goreq | Minimal and simple request library for Go language | REST library
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QUESTION
So a struct holds data that could get mutated. Is there some trick or technique in Golang that can tell a func that it must accept a new instance of a struct? In other words, try to best avoid reusing data that may have been mutated before the fact or may get mutated during func lifecycle. (I could avoid mutating stuff, but other devs on my team might not get the memo).
To illustrate:
type CMRequest struct {
Endpoint string
Method string
}
func (cmreq CMRequest) Run() (res *goreq.Response) {
/// this could mutate cmreq
}
obviously Run() could mutate the data in cmreq, so I am wondering if there is a good pattern to force the creation of fresh data every time? The only thing I can think of is to keep the struct private, and do something like this:
type cmrequest struct {
Endpoint string
Method string
}
func (cmreq cmrequest) Run() (res *goreq.Response) {
/// this could mutate cmreq
}
and then expose a helper func:
func MakeRequestAndUnmarshalBody(d CMRequestNoReuse) (*goreq.Response) {
// check that d has a unique memory location?
cmreq := NewCPRequest(d)
res := cmreq.Run()
return res
}
so the helper func would be public, and it would create a new instance of the struct every time? is there any other way to go about it? I still can't force the user to pass in new data every time, although I could check to see if the memory location of d CMRequestNoReuse is unique?
ANSWER
Answered 2020-Apr-03 at 17:43Actually, no, in your example, this doesn't mutate data in CMRequest instance:
func (cmreq CMRequest) Run() (res *goreq.Response) {
/// this could mutate cmreq
}
When you pass object by value, you actually get copy of it, not reference to it.
To mutate, you need to pass by pointer. I.e. Run method would look like:
func (cmreq *CMRequest) Run() (res *goreq.Response) {
/// this actually can mutate cmreq
}
Then you actually have access to original object via pointer and can mutate it.
Here is example in go playground. But note, if one of the fields of struct is pointer -- you still can mutate it.
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