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 <vector>
#include <list>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <utility>
#include <functional>
#include <sstream>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <climits>
#include <cassert>
using namespace std;
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();}
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define EXIST2(s,e) (find(ALL(s),(e))!=(s).end())
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define EACH(t,i,c) for(t::iterator i=(c).begin(); i!=(c).end(); ++i)
const double EPS = 1e-10;
const double PI  = acos(-1.0);

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

int dfs(vs &f, vvi &visited,int x,int y){
	int ret=1;
	visited[y][x]=1;
	REP(i,4){
		int xx=x+dx[i],yy=y+dy[i];
		if(xx>=0&&yy>=0&&xx<w&&yy<h){
			if(f[yy][xx]!='#'&&!visited[yy][xx]){
				ret+=dfs(f,visited,xx,yy);
			}
		}
	}
	return ret;
}

int main(){
	while(cin>>w>>h,w){
		vs f(h,string(w,' '));
		int sx,sy;
		REP(i,h){
			REP(j,w){
				cin>>f[i][j];
				if(f[i][j]=='@'){
					sx=j;sy=i;
				}
			}
		}
		vvi visited(h,vi(w));
		cout<<dfs(f,visited,sx,sy)<<endl;
	}
}

C++: #include<bits/stdc++.h>
using namespace std;
int main(){
  int mp[21][21];
  int w,h;
  int x[4]={1,-1,0,0};
  int y[4]={0,0,1,-1};
  int m;
  bool b;
  char s;
  int ans;
  while(1){
    cin>>w>>h;
    if(w==0 && h==0) break;
    ans=0;
    for(int i=0;i<21;i++){
      for(int j=0;j<21;j++){
	mp[i][j]=0;
      }
    }
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	cin>>s;
	if(s=='#') mp[i][j]=1;
	else if(s=='@') mp[i][j]=2;
      }
    }
    b=true;
    while(b){
      b=false;
      for(int i=0;i<h;i++){
	for(int j=0;j<w;j++){
	  if(mp[i][j]==2){
	    /*for(int k=0;k<4;k++){*/
	      if(mp[i+1][j]==0){
		mp[i+1][j]=2;
		b=true;
	      }
	      if(mp[i-1][j]==0){
		mp[i-1][j]=2;
		b=true;
	      }
	      if(mp[i][j+1]==0){
		mp[i][j+1]=2;
		b=true;
	      }
	      if(mp[i][j-1]==0){
		mp[i][j-1]=2;
		b=true;
	      }
	  }
	}
      }
    }
    for(int i=0;i<h;i++){
      for(int j=0;j<w;j++){
	/*cout<<mp[i][j]<<" ";*/
	if(mp[i][j]==2) ans++;
      }
      /* cout<<endl;*/
    }
    cout<<ans<<endl;
  }
  return 0;
}

C++: #include <iostream> 
#include <string> 
#include <vector> 
#include <cmath> 
#include <algorithm> 
#include <cstdlib> 
#include <ctime> 
#include <cstdio> 
#include <functional> 
#include <set> 
#include <sstream> 
#include <cctype>
#include <stack>
#include <queue>
#include <cstring>
#include <map>
 
using namespace std;

bool vis[30][30];
int cnt;
int dy[]={-1,0,1,0};
int dx[]={0,1,0,-1};
int w,h;

void dfs(vector<string> &vs,int y,int x){

	if(vis[y][x]) return;
	vis[y][x]=true;
	cnt++;

	for(int i=0;i<4;i++){
		int ny=y+dy[i],nx=x+dx[i];
		if( 0<=ny && ny<h && 0<=nx && nx<w && vs[ny][nx]=='.') dfs(vs,ny,nx);
	}

}

int main(){

	while(cin>>w>>h,w||h){

		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++) vis[i][j]=false;
		}
		cnt=0;

		vector<string> vs(h);
		int x,y;
		for(int i=0;i<h;i++){
			cin>>vs[i];
			for(int j=0;j<w;j++){
				if(vs[i][j]=='@') y=i,x=j;
			}
		}

		dfs(vs,y,x);
		cout<<cnt<<endl;

	}

	return 0;
}

C++: #include <iostream>
#include <algorithm>

using namespace std;

int w,h;
char map[20][20];
int cnt;

int dirx[4] = {1,0,-1,0};
int diry[4] = {0,1,0,-1};

bool inField(int x,int y){
	return (0<=x) && (x<w) && (0<=y) && (y<h);
}

//深さ優先探索
void dfs(int posx,int posy){
	cnt ++;
	map[posy][posx] = '#';
	for(int i=0;i<4;i++){
		int nextx = posx + dirx[i];
		int nexty = posy + diry[i];
		if(inField(nextx,nexty) && map[nexty][nextx] != '#'){
			dfs(nextx,nexty);
		}
	}
	
}
//名簿
int main(){
	while(true){
		cin >> w >> h;
		if(w==0){
			break;
		}
		cnt =0;
		int posx,posy;
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				cin >> map[i][j];
				if(map[i][j] == '@'){
					posx = j;
					posy = i;
					map[i][j] = '.';
				}
				
			}
		}
		dfs(posx,posy);
		cout << cnt << endl;
	}
}