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 <vector>
#include <queue>
#include <utility>
using namespace std;
int main( void )
{
	const int dx[4] = { 1, 0, -1, 0 };
	const int dy[4] = { 0, -1, 0, 1 };

	int w, h;
	while ( cin>>w>>h && ( w != 0 && h != 0 ) )
	{
		vector < vector <char> > V( h, vector <char>(w, false) );
		queue < pair<int,int> > Q;

		for ( int i = 0; i < h; i++ )
		{
			for ( int j = 0; j < w; j++ )
			{
				cin >> V[i][j];
				if ( V[i][j] == '@' )
					Q.push( make_pair( i, j ) );
			}
		}

		while ( !Q.empty() )
		{
			pair <int,int> now = Q.front();
			Q.pop();

			V[now.first][now.second] = 'v';

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

				if ( ( nx < 0 || nx >= w ) || ( ny < 0 || ny >= h ) )
					continue;

				if ( V[ny][nx] != '.' )
					continue;

				Q.push( make_pair( ny, nx ) );

			}
		}

		int kotae = 0;
		for ( int i = 0; i < h; i++ )
		{
			for ( int j = 0; j < w; j++ )
			{
				if ( V[i][j] == 'v' )
					kotae++;
			}
		}

		cout << kotae << endl;
	}
}

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

using namespace std;

struct Point{
	int x,y;
};

const Point dir[4] = {{1,0},{0,1},{-1,0},{0,-1}};

int w,h;
char map[20][20];
bool visited[20][20];//訪問済みならtrue
int cnt;//黒いタイルを数えるカウンタ

bool inField(Point p){
	return 0<=p.x && p.x < w && 0<=p.y && p.y < h;
}

//幅優先探索
void dfs(Point pos){
	cnt++;
	visited[pos.y][pos.x] = true;
	for(int i=0;i<4;i++){
		Point next = {pos.x+dir[i].x, pos.y+dir[i].y};
		if(inField(next)==true){
			if(visited[next.y][next.x] == false && map[next.y][next.x] == '.'){
				dfs(next);
			}
		}
	}
}

int main(){
	while(true){
		cin >> w >> h;
		if(w==0){
			break;
		}
		Point init;//初期位置
		for(int i=0;i<h;i++){
			for(int j=0;j<w;j++){
				cin >> map[i][j];
				visited[i][j] = false;
				if(map[i][j] == '@'){
					//初期代入以外はキャストする必要がある。
					init = (Point){j,i};
					map[i][j] = '.';
				}
			}
		}
		cnt = 0;
		dfs(init);
		cout << cnt << endl;
	}
	return 0;
}

C++: #if 0
08-162015?????????????????°???????????????????????????

#endif

#include <iostream>
#include <queue>
#include <string.h>
#include <utility>
using namespace std;

int w,h;
const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1}; //??????????§??????¨
char P[30][30]; //??§?¨?????????????????????¨???

bool valid(int x,int y){ //??????????????????
	return (x>=0)&&(x<w)&&(y>=0)&&(y<h)&&(P[x][y]!='#');
}

void bfs(int x,int y){ //???????????¢?´¢
	int tot=1; //?????£????????????(?????????????????????)
	queue<pair<int,int> > q;
	pair<int,int> p;
	p = make_pair(x,y);
	while(!q.empty()){
		p=q.front();
		q.pop();
		for(int i=0;i<4;++i){
			if(valid(p.first+dx[i],p.second+dy[i]))
				tot+=1;
				p.first+=dx[i]; p.second+=dy[i];
				q.push(p);
		}
	}
}


int main(){
	int x1,y1;
	while((cin>>w>>h)&&(w!=0&&h!=0)){
		for(int i=0;i<h;++i){
			for(int j=0;j<w;++j){
				cin>>P[j][i];
				if(P[j][i]=='@')
					x1=j; y1=i; 
			}
		}
		bfs(x1,y1); //??????????????°????????£??\????????¢?´¢????§?
	}
}

C++: #include <iostream>
using namespace std;

int masu[20][20];
int W,H;

int dfs(int x,int y){
     if(x>=W||y>=H||x<0||y<0)return 0;
     if(masu[x][y]==1)return 0;
     masu[x][y]=1;
     return dfs(x+1,y)+dfs(x,y+1)+dfs(x-1,y)+dfs(x,y-1)+1;
}

int main(){
     while(1){
	  cin>>W>>H;
	  if(W==0)break;
	  int pos[2];
	  for(int j=0;j<H;j++){
	       string temp;
	       cin>>temp;
	       for(int i=0;i<W;i++){
		    switch(temp[i]){
		    case '.':
			 masu[i][j]=0;
			 break;
		    case '@':
			 masu[i][j]=0;
			 pos[0]=i;pos[1]=j;
			 break;
		    case '#':
			 masu[i][j]=1;
			 break;
		    }
	       }
	  }
	  int ans=dfs(pos[0],pos[1]);
	  cout<<ans<<endl;
     }
}

C++: #include<iostream>
#include<cmath>
#include<cstdio>
#include<algorithm>
#include<vector>
#include<cstring>

using namespace std;

#define INF 999999999

char map[20][21];
bool ch[20][20];
int ans;

void dfs(int w, int h, char map[20][21], bool c[20][20], int x, int y){
	//cout << x << " " << y << " " << ans << endl;
	ans++;
	c[x][y] = 1;
	if(x>0 && map[x-1][y]=='.' && c[x-1][y]==0){
		dfs(w,h,map,c,x-1,y);
	}
	if(x<h-1 && map[x+1][y]=='.' && c[x+1][y]==0){
		dfs(w,h,map,c,x+1,y);
	}
	if(y>0 && map[x][y-1]=='.' && c[x][y-1]==0){
		dfs(w,h,map,c,x,y-1);
	}
	if(y<w-1 && map[x][y+1]=='.' && c[x][y+1]==0){
		dfs(w,h,map,c,x,y+1);
	}
}

int main(){
	int i, j, w, h, x, y;
	while(1){
		cin >> w >> h;
		if(!w) break;
		for(i=0;i<h;i++){
			cin >> map[i];
			for(j=0;j<w;j++){
				ch[i][j] = 0;
				if(map[i][j] == '@'){
					x = i;
					y = j;
				}
			}
		}
		ans = 0;
		dfs(w,h,map,ch,x,y);
		
		cout << ans << endl;
	}
	return 0;
}