Using Pointers in a for loop

Question

I'm struggling to understand why I have a bug in my code in one state but not the other. It's been a while since I've covered pointers, so I'm probably rusty!

Basically I have a repository structure I'm using to store an object in memory, that has a Store function.

type chartsRepository struct {
    mtx    sync.RWMutex
    charts map[ChartName]*Chart
}

func (r *chartsRepository) Store(c *Chart) error {
    r.mtx.Lock()
    defer r.mtx.Unlock()
    r.charts[c.Name] = c
    return nil
}

So all it does is put a RW mutex lock on and adds the pointer to a map, referenced by an identifier.

Then I've got a function that will basically loop through a slice of these objects, storing them all in the repository.

type service struct {
    charts Repository
}

func (svc *service) StoreCharts(arr []Chart) error {
    hasError := false
    for _, chart := range arr {
        err := svc.repo.Store(&chart)
        // ... error handling
    }
    if hasError {
        // ... Deals with the error object
        return me
    }
    return nil
}

The above doesn't work, it looks like everything works fine at first, but on trying to access the data later, the entries in the map all point to the same Chart object, despite having different keys.

If I do the following and move the pointer reference to another function, everything works as expected:

func (svc *service) StoreCharts(arr []Chart) error {
    // ...
    for _, chart := range arr {
        err := svc.storeChart(chart)
    }
    // ...
}

func (svc *service) storeChart(c Chart) error {
    return svc.charts.Store(&c)
}

I'm assuming the issue is that because the loop overwrites the reference to the chart in the for loop, the pointer reference also changes. When the pointer is generated in an independent function, that reference is never overwritten. Is that right?

I feel like I'm being stupid, but shouldn't the pointer be generated by &chart and that's independent of the chart reference? I also tried creating a new variable for the pointer p := &chart in the for loop and that didn't work either.

Should I just avoid generating pointers in loops?

Answer 1

Waitting for answers