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 <algorithm>
#include <bitset>
#include <cmath>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

const int MOD = 1e9 + 7;
const int iINF = 1000000000;
const long long int llINF = 1000000000000000000;

using namespace std;
using ll = long long int;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vvvl = vector<vector<vector<ll>>>;

typedef pair<ll, ll> pll;
bool paircomp(const pll &a, const pll &b) {
  if (a.first == b.first)
    return a.second < b.second;
  return a.first < b.first;
}
struct multi {
  ll first;
  ll second;
  ll third;
};
bool multicomp(const multi &a, const multi &b) {
  if (a.first == b.first)
    return a.second < b.second;
  return a.first < b.first;
}
#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define AUTO(i, m) for (auto &i : m)
#define ALL(a) (a).begin(), (a).end()
#define MAX(vec) *std::max_element(vec.begin(), vec.end())
#define MIN(vec) *std::min_element(vec.begin(), vec.end())
#define ARGMAX(vec)                                                            \
  std::distance(vec.begin(), std::max_element(vec.begin(), vec.end()))
#define ARGMIN(vec)                                                            \
  std::distance(vec.begin(), std::min_element(vec.begin(), vec.end()))
#define REV(T) greater<T>()
#define PQ(T) priority_queue<T, vector<T>, greater<T>>
#define VVL(a, b, c) vector<vector<ll>>(a, vector<ll>(b, c))
#define VVVL(a, b, c, d)                                                       \
  vector<vector<vector<ll>>>(a, vector<vector<ll>>(b, vector<ll>(c, d)))
#define SP(a) setprecision(a)
#define SQRT(a) sqrt((long double)(a))
#define DPOW(a, b) pow((long double)(a), (long double)(b))
#define UNIQUE(vec)                                                            \
  do {                                                                         \
    sort(ALL((vec)));                                                          \
    (vec).erase(std::unique(ALL((vec))), (vec).end());                         \
  } while (0)

ll POW(ll n, ll m) {
  if (m == 0) {
    return 1;
  } else if (m % 2 == 0) {
    ll tmp = POW(n, m / 2);
    return (tmp * tmp);
  } else {
    return (n * POW(n, m - 1));
  }
}

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

void func(vvl &S, vector<string> &F, ll &w, ll &h, ll y, ll x) {
  if (y < 0 || y >= h || x < 0 || x >= w || S[y][x] != 0) {
    return;
  } else if (F[y][x] == '#') {
    S[y][x] = -1;
    return;
  } else {
    S[y][x] = 1;
    func(S, F, w, h, y, x + 1);
    func(S, F, w, h, y, x - 1);
    func(S, F, w, h, y + 1, x);
    func(S, F, w, h, y - 1, x);
    return;
  }
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  vl all_ans;
  while (true) {
    ll h, w;
    cin >> w >> h;
    if (h == 0 || w == 0)
      break;
    vvl S = VVL(h, w, 0);
    vector<string> F(h);
    REP(i, h) cin >> F[i];
    ll x = 0, y = 0;
    REP(i, h) REP(j, w) {
      if (F[i][j] == '@') {
        x = j;
        y = i;
      }
    }
    func(S, F, w, h, y, x);
    ll ans = 0;
    REP(i, h) REP(j, w) {
      if (S[i][j] == 1)
        ans++;
    }
    all_ans.push_back(ans);
  }
  AUTO(ans, all_ans) cout << ans << endl;

  return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<queue>
#include<cstring>
using namespace std;
char cm[200][200];
int im[200][200];
 int W,H;
bool visited[200][200];
int sy,sx;
int dx[4] = {1,0,-1,0};
int dy[4] = {0,1,0,-1};
typedef pair<int,int> P;

int bfs();

int main(){
 

  while(1)
    {
      cin >> W >> H;
      if(W == 0 && H == 0)break;
      memset(im,0,sizeof(im));
      memset(visited,false,sizeof(visited));
      for(int i=0;i<H;i++)
	{
	  for(int j=0;j<W;j++)
	    {
	      cin >> cm[i][j];
	      if(cm[i][j] == '#'){
		im[i][j] = 8;
	      }
	      else if(cm[i][j] == '@')
		{
		  sy = i;
		  sx = j;
		  im[i][j] = 1;
		}
	    }
	}

      cout << bfs() << endl;
      /*     
 for(int i=0;i<H;i++){
	for(int j=0;j<W;j++)cout << im[i][j];
	cout << endl;
      }
      */

    }
}

int bfs()
{
  int maxx = 0;
  queue<P> que;
  que.push(P(sx,sy)); 
  im[sy][sx] = 1;
  int cou=1;
  visited[sy][sx] = true;

  while(que.size())
    {
      P p = que.front();
      que.pop();

      for(int i=0;i<4;i++)
	{
	  int nx = p.first + dx[i],ny = p.second + dy[i];

	  if(0 <= nx && nx < W && 0 <= ny && ny < H && cm[ny][nx] != '#' && !visited[ny][nx])
	    {
	      visited[ny][nx] = true;
	      cou++;
	      im[ny][nx]=cou;
	      maxx = max(maxx,im[ny][nx]);
	      que.push(P(nx,ny));
	    }
	}
      
    }
return cou;
}