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mutex-zero-value.md

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Zero-value Mutexes are Valid

The zero-value of sync.Mutex and sync.RWMutex is valid, so you almost never need a pointer to a mutex.

BadGood
 
mu := new(sync.Mutex)
mu.Lock()
 
var mu sync.Mutex
mu.Lock()

If you use a struct by pointer, then the mutex should be a non-pointer field on it. Do not embed the mutex on the struct, even if the struct is not exported.

BadGood
 
type SMap struct {
  sync.Mutex

  data map[string]string
}

func NewSMap() *SMap {
  return &SMap{
    data: make(map[string]string),
  }
}

func (m *SMap) Get(k string) string {
  m.Lock()
  defer m.Unlock()

  return m.data[k]
}
 
type SMap struct {
  mu sync.Mutex

  data map[string]string
}

func NewSMap() *SMap {
  return &SMap{
    data: make(map[string]string),
  }
}

func (m *SMap) Get(k string) string {
  m.mu.Lock()
  defer m.mu.Unlock()

  return m.data[k]
}

The Mutex field, and the Lock and Unlock methods are unintentionally part of the exported API of SMap.

The mutex and its methods are implementation details of SMap hidden from its callers.