope if you simply pass by value. However, when you pass a pointer, you're able to modify the "real" variable rather than just the copy that exists inside the function.

Answer 4

While most of the answers here capture exactly what happens, I wanted to dig a bit into how/why that's happening. I started with a couple small snippets of code:
pointer method
package main
import "fmt"
type Bar struct{}
func (b *Bar) Print() {
fmt.Println("debosmit ray")
}
func main() {
b := Bar{}
b.Print()
}
value method
package main
import "fmt"
type Bar struct{}
func (b Bar) Print() {
fmt.Println("debosmit ray")
}
func main() {
b := Bar{}
b.Print()
}
Then, I wanted to look at the assembly for just the files (generated using go tool compile -S filename.go > filename.S, for each of the files. Both the outputs are available here (should be available forever).
Let's take a look at the output of diff pointer.S value.S (pointer -> has pointer method, value -> has value method).
14,15c14,15
< "".(*Bar).Print STEXT size=138 args=0x8 locals=0x58
< 0x0000 00000 (bar.go:7) TEXT "".(*Bar).Print(SB), ABIInternal, $88-8
---
> "".Bar.Print STEXT size=138 args=0x0 locals=0x58
> 0x0000 00000 (bar.go:7) TEXT "".Bar.Print(SB), ABIInternal, $88-0
24c24
< 0x001d 00029 (bar.go:7) FUNCDATA $0, gclocals·2a5305abe05176240e61b8620e19a815(SB)
---
> 0x001d 00029 (bar.go:7) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
26c26
< 0x001d 00029 (bar.go:7) FUNCDATA $3, "".(*Bar).Print.stkobj(SB)
---
> 0x001d 00029 (bar.go:7) FUNCDATA $3, "".Bar.Print.stkobj(SB)
126a127,200
> "".(*Bar).Print STEXT dupok size=187 args=0x8 locals=0x58
> 0x0000 00000 (<autogenerated>:1) TEXT "".(*Bar).Print(SB), DUPOK|WRAPPER|ABIInternal, $88-8
> 0x0000 00000 (<autogenerated>:1) MOVQ (TLS), CX
> 0x0009 00009 (<autogenerated>:1) CMPQ SP, 16(CX)
> 0x000d 00013 (<autogenerated>:1) PCDATA $0, $-2
> 0x000d 00013 (<autogenerated>:1) JLS 154
> 0x0013 00019 (<autogenerated>:1) PCDATA $0, $-1
> 0x0013 00019 (<autogenerated>:1) SUBQ $88, SP
> 0x0017 00023 (<autogenerated>:1) MOVQ BP, 80(SP)
> 0x001c 00028 (<autogenerated>:1) LEAQ 80(SP), BP
> 0x0021 00033 (<autogenerated>:1) MOVQ 32(CX), BX
> 0x0025 00037 (<autogenerated>:1) TESTQ BX, BX
> 0x0028 00040 (<autogenerated>:1) JNE 165
> 0x002a 00042 (<autogenerated>:1) NOP
> 0x002a 00042 (<autogenerated>:1) FUNCDATA $0, gclocals·1a65e721a2ccc325b382662e7ffee780(SB)
> 0x002a 00042 (<autogenerated>:1) FUNCDATA $1, gclocals·2589ca35330fc0fce83503f4569854a0(SB)
> 0x002a 00042 (<autogenerated>:1) FUNCDATA $3, "".(*Bar).Print.stkobj(SB)
> 0x002a 00042 (<autogenerated>:1) CMPQ ""..this+96(SP), $0
> 0x0030 00048 (<autogenerated>:1) JEQ 148
> 0x0032 00050 (<unknown line number>) NOP
> 0x0032 00050 (bar.go:8) XORPS X0, X0
> 0x0035 00053 (bar.go:8) MOVUPS X0, ""..autotmp_13+64(SP)
> 0x003a 00058 (bar.go:8) LEAQ type.string(SB), AX
> 0x0041 00065 (bar.go:8) MOVQ AX, ""..autotmp_13+64(SP)
> 0x0046 00070 (bar.go:8) LEAQ ""..stmp_2(SB), AX
> 0x004d 00077 (bar.go:8) MOVQ AX, ""..autotmp_13+72(SP)
> 0x0052 00082 (<unknown line number>) NOP
> 0x0052 00082 ($GOROOT/src/fmt/print.go:274) MOVQ os.Stdout(SB), AX
> 0x0059 00089 ($GOROOT/src/fmt/print.go:274) LEAQ go.itab.*os.File,io.Writer(SB), CX
> 0x0060 00096 ($GOROOT/src/fmt/print.go:274) MOVQ CX, (SP)
> 0x0064 00100 ($GOROOT/src/fmt/print.go:274) MOVQ AX, 8(SP)
> 0x0069 00105 ($GOROOT/src/fmt/print.go:274) LEAQ ""..autotmp_13+64(SP), AX
> 0x006e 00110 ($GOROOT/src/fmt/print.go:274) MOVQ AX, 16(SP)
> 0x0073 00115 ($GOROOT/src/fmt/print.go:274) MOVQ $1, 24(SP)
> 0x007c 00124 ($GOROOT/src/fmt/print.go:274) MOVQ $1, 32(SP)
> 0x0085 00133 ($GOROOT/src/fmt/print.go:274) PCDATA $1, $1
> 0x0085 00133 ($GOROOT/src/fmt/print.go:274) CALL fmt.Fprintln(SB)
> 0x008a 00138 (bar.go:8) MOVQ 80(SP), BP
> 0x008f 00143 (bar.go:8) ADDQ $88, SP
> 0x0093 00147 (bar.go:8) RET
> 0x0094 00148 (<autogenerated>:1) CALL runtime.panicwrap(SB)
> 0x0099 00153 (<autogenerated>:1) XCHGL AX, AX
> 0x009a 00154 (<autogenerated>:1) NOP
> 0x009a 00154 (<autogenerated>:1) PCDATA $1, $-1
> 0x009a 00154 (<autogenerated>:1) PCDATA $0, $-2
> 0x009a 00154 (<autogenerated>:1) CALL runtime.morestack_noctxt(SB)
> 0x009f 00159 (<autogenerated>:1) PCDATA $0, $-1
> 0x009f 00159 (<autogenerated>:1) NOP
> 0x00a0 00160 (<autogenerated>:1) JMP 0
> 0x00a5 00165 (<autogenerated>:1) LEAQ 96(SP), DI
> 0x00aa 00170 (<autogenerated>:1) CMPQ (BX), DI
> 0x00ad 00173 (<autogenerated>:1) JNE 42
> 0x00b3 00179 (<autogenerated>:1) MOVQ SP, (BX)
> 0x00b6 00182 (<autogenerated>:1) JMP 42
> 0x0000 65 48 8b 0c 25 00 00 00 00 48 3b 61 10 0f 86 87 eH..%....H;a....
> 0x0010 00 00 00 48 83 ec 58 48 89 6c 24 50 48 8d 6c 24 ...H..XH.l$PH.l$
> 0x0020 50 48 8b 59 20 48 85 db 75 7b 48 83 7c 24 60 00 PH.Y H..u{H.|$`.
> 0x0030 74 62 0f 57 c0 0f 11 44 24 40 48 8d 05 00 00 00 tb.W...D$#H.....
> 0x0040 00 48 89 44 24 40 48 8d 05 00 00 00 00 48 89 44 .H.D$#H......H.D
> 0x0050 24 48 48 8b 05 00 00 00 00 48 8d 0d 00 00 00 00 $HH......H......
> 0x0060 48 89 0c 24 48 89 44 24 08 48 8d 44 24 40 48 89 H..$H.D$.H.D$#H.
> 0x0070 44 24 10 48 c7 44 24 18 01 00 00 00 48 c7 44 24 D$.H.D$.....H.D$
> 0x0080 20 01 00 00 00 e8 00 00 00 00 48 8b 6c 24 50 48 .........H.l$PH
> 0x0090 83 c4 58 c3 e8 00 00 00 00 90 e8 00 00 00 00 90 ..X.............
> 0x00a0 e9 5b ff ff ff 48 8d 7c 24 60 48 39 3b 0f 85 77 .[...H.|$`H9;..w
> 0x00b0 ff ff ff 48 89 23 e9 6f ff ff ff ...H.#.o...
> rel 5+4 t=17 TLS+0
> rel 61+4 t=16 type.string+0
> rel 73+4 t=16 ""..stmp_2+0
> rel 85+4 t=16 os.Stdout+0
> rel 92+4 t=16 go.itab.*os.File,io.Writer+0
> rel 134+4 t=8 fmt.Fprintln+0
> rel 149+4 t=8 runtime.panicwrap+0
> rel 155+4 t=8 runtime.morestack_noctxt+0
139,143c213,217
< go.info."".(*Bar).Print$abstract SDWARFINFO dupok size=26
< 0x0000 04 2e 28 2a 42 61 72 29 2e 50 72 69 6e 74 00 01 ..(*Bar).Print..
< 0x0010 01 11 62 00 00 00 00 00 00 00 ..b.......
< rel 0+0 t=24 type.*"".Bar+0
< rel 21+4 t=29 go.info.*"".Bar+0
---
> go.info."".Bar.Print$abstract SDWARFINFO dupok size=23
> 0x0000 04 2e 42 61 72 2e 50 72 69 6e 74 00 01 01 11 62 ..Bar.Print....b
> 0x0010 00 00 00 00 00 00 00 .......
> rel 0+0 t=24 type."".Bar+0
> rel 18+4 t=29 go.info."".Bar+0
297c371,392
< type."".Bar SRODATA size=96
---
> type..namedata.*func(main.Bar)- SRODATA dupok size=18
> 0x0000 00 00 0f 2a 66 75 6e 63 28 6d 61 69 6e 2e 42 61 ...*func(main.Ba
> 0x0010 72 29 r)
> type.*func("".Bar) SRODATA dupok size=56
> 0x0000 08 00 00 00 00 00 00 00 08 00 00 00 00 00 00 00 ................
> 0x0010 7f 95 9a 2f 08 08 08 36 00 00 00 00 00 00 00 00 .../...6........
> 0x0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> 0x0030 00 00 00 00 00 00 00 00 ........
> rel 24+8 t=1 runtime.memequal64·f+0
> rel 32+8 t=1 runtime.gcbits.01+0
> rel 40+4 t=5 type..namedata.*func(main.Bar)-+0
> rel 48+8 t=1 type.func("".Bar)+0
> type.func("".Bar) SRODATA dupok size=64
> 0x0000 08 00 00 00 00 00 00 00 08 00 00 00 00 00 00 00 ................
> 0x0010 b4 2e bc 27 02 08 08 33 00 00 00 00 00 00 00 00 ...'...3........
> 0x0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> 0x0030 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
> rel 32+8 t=1 runtime.gcbits.01+0
> rel 40+4 t=5 type..namedata.*func(main.Bar)-+0
> rel 44+4 t=6 type.*func("".Bar)+0
> rel 56+8 t=1 type."".Bar+0
> type."".Bar SRODATA size=112
303c398,399
< 0x0050 00 00 00 00 00 00 00 00 10 00 00 00 00 00 00 00 ................
---
> 0x0050 00 00 00 00 01 00 01 00 10 00 00 00 00 00 00 00 ................
> 0x0060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
309a406,409
> rel 96+4 t=5 type..namedata.Print.+0
> rel 100+4 t=25 type.func()+0
> rel 104+4 t=25 "".(*Bar).Print+0
> rel 108+4 t=25 "".Bar.Print+0
320a421,423
> ""..stmp_2 SRODATA size=16
> 0x0000 00 00 00 00 00 00 00 00 0c 00 00 00 00 00 00 00 ................
> rel 0+8 t=1 go.string."debosmit ray"+0
325,326c428,429
< gclocals·2a5305abe05176240e61b8620e19a815 SRODATA dupok size=9
< 0x0000 01 00 00 00 01 00 00 00 00 .........
---
> gclocals·33cdeccccebe80329f1fdbee7f5874cb SRODATA dupok size=8
> 0x0000 01 00 00 00 00 00 00 00 ........
329c432
< "".(*Bar).Print.stkobj SRODATA size=24
---
> "".Bar.Print.stkobj SRODATA size=24
333,334d435
< gclocals·33cdeccccebe80329f1fdbee7f5874cb SRODATA dupok size=8
< 0x0000 01 00 00 00 00 00 00 00 ........
338a440,447
> gclocals·1a65e721a2ccc325b382662e7ffee780 SRODATA dupok size=10
> 0x0000 02 00 00 00 01 00 00 00 01 00 ..........
> gclocals·2589ca35330fc0fce83503f4569854a0 SRODATA dupok size=10
> 0x0000 02 00 00 00 02 00 00 00 00 00 ..........
> "".(*Bar).Print.stkobj SRODATA dupok size=24
> 0x0000 01 00 00 00 00 00 00 00 f0 ff ff ff ff ff ff ff ................
> 0x0010 00 00 00 00 00 00 00 00 ........
> rel 16+8 t=1 type.[1]interface {}+0
Here, it is evident that for the value method case:
a copy of b was created due to the b.Print() invocation
a copy of the string rel 0+8 t=1 go.string."debosmit ray"+0 was set on the copied struct.
Therefore, this further concretizes that when you use value pointers:
a copy of the object is made when you invoke methods on that object
any mutations to internal state, will only reflect on that copy of the object