Go: package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	sc = bufio.NewScanner(os.Stdin)
)

type Color int

const (
	ColorBlack Color = iota
	ColorRed
)

var strByColor = map[Color]string{
	ColorBlack: "black",
	ColorRed:   "red",
}

func (c Color) String() string {
	return strByColor[c]
}

type Node struct {
	ID      int
	X, Y    int
	Color   Color
	NextIDs map[int]struct{}
}

func (n *Node) String() string {
	d, _ := json.Marshal(n)
	return string(d)
}

type Map struct {
	Width, Height int
	Nodes         map[int]*Node
}

func (m *Map) GetByID(id int) *Node {
	return m.Nodes[id]
}

func (m *Map) Get(x, y int) *Node {
	return m.Nodes[m.getID(x, y)]
}

func (m *Map) SetColor(x, y int, c Color) {
	m.Nodes[m.getID(x, y)].Color = c
}

func (m *Map) getID(x, y int) int {
	return x + y*m.Width
}

func (m *Map) init() {
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			id := m.getID(x, y)
			m.Nodes[id] = &Node{ID: id, X: x, Y: y}
		}
	}
	for x := 1; x <= m.Width; x++ {
		for y := 1; y <= m.Height; y++ {
			nextIDs := map[int]struct{}{}
			if x > 1 {
				nextIDs[m.getID(x-1, y)] = struct{}{}
			}
			if x < m.Width {
				nextIDs[m.getID(x+1, y)] = struct{}{}
			}
			if y > 1 {
				nextIDs[m.getID(x, y-1)] = struct{}{}
			}
			if y < m.Height {
				nextIDs[m.getID(x, y+1)] = struct{}{}
			}
			m.Get(x, y).NextIDs = nextIDs
		}
	}
}

func NewMap(w, h int) *Map {
	m := &Map{Width: w, Height: h, Nodes: map[int]*Node{}}
	m.init()
	return m
}

type Searcher struct {
	Map        *Map
	queue      []*Node
	touchedIDs map[int]struct{}
	visitedIDs map[int]struct{}
}

func NewSearcher(m *Map) *Searcher {
	return &Searcher{
		Map:        m,
		queue:      []*Node{},
		touchedIDs: map[int]struct{}{},
		visitedIDs: map[int]struct{}{},
	}
}

func (s *Searcher) Search(start *Node) {
	s.enqueue(start)

	for {
		if len(s.queue) == 0 {
			break
		}
		for nextID := range s.dequeue().NextIDs {
			n := s.Map.GetByID(nextID)
			if n.Color == ColorRed || s.hasTouched(n) {
				continue
			}
			s.enqueue(n)
		}
	}
}

func (s *Searcher) VisitedNodeCount() int {
	return len(s.visitedIDs)
}

func (s *Searcher) hasTouched(n *Node) bool {
	_, ok := s.touchedIDs[n.ID]
	return ok
}

func (s *Searcher) enqueue(n *Node) {
	s.queue = append(s.queue, n)
	s.touchedIDs[n.ID] = struct{}{}
}

func (s *Searcher) dequeue() *Node {
	n := s.queue[0]
	s.queue = s.queue[1:]
	s.visitedIDs[n.ID] = struct{}{}
	return n
}

func main() {
	for sc.Scan() {
		if sc.Text() == "0 0" {
			break
		}
		chunks := strings.Split(sc.Text(), " ")
		w, _ := strconv.Atoi(chunks[0])
		h, _ := strconv.Atoi(chunks[1])

		m := NewMap(w, h)

		// Set colors
		var start *Node
		for y := 1; y <= h; y++ {
			sc.Scan()
			for i, c := range sc.Text() {
				switch c {
				case '.', '@':
					m.SetColor(i+1, y, ColorBlack)
					if c == '@' {
						start = m.Get(i+1, y)
					}
				case '#':
					m.SetColor(i+1, y, ColorRed)
				}
			}
		}

		// Search
		s := NewSearcher(m)
		s.Search(start)
		fmt.Println(s.VisitedNodeCount())
	}
}

C++: #include<iostream>
using namespace std;
void counts(int h,int w);
int check[31][31]={0};
char s[31][31];
int dh[4]={-1,0,1,0};
int dw[4]={0,1,0,-1};
int ww,hh;
int counta=0;
main(){
  int sw,sh,countx;
  while(1){
    cin>>ww>>hh;
    if(ww==0&&hh==0) break;
    for(int i=1;i<=hh;i++){
      for(int j=1;j<=ww;j++){
	cin>>s[i][j];
	if(s[i][j]=='@'){
	  sh=i;
	  sw=j;
	}
      }
    }
    counts(sh,sw);
    cout<<counta<<endl;
    for(int i=0;i<=30;i++){
      for(int j=0;j<=30;j++){
	check[i][j]=0;
      }
    }
    counta=0;
  }
}
void counts(int h,int w){
  check[h][w]=1;
  counta++;
  for(int i=0;i<8;i++){
    int nh=h+dh[i];
    int nw=w+dw[i];
    if(nh>=1&&nh<=hh&&nw>=1&&nw<=ww&&s[nh][nw]=='.'&&check[nh][nw]==0){
      counts(nh,nw);
    }
  }
  return;
}

C++: #include<iostream>
using namespace std;

bool map[20][20];
int w,h;


int Search(int x,int y, int sum){

  sum++;
  map[y][x]=false;

  cout << "x,y=" << x << ',' << y << endl;

  if(x!=w-1){
    if(map[y][x+1]==true){
      sum = Search(x+1,y,sum);
    }
  }
  if(x!=0){
    if(map[y][x-1]==true){
      sum = Search(x-1,y,sum);
    }
  }
  if(y!=h-1){
    if(map[y+1][x]==true){
      sum = Search(x,y+1,sum);
    }
  }
  if(y!=0){
    if(map[y-1][x]==true){
      sum = Search(x,y-1,sum);
    }
  }

  return sum;

}


int main(){

  int x,y;
  char info;

  while(cin >> w >> h){
    if(w==0 && h==0){ break; }

    for(int i=0; i<h; i++){
        for(int j=0; j<w; j++){
          cin >> info;
          if(info == '.'){ map[i][j]=true;}
          else if(info == '#'){ map[i][j]=false;}
          else{ x=j; y=i; map[i][j]=false;}
        }
    }

    cout << Search(x,y,0) << endl;
  }

  return 0;

}