Struct embedding includes an anonymous field in a Go struct, promoting the embedded field’s methods and data to the containing type. It supports composition rather than inheritance: the types remain distinct, are not interchangeable, and do not form a runtime type hierarchy. Use interfaces when runtime polymorphism is required. section 1.5.1 section 9.11
Use embedding to extend focused components while keeping the surrounding design composable:
- Embed a type or interface anonymously to promote its fields and methods to the containing struct. section 9.11
- Override a promoted method when the containing type needs custom behavior; other calls can continue forwarding to the embedded value. section 9.3.1
- Embedding an interface can help satisfy another interface, but it can also hide behavior or alter serialization when the embedded type implements json.Marshaler. section 9.3.1 section 9.4
- Prefer small composable pieces and wrappers over tightly coupled hierarchies; expose a path to underlying behavior when optional capabilities matter. section 9.0 section 9.11
Intercepting HTTP response writes
type Interceptor struct { http.ResponseWriter OnWriteHeader func(code int) } func (ic *Interceptor) WriteHeader(code int) { if ic.OnWriteHeader != nil { ic.OnWriteHeader(code) } ic.ResponseWriter.WriteHeader(code) } The embedded http.ResponseWriter promotes its methods, allowing Interceptor to satisfy ResponseWriter. The custom WriteHeader method observes the status code, then forwards the call to the original writer. This building block can be combined with middleware to add behavior without modifying handlers. section 9.3.1 section 9.3.4
Listing 9.6 Intercepting method calls (link/kit/hlog/hlog.go)
// Interceptor provides hooks to intercept response writes.
type Interceptor struct {
http.ResponseWriter #1
OnWriteHeader func(code int) #2
}
// WriteHeader calls [Interceptor.OnWriteHeader] if provided and
// then calls the embedded [http.ResponseWriter.WriteHeader].
func (ic *Interceptor) WriteHeader(code int) {
if ic.OnWriteHeader != nil { #3
ic.OnWriteHeader(code) #3
} #3
ic.ResponseWriter.WriteHeader(code) #4
}
Inspect the complete Interceptor listing to see anonymous ResponseWriter embedding, method overriding, callback handling, and forwarding together. section 9.3.1
