I have a struct type that consists of maps of other structs, which I want
to manipulate through methods. Here is a simplification:
type Reconciler struct {
registers map[string]CashRegister
}
func(r *Reconciler) AddEntry(register string, amount int) {
reg, ok := r.registers[register]
if !ok {
// make a new CashRegister for register, assign it to "reg"
}
reg.Add(amount)
}
type CashRegister struct {
// some fields
}
func(c *CashRegister) Add(amount int) {
// mutate some state in the CashRegister
}
Now the program creates a Reconciler, goes through a list of entries and
adds them, then puts the Reconciler onto a channel for something else to
work with. The problem (you probably see it already) is that the
Reconciler's state never changes. The line "reg.Add(amount)" changes a copy
of the value stored in the map, not the value stored in the map. But I
can't say "r.registers[register].Add(amount)" because the compiler
complains that isn't a pointer type. Clearly I need to operate on the
actual values, not copies of them.
My next thought is that I need to change the definition of Reconciler so I
manipulate pointers, not values:
type Reconciler struct {
registers map[string]*CashRegister
}
Is that the solution? Is that going to cause any problems with sending the
value on the channel? I thought I saw somewhere that you should only send
values on channels, not pointers, or there could be problems with
concurrency and safety.
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