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>
using namespace std;
typedef pair<int, int> pii;
pii dxdy[]={{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
char fld[20][20];
bool used[20][20];
int W, H;

void rec(int i, int j){
   used[i][j]=1;
   for(pii dd: dxdy){
      int ni=i+dd.first;
      int nj=j+dd.second;
      if(!(0<=ni&&ni<H&&0<=nj&&nj<W)) continue;
      if(used[ni][nj]) continue;
      if(fld[ni][nj]!='.') continue;
      rec(ni, nj);
   }
}

int main(){

   while(1){
      cin>> W>> H;
      if(!(H||W)) break;
      int sx, sy;
      for(int i=0; i<H; i++){
         for(int j=0; j<W; j++){
            cin>> fld[i][j];
            if(fld[i][j]=='@'){
               sy=i;
               sx=j;
            }
         }
      }

      for(int i=0; i<20; i++)for(int j=0; j<20; j++) used[i][j]=0;
      used[sy][sx]=1;
      rec(sy, sx);
      int ans=0;
      for(int i=0; i<H; i++)for(int j=0; j<W; j++){
         ans+=used[i][j];
      }
      cout<< ans<< endl;

   }

   return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <iostream>
#include <sstream>
#include <vector>
#include <string>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <algorithm>
#include <functional>

using namespace std;

#define rep(i,n) for((i)=0;(i)<(int)(n);(i)++)
#define foreach(c,itr) for(__typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)

int W,H,res;
string board[30];

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

void dfs(int y, int x){
  int i;

  res++;
  board[y][x] = '#';
  rep(i,4){
    int my = y + dy[i];
    int mx = x + dx[i];
    if(0 <= my && my < H && 0 <= mx && mx < W && board[my][mx] == '.') dfs(my,mx);
  }
}

int main(){
  int i,j;

  while(1){
    scanf("%d %d",&W,&H);
    if(W == 0 && H == 0) break;

    rep(i,H) cin >> board[i];

    res = 0;
    rep(i,H) rep(j,W) if(board[i][j] == '@') dfs(i,j);
    printf("%d\n",res);
  }

  return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;
int cnt = 0;
void depth_first_search(int,int,vector< vector<char> > &);
int main(){

  while(true){
    int w,h;
    cin >> w;
    cin >> h;
    
    int fx;
    int fy;
    if(w == 0 && h ==0 )return 0;
    vector< vector<char> > map(w, vector<char>(h));
    for(int i = 0;i < h;i++){
      for(int j = 0;j < w;j++){
	cin >> map[j][i];
	
	if(map[j][i] == '@'){
	  fx = j;
	  fy = i;
	}
      }
    }
    cnt = 1;
    depth_first_search(fx,fy,map);
    cout << cnt << endl;
  }
  return 0;
}
void depth_first_search(int x,int y,vector< vector<char> > &map){
  int dx[4] = {0,1,-1,0};
  int dy[4] = {1,0,0,-1};

  for(int i = 0;i < 4;i++){
    if(!(x+dx[i] == -1 
	|| x+dx[i] == map.size() 
	|| y+dy[i] == -1 
	|| y+dy[i] == map[0].size()
	)
	&& map[x+dx[i]][y+dy[i]] == '.' 
       ){
      map[x+dx[i]][y+dy[i]] = '1';
      cnt++;
      
      depth_first_search(x+dx[i],y+dy[i],map);
      
    }
  }
}

C++: #include <iostream>
#include <vector>
using namespace std;
int solve(bool visited[20][20],vector<string> v,int x,int y,int w,int h){
	if(x<0||y<0||w<=x||h<=y) return 0;
	if(visited[y][x]==true||v[y][x]=='#') return 0;
	int sum=1;
	visited[y][x]=true;
	sum+=solve(visited,v,x+1,y,w,h);
	sum+=solve(visited,v,x,y+1,w,h);
	sum+=solve(visited,v,x-1,y,w,h);
	sum+=solve(visited,v,x,y-1,w,h);
	return sum;
}
int main(void){
	int w,h;
	while(cin>>w>>h,w||h){
		vector<string> v;
		bool visited[20][20];
		for(int i=0;i<20;++i) for(int j=0;j<20;++j)visited[i][j]=false;
		int x,y;
		for(int i=0;i<h;++i){
			string s;cin>>s;
			if(s.find("@")!=-1){x=s.find("@");y=i;}
			v.push_back(s);
		}
		cout<<solve(visited,v,x,y,w,h)<<endl;
	}
	return 0;
}