Constructors in Go

a.go

package tmp

import "net"

type ServerA struct {
	Clients map[string]net.Conn
}

func (srv *ServerA) Accept(conn net.Conn) error {
	key := conn.RemoteAddr().String()
	if _, ok := srv.Clients[key]; ok {
		return ErrCollision
	}
	srv.Clients[key] = conn
	return nil
}

func init() {
	// Construct it yourself!
	srv := &ServerA{
		Clients: map[string]net.Conn{},
	}
	Start(srv)
}

b.go

package tmp

import "net"

type serverB struct {
	clients map[string]net.Conn
}

func (srv *serverB) Accept(conn net.Conn) error {
	key := conn.RemoteAddr().String()
	if _, ok := srv.clients[key]; ok {
		return ErrCollision
	}
	srv.clients[key] = conn
	return nil
}

func NewServerB() Server {
	return &serverB{
		clients: map[string]net.Conn{},
	}
}

func init() {
	// Here's a handy helper constructor.
	srv := NewServerB()
	Start(srv)
}

c.go

package tmp

import "net"

type ServerC struct {
	clients map[string]net.Conn
}

func (srv *ServerC) Init() {
	if srv.clients == nil {
		srv.clients = map[string]net.Conn{}
	}
}

func (srv *ServerC) Accept(conn net.Conn) error {
	key := conn.RemoteAddr().String()
	if _, ok := srv.clients[key]; ok {
		return ErrCollision
	}
	srv.clients[key] = conn
	return nil
}

func init() {
	// Construct it yourself, but don't forget to Init!
	srv := ServerC{}
	srv.Init()
	Start(&srv)
}

main.go

package tmp

import (
	"errors"
	"net"
)

var ErrCollision = errors.New("already connected")

type Server interface {
	Accept(net.Conn) error
}

func Start(srv Server) {
	// ...
}