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 <stack>
#include <queue>
#include <fstream>
using namespace std;

int main(){
  std::ifstream in("input.txt");
  std::cin.rdbuf(in.rdbuf());
  int ans[1024] = {0};
  int dataCounter = 0;
  int i = 0;
  while(1){
    int visited[20][20] = {0};
    int data[20][20];
    int x,y;
    int w = 0;
    int h = 0;
    queue<int> xst;
    queue<int> yst;
    cin >> w;
    cin >> h;
    if(w == 0 && h == 0) break;
    for(i=0;i<h;i++){
      int inputCounter = 0;
      char d;
      while(cin >> d){
        if(d == '.')
          data[i][inputCounter] = 0;
        else if(d == '#')
          data[i][inputCounter] = 1;
        else if(d == '@'){
          data[i][inputCounter] = 2;
          x = inputCounter;
          y = i;
        }
        if(inputCounter == w-1)break;
        inputCounter++;
      }
    }
    int j=0;
    xst.push(x);
    yst.push(y);
    while(!xst.empty()){
      x = xst.front();
      y = yst.front();
      xst.pop();
      yst.pop();
      if(x < 0 || w <= x || y < 0 || h <= y)continue;
      if(visited[y][x]==1 || data[y][x] == 1)continue;
      visited[y][x] = 1;
      ans[dataCounter]++;
      xst.push(x+1);
      yst.push(y);
      xst.push(x);
      yst.push(y+1);
      xst.push(x-1);
      yst.push(y);
      xst.push(x);
      yst.push(y-1);
    }

    dataCounter++;
  }
  for(i=0;i<dataCounter;i++){
    cout << ans[i] << endl;
  }
  return 0;
}

C++: #include <iostream>
using namespace std;
int W,H;
int x[4]={0,0,1,-1};
int y[4]={1,-1,0,0};
int i,j;
int count=0;
char room[100][100];

int dfs(int, int);

int main(){
  while(1){
    int sy,sx;
    cin >> W >> H;
    if(W==0&&H==0)break;
    for(i = 0 ; i < H ; i++){
      for(j = 0 ; j < W ; j++){
	cin >> room[i][j];
	if(room[i][j] == '@'){
	  sy = i;
	  sx = j; 
	}
      }
    }
    cout << dfs(sx,sy) << endl;
  }
  return 0;
}

int dfs(int ax,int ay){
  if(ax<0||ay<0)return 0;
  if(ax>=W||ay>=H)return 0;
  if(room[ay][ax]=='#')return 0;
  room[ay][ax]='#';
  int cnt=1;
  int i;
  for(i=0;i<4;i++){
    int nx=ax+x[i];
    int ny=ay+y[i];
    cnt+=dfs(nx,ny);
  }
  return cnt;
}

C++: #include<bits/stdc++.h>
using namespace std;
char n[100][100];int k[100][100],l[100][100],sum,H,W,i,j,m,s;
#define A(B,C) if(k[B][C]!=1){l[B][C]=1;}
int main(){while(true){s=0;memset(l,0,sizeof(l));cin>>W>>H;if(W==0){break;}for(i=1;i<=H;i++){for(j=1;j<=W;j++){cin>>n[i][j];if(n[i][j]=='.'){k[i][j]=0;}else if(n[i][j]=='@'){k[i][j]=2;l[i][j]=1;}else{k[i][j]=1;}}}
for(i=0;i<2500;i++){for(j=1;j<=H;j++){for(m=1;m<=W;m++){if(l[j][m]==1){A(j-1,m);A(j+1,m);A(j,m-1);A(j,m+1);}}}}
for(j=1;j<=H;j++){for(m=1;m<=W;m++){if(l[j][m]==1){s++;}}}cout<<s<<endl;}}

C++: #include <bits/stdc++.h>
using namespace std;
vector<string> s;
vector<vector<bool> > done;
int w,h;
int dy[] = {-1,1,0,0};
int dx[] = {0,0,-1,1};
int ans = 0;

void dfs(int y, int x)
{
		ans++;
        done[y][x] = true;
        for(int i=0;i<4;i++)
        {
                int ny = y + dy[i];
                int nx = x + dx[i];
                if(0<=ny && ny<h && 0<=nx && nx<w && s[ny][nx]=='.' && !done[ny][nx])
                {
                        dfs(ny,nx);
                }
        }
}
 
signed
main()
{
    while(cin >> w >> h and !(w==0 & h==0))
    {
            s = vector<string>(h);
            done = vector<vector<bool> >(h,vector<bool>(w, false));

			int sy,sx;
            for(int y=0;y<h;y++)
			{
					cin >> s[y];
					for(int x=0;x<w;x++)
					{
						if(s[y][x]=='@')
						{
								sy = y;
								sx = x;
						}
					}
			}
 			dfs(sy,sx);
            cout << ans << endl;
			ans = 0;
    }
}