How do you use wrapper types to adapt existing types to satisfy interfaces in Go?

Sometimes an existing type almost satisfies an interface but has a slightly different method signature. Rather than modifying the original type (which may be in another package), you can wrap it in a new type that adapts the interface.

// Standard logger — method signature doesn't match our interface
// log.Logger has: func (l *Logger) Printf(format string, v ...any)

// Our interface for injecting loggers
type Logger interface {
    Log(msg string)
}

// Adapter: wraps *log.Logger to satisfy Logger
type StdLogger struct{ l *log.Logger }

func NewStdLogger(l *log.Logger) Logger { return &StdLogger{l: l} }
func (s *StdLogger) Log(msg string) { s.l.Println(msg) }

// Now log.Logger can be used anywhere Logger is expected
svc := NewService(NewStdLogger(log.Default()))

// Adapter for third-party HTTP client → our HTTPClient interface
type HTTPClient interface {
    Do(req *http.Request) (*http.Response, error)
}
// *http.Client already has Do() — no adapter needed!
var client HTTPClient = http.DefaultClient // satisfied out of the box

// Reverse adapter: wrap an interface to add a method
// LimitedReader wraps io.Reader and adds a counter
type CountingReader struct {
    r     io.Reader
    count int64
}
func (cr *CountingReader) Read(p []byte) (int, error) {
    n, err := cr.r.Read(p)
    cr.count += int64(n)
    return n, err
}
func (cr *CountingReader) BytesRead() int64 { return cr.count }

cr := &CountingReader{r: os.Stdin}
io.Copy(os.Stdout, cr)
fmt.Println("read", cr.BytesRead(), "bytes")

The adapter pattern in Go is simpler than in Java because Go interfaces are implicit. A wrapper struct that delegates to the original type and has the right method signatures automatically satisfies any compatible interface — no import of the interface's package, no extends Adapter.

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