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 <stack>
using namespace std;

int main(){
  int W, H;
  while(cin >> W >> H && W>0 && H>0){
    int map[20][20];
    for(int i=0;i<20;i++)
      for(int j=0;j<20;j++)
        map[i][j]=0;
    int at[2];

    for(int i=0;i<H;i++){
      string s;
      cin >> s;
      for(int j=0;j<W;j++){
        map[i][j]=(s[j]=='.'?1:(s[j]=='@'?2:0));
        if(s[j]=='@'){
          at[0]=i; at[1]=j;
        }
      }
    }

    vector<int> list[400];  //??£??\?????????????????????????????????20*i+j?????????
    for(int i=0;i<400;i++){
      if(i>=20) //???
        if(map[i/20-1][i%20]>0) list[i].push_back(i-20);
      if(i%20>0) //???
        if(map[i/20][(i-1)%20]>0) list[i].push_back(i-1);
      if(i%20<19) //???
        if(map[i/20][(i+1)%20]>0) list[i].push_back(i+1);
      if(i<180) //???
        if(map[i/20+1][i%20]>0) list[i].push_back(i+20);
    }

    bool d[400];
    for(int i=0;i<400;i++) d[i]=false;

    stack<int> cur;
    cur.push(at[0]*20+at[1]);
    d[at[0]*20+at[1]]=true;

    while(!cur.empty()){
      bool flg=false;
      for(vector<int>::iterator it=list[cur.top()].begin();it!=list[cur.top()].end();it++){
        if(!d[*it]){
          flg=true;
          cur.push(*it);
          d[*it]=true;
          break;
        }
      }
      if(flg) continue;
      cur.pop();
    }

    int cnt=0;
    for(int i=0;i<400;i++)
      if(d[i]) cnt++;

    cout << cnt << endl;
  }

  return 0;  
}

C++: #include <bits/stdc++.h>
using namespace std;
string place[20];
int dx[4]={0,1,0,-1},dy[4]={-1,0,1,0},n,m;
int serch(int y,int x)
{
	int ans=1;
	place[y][x]='#';
	for(int i=0,ax,ay;i<4;i++)if((ay=dy[i]+y)<m&&(ax=dx[i]+x)<n&&ay>=0&&ax>=0&&place[ay][ax]=='.')ans+=serch(ay,ax);
	return ans;
}

int main() {
	while(cin>>n>>m,n,m){
		for(int i=0;i<m;i++)cin>>place[i];
		for(int i=0;i<m;i++)for(int j=0;j<n;j++)if(place[i][j]=='@')cout<<serch(i,j)<<endl;
	}
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<algorithm>
using namespace std;
#define REP(i, j) for(int i = 0; i < j; i++)

int w, h;

int dfs(int y, int x, vector<string> &v){
  int my[] = {0, 0, 1, -1};
  int mx[] = {1, -1, 0, 0};
  int ret = 1;
  REP(i, 4){
    int ny = y + my[i], nx = x + mx[i];
    if(ny >= 0 && nx >= 0 && ny < h && nx < w && v[ny][nx] == '.'){
      v[ny][nx] = '#';
      ret += dfs(ny, nx, v);
    }
  }
  return ret;
}

int main(){
  while(cin >>w >>h && w){
    vector<string> v(h);
    REP(i, h) cin >>v[i];
    REP(i, h) REP(j, w) if(v[i][j] == '@') cout <<dfs(i, j, v) <<endl;
  }
  return 0;
}