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container-copy.md

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Copy Slices and Maps at Boundaries

Slices and maps contain pointers to the underlying data so be wary of scenarios when they need to be copied.

Receiving Slices and Maps

Keep in mind that users can modify a map or slice you received as an argument if you store a reference to it.

Bad Good
 
func (d *Driver) SetTrips(trips []Trip) {
  d.trips = trips
}

trips := ...
d1.SetTrips(trips)

// Did you mean to modify d1.trips?
trips[0] = ...
 
func (d *Driver) SetTrips(trips []Trip) {
  d.trips = make([]Trip, len(trips))
  copy(d.trips, trips)
}

trips := ...
d1.SetTrips(trips)

// We can now modify trips[0] without affecting d1.trips.
trips[0] = ...

Returning Slices and Maps

Similarly, be wary of user modifications to maps or slices exposing internal state.

BadGood
 
type Stats struct {
  mu sync.Mutex
  counters map[string]int
}

// Snapshot returns the current stats.
func (s *Stats) Snapshot() map[string]int {
  s.mu.Lock()
  defer s.mu.Unlock()

  return s.counters
}

// snapshot is no longer protected by the mutex, so any
// access to the snapshot is subject to data races.
snapshot := stats.Snapshot()
 
type Stats struct {
  mu sync.Mutex
  counters map[string]int
}

func (s *Stats) Snapshot() map[string]int {
  s.mu.Lock()
  defer s.mu.Unlock()

  result := make(map[string]int, len(s.counters))
  for k, v := range s.counters {
    result[k] = v
  }
  return result
}

// Snapshot is now a copy.
snapshot := stats.Snapshot()