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<queue>
#include<string>
#include<map>
#define F first
#define S second
using namespace std;

char c[33][33];
int w,h;

int solve(int x, int y){
  typedef pair < int, int > P;
  queue < P > que;
  int count = 0;

  que.push( P(x, y) );
  
  while(!que.empty()){
    P q = que.front(); que.pop();
    
    int dx[] = {0, 0, 1, -1};
    int dy[] = {1, -1, 0, 0};
    
    for(int i=0;i<4;i++){
      int xx = dx[i] + q.F;
      int yy = dy[i] + q.S;

      if(xx < 0 || yy < 0 || xx == w || yy == h) continue;
      if(c[yy][xx] == '#') continue;

      c[yy][xx] = '#';
      que.push( P(xx, yy) );
      count++;
    }

  }

  return count;
}

int main(){

  int sx, sy;
  while(true){
    cin >> w >> h;
    if(!w && !h) break;

    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin >> c[i][j];
	if(c[i][j] == '@') sx = j, sy = i;
      }
    }

    cout << solve(sx, sy) << endl;
  }

}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <utility>
#include <cstring>
#include <iomanip>
#include <numeric>
#include <limits>
#include <cmath>
#include <cassert>
#include <queue>

using namespace std;
using ll = long long;

const int INF = 1<<30;
const int MOD = (int)1e9 + 7;
const int MAX_N = (int)1e5 + 5;
const int dy[] = {1, 0, -1, 0};
const int dx[] = {0, 1, 0, -1};
#define debug(x) cout << #x << ": " << x << endl

int W, H;

int solve(int x, int y, const vector<string> &table)
{
    queue<pair<int, int>> que;
    que.push(make_pair(y, x));
    vector<vector<int>> used(H, vector<int>(W, 0));
    used[y][x] = 1;
    int res = 0;
    while(que.size())
    {
        pair<int, int> p = que.front(); que.pop();
        res++;
        for(int i = 0; i < 4; i++)
        {
            int ny = p.first + dy[i];
            int nx = p.second + dx[i];
            if(nx < 0 or ny < 0 or nx >= W or ny >= H) continue;
            if(table[ny][nx] == '#' or used[ny][nx]) continue;
            used[ny][nx] = 1;
            que.push(make_pair(ny, nx));
        }
    }
    return res;
}

signed main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    while(cin >> W >> H, W or H)
    {
        vector<string> table(H);
        int sx, sy;
        for(int i = 0; i < H; i++)
        {
            cin >> table[i];
            for(int j = 0; j < table[i].size(); j++)
            {
                if(table[i][j] == '@') sx = j, sy = i;
            }
        }
        cout << solve(sx, sy, table) << endl;
    }
    return 0;
}

C++: #include <stdio.h>

void search(int x, int y, int w, int h, int tile[20][20], int& num){
  if(tile[y][x] != 0){
    num++;
    tile[y][x] = 0;
  
    if(x-1 >= 0){
      search(x-1, y, w, h, tile, num);
    }
    if(x+1 < w){
      search(x+1, y, w, h, tile, num);
    }
    if(y-1 >= 0){
      search(x, y-1, w, h, tile, num);
    }
    if(y+1 < h){
      search(x, y+1, w, h, tile, num);
    }
  }
}

int main(void){
  int w, h, x, y;
  char c[20][21];
  int tile[20][20];
  while(1){
    scanf("%d%d", &w, &h);
    if(w==0 && h==0){
      break;
    }
    int num = 0;
    for(int i=0; i<h; i++){
      scanf("%s", c[i]);
    }
    for(int i=0; i<h; i++){
      for(int j=0; j<w; j++){
	if(c[i][j] == '.'){
	  tile[i][j] = 1;
	}else if(c[i][j] == '@'){
	  tile[i][j] = 1;
	  x = j;
	  y = i;
	}else if(c[i][j] == '#'){
	  tile[i][j] = 0;
	}
      }
    }
    search(x, y, w, h, tile, num);
    printf("%d\n", num);
  }
  return 0;
}