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>
#include<string>
#include<algorithm>
#include<vector>
#include<cstdio>
#include<cstring>
using namespace std;

#define rep(i,n) for(int i=0;i<n;i++)
#define REP(n) rep(i,n)
#define all(n) n.begin(),n.end()

const int MAXW = 25, MAXH = 25;
const char B = '.', R = '#';
char tile[MAXW][MAXH];
int dx[] = {-1, 1, 0, 0};
int dy[] = {0, 0, -1, 1};

int solve(int x, int y)
{
    int ans = 1;

    REP(4)if(tile[x + dx[i]][y + dy[i]] == B)
    {
        tile[x + dx[i]][y + dy[i]] = R;
        ans += solve(x + dx[i], y + dy[i]);
    }

    return ans;
}

int main()
{
    int w, h;
    while(cin >> w >> h && ( w || h))
    {
        memset(tile, R, sizeof(tile));

        int x, y; char t;
        rep(j, h)rep(i, w)
        {
            cin >> tile[i + 1][j + 1];
            if(tile[i + 1][j + 1] == '@'){ x = i + 1; y = j + 1; tile[i + 1][j + 1] = R;}
        }
            cout << solve(x, y) << endl;
    }

    return 0;
}

C++: #include <stdio.h>

#define W 21
#define H 20

int tileCount(char room[H][W], int row, int col, int h, int w);

int main()
{
  char room[H][W];
  int h;    // 部屋の行方向のタイルの枚数
  int w;    // 部屋の列方向のタイルの枚数
  int row;  // 最初に人が立っている行位置
  int col;  // 最初に人が立っている列位置
  int i, j;

  // 部屋の情報を、配列room[]に文字列として読み込む
  scanf("%d%d", &w, &h);
  while (h != 0 && w != 0) {
    for (i = 0; i < h; i++) {
      scanf("%s", room[i]);
      for (j = 0; j < w; j++) {
        if (room[i][j] == '@') {
          row = i;  // 最初に人が立っている行位置
          col = j;  // 最初に人が立っている列位置
        }
      }
    }
    printf("%d\n", tileCount(room, row, col, h, w));

    scanf("%d%d", &w, &h);
  }
  
  return(0);
}

int tileCount(char room[H][W], int row, int col, int h, int w){
  int dx[4]={0,1,0,-1};
  int dy[4]={1,0,-1,0};
  int nx,ny;
  int i,j;
  int cnt=1;
  if(row<0 || col<0 || col>=w || row>=h || room[row][col]=='#')return 0;
  room[row][col]='#';
  for(i=0;i<4;i++){
    nx=col+dx[i];
    ny=row+dy[i];
    cnt+=tileCount(room,ny,nx,h,w);
  }
  return cnt;
}

C++: #include<iostream>
#include<algorithm>

using namespace std;

char tyle[30][30]={'0'};

int find_tyle(int i,int j){
  if(tyle[i+1][j]=='.'){
    tyle[i+1][j]='a';
    find_tyle(i+1,j);
  }
  if(tyle[i][j+1]=='.'){
    tyle[i][j+1]='a';
    find_tyle(i,j+1);
  }
  if(tyle[i-1][j]=='.'){
    tyle[i-1][j]='a';
    find_tyle(i-1,j);
  }
  if(tyle[i][j-1]=='.'){
    tyle[i][j-1]='a';
    find_tyle(i,j-1);
  }
}

int main(){
  
  int w,h;
  
  while(cin>>w>>h,w){
    int ans=0;
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
      cin>>tyle[i][j];
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='@'){
	  find_tyle(i,j);
	  tyle[i][j]='a';
	}
      }
    }
    for(int i=1;i<=h;++i){
      for(int j=1;j<=w;++j){
	if(tyle[i][j]=='a'){
	  ans++;
      }
    }
  }
    cout<<ans<<endl;

   for(int i=0;i<30;++i){
      for(int j=0;j<=30;++j){
      tyle[i][j]='0';
      }
    }
    
  }
}