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Field and method promotion

As explained in 100 Go Mistakes and How to Avoid Them

Field and method promotion is the Go behavior that makes members of an embedded type accessible through the containing type.

Embedding promotes an embedded type’s fields and methods through the containing type. The convenience can accidentally expose implementation details, especially when embedding a mutex. section 2.10

Accidentally exporting a mutex

type InMem struct { sync.Mutex m map[string]int } func (i *InMem) Get(key string) (int, bool) { i.Lock() v, ok := i.m[key] i.Unlock() return v, ok } Because Lock and Unlock are promoted, clients can call inmem.New().Lock(). That exposes synchronization the type should normally encapsulate. section 2.10

Choose embedding according to the interface the containing type should expose:

  • Use an unexported named field when the embedded behavior is implementation detail: mu sync.Mutex. section 2.10
  • Embed when promotion intentionally supplies the containing type’s public behavior, such as Logger embedding io.WriteCloser and satisfying its interface. section 2.10
type Foo struct {
    Bar              #1
}
 
type Bar struct {
    Baz int
}

Inspect how an embedded Bar makes Baz directly accessible through Foo, as in foo.Baz = 42. section 2.10

How 100 Go Mistakes and How to Avoid Them covers it

How deep it goes
Explains promoted fields and methods through concrete Go examples, then stops at the embedding-versus-subclassing distinction.
Examples
Uses nested structs, an embedded sync.Mutex, an encapsulated mutex field, and an io.WriteCloser-based Logger.
What it assumes
Assumes familiarity with Go structs, methods, interfaces, and mutex-based synchronization.
What you can do after
Evaluate whether embedding intentionally defines a public API or unintentionally exposes internal behavior.
How it connects
Connects embedding with encapsulation, interfaces, mutexes, and object-oriented subclassing.
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of 100 Go Mistakes and How to Avoid Them. The section links point to the parts of the book it draws on.