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 <string>
#include <map>
#include <algorithm>
#include <queue>

#define REP(i,n) for(int i=0;i<(int)(n);i++)

using namespace std;

int main() {
  while(1){
    int w,h;
    cin>>w>>h;
    if(!w)break;
    vector<string> t(h);
    REP(i,h)cin>>t[i];
    int si,sj;
    REP(i,h)REP(j,w){
      if(t[i][j] == '@'){
        si = i;
        sj = j;
      }
    }
    vector<vector<bool>> vis(h,vector<bool>(w,false));
    vis[si][sj] = true;
    queue<pair<int,int>> q;
    q.push(make_pair(si,sj));
    while(!q.empty()){
      auto p = q.front();q.pop();
      int i,j;
      tie(i,j) = p;
      int di[] = {1,0,-1,0};
      int dj[] = {0,1,0,-1};
      REP(k,4){
        int ni=i+di[k];
        int nj=j+dj[k];
        if(ni<0||ni>=h||nj<0|nj>=w)continue;
        if(t[ni][nj] == '#')continue;
        if(vis[ni][nj]) continue;
        q.push(make_pair(ni,nj));
        vis[ni][nj] = true;
      }
    }
    int cnt = 0;
    REP(i,h)REP(j,w){
      if(vis[i][j])++cnt;
    }
    cout << cnt << endl;
  }
  return 0;
}

C++: #include<cstdio>


int ans[1024];
int w,h,k;
bool b[22][22];//??°???

int d(int,int);

int main(void){
  int i,j;
  char c[20][20];
  int p,q;//???????????§?¨?

    k =0;
  while(true){
    //??\???
    scanf("%d%d",&w,&h);
    if(w==0 && h==0)break;
    
    for(i=0;i<h;i++){
	scanf("%s",c[i]);
//printf("%d,%s\n",i,c[i]);
      }
//printf("%c\n",c[0][1]);
    for(i=0;i<h;i++){
      for(j=0;j<w;j++){
        if(c[i][j] =='@'){
            p=i;
            q=j;
        }
          if(c[i][j] =='#'){
              b[i][j] = false;
            
          }else {
              b[i][j] = true;
          }
      }
    }
      ans[k] = 1;
      //printf("Ok");
    d(p,q);
      k++;
  }
    for (i=0;i<k;i++){
        printf("%d\n",ans[i]);
    }
  return 0;
}
int d(int p,int q){
  int i,j,p1,q1;
    b[p][q]  = false;
  for(i=0;i<4;i++){
    if(i==0){
      p1=p+1;
      q1=q;
    } else if(i==1){
      p1=p-1;
      q1=q;
    }else if(i==2){
      p1=p;
      q1=q+1;
    }else {
      p1=p;
      q1=q-1;
    }
    if(p1>=0 && p1<h && q1>=0 && q1<w){
      if(b[p1][q1] == true){
	ans[k]++;
          
        d(p1,q1);
      }
    }
  }
  return 0;
}

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

#endif

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

int w,h;
int tot=1; //?????£????????????(?????????????????????)
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){ //???????????¢?´¢
	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); //??????????????°????????£??\????????¢?´¢????§?
		cout << tot << endl;
	}
}

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

void rec(int x,int y);
char map[30][30];
bool isVisit[30][30];
int count=0;
//start_pos
int start_x;
int start_y;
int h,w;

int main()
{
    while (cin >> w >> h) 
    {
        count = 0;
        //start pos

        for (int i = 0; i < h; i++) 
        {
            for (int j = 0; j < w; j++) 
            {
                cin >> map[j][i];
                if (map[j][i] == '@') 
                {
                    start_x=j;
                    start_y=i;
                }
                //ついでにisVisit初期化
                isVisit[j][i]=false;
            }
        }
        rec(start_x,start_y);
        cout <<count<<endl;
    }

}

void rec(int x,int y)
{
    if ( y<0 || x<0 || h<=y || w<=x) {return;}
    if ( map[x][y] == '#') {return;}
    if (isVisit[x][y]) { return;}
    isVisit[x][y] = true;
    count++;

    rec(x, y-1);
    rec(x+1, y);
    rec(x, y+1);
    rec(x-1, y);

    return;
}