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
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <queue>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const int DX[] = {0, 1, 0, -1};
const int DY[] = {-1, 0, 1, 0};


//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

//debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

int main(int argc, char const *argv[])
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	for(int h, w; cin >> w >> h, w | h;){
		VVI table(h, VI(w));
		PII goal;
		REP(i, h){
			string line;
			cin >> line;
			REP(j, w){
				char letter = line[j];
				int value = 0;
				if(letter == '@'){
					goal = MP(i, j);
					value = -1;
				}else if(letter == '#'){
					value = -1;
				}
				table[i][j] = value;
			}
		}
		queue<PII> q;
		q.push(goal);
		int total = 1;
		while(!q.empty()){
			PII p = q.front();
			q.pop();
			int x = p.second, y = p.first;
			int cv = table[y][x];
			REP(i, 4){
				int dx = x + DX[i];
				int dy = y + DY[i];
				if(0 <= dx && dx < w &&
					0 <= dy && dy < h &&
					table[dy][dx] == 0){
					total++;
					table[dy][dx] = 1;
					q.push(MP(dy, dx));
				}
			}
		}

		cout << total << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <string>
using namespace std;
vector<int> x;
vector<int> y;
vector<int> nextx;
vector<int> nexty;
int w,h;
int count;
bool check(int a,int b){
  for (int i=0;i<nextx.size();i++){
    if (nextx[i]==a&&nexty[i]==b)
      return false;
  }
  return true;
}
int search(int **field){
  x.clear();
  y.clear();
  for (int i=0;i<nextx.size();i++){
    x.push_back(nextx[i]);
    y.push_back(nexty[i]);
  }
  nextx.clear();
  nexty.clear();
  for (int i=0;i<x.size();i++){
    field[x[i]][y[i]]=0;
    count++;
  }
  for (int i=0;i<x.size();i++){
    if (field[x[i]-1][y[i]]==1&&x[i]-1>=0&&check(x[i]-1,y[i])){
      nextx.push_back(x[i]-1);
      nexty.push_back(y[i]);
    }
    if (x[i]+1<h&&field[x[i]+1][y[i]]==1&&check(x[i]+1,y[i])){
      nextx.push_back(x[i]+1);
      nexty.push_back(y[i]);
    }
    if (field[x[i]][y[i]-1]==1&&y[i]-1>=0&&check(x[i],y[i]-1)){
      nextx.push_back(x[i]);
      nexty.push_back(y[i]-1);
    }
    if (field[x[i]][y[i]+1]==1&&y[i]+1<w&&check(x[i],y[i]+1)){
      nextx.push_back(x[i]);
      nexty.push_back(y[i]+1);
    }
  }
  if (nextx.empty()){
    return -1;
  }
  int *f[h];
  for (int i=0;i<h;i++)
    f[i]=field[i];
  return search(f);
}
int main(){
  string temp;
  while (cin>>w>>h,w|h!=0){
    count=0;
    int field[h][w];
    for (int i=0;i<h;i++){
      cin>>temp;
      for (int j=0;j<w;j++){
	if (temp[j]=='.')
	  field[i][j]=1;
	if (temp[j]=='#')
	  field[i][j]=0;
	if (temp[j]=='@'){
	  field[i][j]=1;
	  nextx.push_back(i);
	  nexty.push_back(j);
	}
      }
    }
    int *f[h];
    for (int i=0;i<h;i++)
      f[i]=field[i];
    search(f);
    cout<<count<<endl;
  }
}